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add vipsthumbnail page

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John Cupitt 2017-03-31 13:26:25 +01:00
parent 30829ef003
commit 27e6c60967
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2
autogen.sh
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@ -27,6 +27,8 @@ find doc -depth \( \
-o -path 'doc/images/*' \
-o -name '*.xml' ! -name libvips-docs.xml ! -path 'doc/xml/*' \
-o -name '*.py' \
-o -name '*.md' \
-o -name '*.docbook' \
\) -prune -or \( \
-type f \
-o -type d -empty \

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@ -1 +0,0 @@
How-it-works.xml

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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<refentry id="How-it-works.md">
<refmeta>
<refentrytitle>How-it-works.md</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>libvips</refmiscinfo>
</refmeta>
<refnamediv>
<refname>libvips</refname>
<refpurpose>How-it-works.md</refpurpose>
</refnamediv>
<para>
Compared to most image processing libraries, VIPS needs little RAM and runs quickly, especially on machines with more than one CPU. VIPS achieves this improvement by only keeping the pixels currently being processed in RAM and by having an efficient, threaded image IO system. This page explains how these features are implemented.
</para>
<refsect3 id="images">
<title>Images</title>
<para>
VIPS images have three dimensions: width, height and bands. Bands usually (though not always) represent colour. These three dimensions can be any size up to 2 ** 31 elements. Every band element in an image has to have the same format. A format is an 8-, 16- or 32-bit int, signed or unsigned, 32- or 64-bit float, and 64- or 128-bit complex.
</para>
</refsect3>
<refsect3 id="regions">
<title>Regions</title>
<para>
An image can be very large, much larger than the available memory, so you can’t just access pixels with a pointer *.
</para>
<para>
Instead, you read pixels from an image with a region. This is a rectangular sub-area of an image. In C, the API looks like:
</para>
<programlisting language="c">
VipsImage *image = vips_image_new_from_file( filename, NULL );
VipsRegion *region = vips_region_new( image );
// ask for a 100x100 pixel region at 0x0 (top left)
VipsRect r = { left: 0, top: 0, width: 100, height: 100 };
if( vips_region_prepare( region, &amp;r ) )
vips_error( ... );
// get a pointer to the pixel at x, y, where x, y must
// be within the region
// as long as you stay within the valid area for the region,
// you can address pixels with regular pointer arithmetic
// compile with -DDEBUG and the macro will check bounds for you
// add VIPS_REGION_LSKIP() to move down a line
VipsPel *pixel = VIPS_REGION_ADDR( region, x, y );
// you can call vips_region_prepare() many times
// everything in libvips is a GObject ... when you're done,
// just free with
g_object_unref( region );
</programlisting>
<para>
The action that <literal>vips_region_prepare()</literal> takes varies with the type of image. If the image is a file on disc, for example, then VIPS will arrange for a section of the file to be read in.
</para>
<para>
(* there is an image access mode where you can just use a pointer, but it’s rarely used)
</para>
</refsect3>
<refsect3 id="partial-images">
<title>Partial images</title>
<para>
A partial image is one where, instead of storing a value for each pixel, VIPS stores a function which can make any rectangular area of pixels on demand.
</para>
<para>
If you use <literal>vips_region_prepare()</literal> on a region created on a partial image, VIPS will allocate enough memory to hold the pixels you asked for and use the stored function to calculate values for just those pixels *.
</para>
<para>
The stored function comes in three parts: a start function, a generate function and a stop function. The start function creates a state, the generate function uses the state plus a requested area to calculate pixel values and the stop function frees the state again. Breaking the stored function into three parts is good for SMP scaling: resource allocation and synchronisation mostly happens in start functions, so generate functions can run without having to talk to each other.
</para>
<para>
VIPS makes a set of guarantees about parallelism that make this simple to program. Start and stop functions are mutually exclusive and a state is never used by more than one generate. In other words, a start / generate / generate / stop sequence works like a thread.
</para>
<figure>
<mediaobject>
<imageobject>
<imagedata fileref="Sequence.png" />
</imageobject>
<textobject><phrase></phrase></textobject>
</mediaobject>
</figure>
<para>
(* in fact VIPS keeps a cache of calculated pixel buffers and will return a pointer to a previously-calculated buffer if it can)
</para>
</refsect3>
<refsect3 id="operations">
<title>Operations</title>
<para>
VIPS operations read input images and write output images, performing some transformation on the pixels. When an operation writes to an image the action it takes depends upon the image type. For example, if the image is a file on disc then VIPS will start a data sink to stream pixels to the file, or if the image is a partial one then it will just attach start / generate / stop functions.
</para>
<para>
Like most threaded image processing systems, all VIPS operations have to be free of side-effects. In other words, operations cannot modify images, they can only create new images. This could result in a lot of copying if an operation is only making a small change to a large image so VIPS has a set of mechanisms to copy image areas by just adjusting pointers. Most of the time no actual copying is necessary and you can perform operations on large images at low cost.
</para>
</refsect3>
<refsect3 id="run-time-code-generation">
<title>Run-time code generation</title>
<para>
VIPS uses <ulink url="http://code.entropywave.com/orc/">Orc</ulink>, a run-time compiler, to generate code for some operations. For example, to compute a convolution on an 8-bit image, VIPS will examine the convolution matrix and the source image and generate a tiny program to calculate the convolution. This program is then <quote>compiled</quote> to the vector instruction set for your CPU, for example SSE3 on most x86 processors.
</para>
<para>
Run-time vector code generation typically speeds operations up by a factor of three or four.
</para>
</refsect3>
<refsect3 id="joining-operations-together">
<title>Joining operations together</title>
<para>
The region create / prepare / prepare / free calls you use to get pixels from an image are an exact parallel to the start / generate / generate / stop calls that images use to create pixels. In fact, they are the same: a region on a partial image holds the state created by that image for the generate function that will fill the region with pixels.
</para>
<figure>
<mediaobject>
<imageobject>
<imagedata fileref="Combine.png" />
</imageobject>
<textobject><phrase></phrase></textobject>
</mediaobject>
</figure>
<para>
VIPS joins image processing operations together by linking the output of one operation (the start / generate / stop sequence) to the input of the next (the region it uses to get pixels for processing). This link is a single function call, and very fast. Additionally, because of the the split between allocation and processing, once a pipeline of operations has been set up, VIPS is able to run without allocating and freeing memory.
</para>
<para>
This graph (generated by <literal>vipsprofile</literal>, the vips profiler) shows memory use over time for a vips pipeline running on a large image. The bottom trace shows total memory, the upper traces show threads calculating useful results (green), threads blocked on synchronisation (red) and memory allocations (white ticks).
</para>
<figure>
<mediaobject>
<imageobject>
<imagedata fileref="Memtrace.png" />
</imageobject>
<textobject><phrase></phrase></textobject>
</mediaobject>
</figure>
<para>
Because the intermediate image is just a small region in memory, a pipeline of operations running together needs very little RAM. In fact, intermediates are small enough that they can fit in L2 cache on most machines, so an entire pipeline can run without touching main memory. And finally, because each thread runs a very cheap copy of just the writeable state of the entire pipeline, threads can run with few locks. VIPS needs just four lock operations per output tile, regardless of the pipeline length or complexity.
</para>
</refsect3>
<refsect3 id="data-sources">
<title>Data sources</title>
<para>
VIPS has data sources which can supply pixels for processing from a variety of sources. VIPS can stream images from files in VIPS native format, from tiled TIFF files, from binary PPM/PGM/PBM/PFM, from Radiance (HDR) files, from FITS images and from tiled OpenEXR images. VIPS will automatically unpack other formats to temporary disc files for you but this can obviously generate a lot of disc traffic. It also has a special sequential mode for streaming operations on non-random-access formats. A post on the libvips blog <ulink url="http://libvips.blogspot.co.uk/2012/06/how-libvips-opens-file.html">explains how libvips opens a file</ulink>. One of the sources uses the <ulink url="http://www.imagemagick.org">ImageMagick</ulink> (or optionally <ulink url="http://www.graphicsmagick.org">GraphicsMagick</ulink>) library, so VIPS can read any image format that these libraries can read.
</para>
<para>
VIPS images are held on disc as a 64-byte header containing basic image information like width, height, bands and format, then the image data as a single large block of pixels, left-to-right and top-to-bottom, then an XML extension block holding all the image metadata, such as ICC profiles and EXIF blocks.
</para>
<para>
When reading from a large VIPS image (or any other format with the same structure on disc, such as binary PPM), VIPS keeps a set of small rolling windows into the file, some small number of scanlines in size. As pixels are demanded by different threads VIPS will move these windows up and down the file. As a result, VIPS can process images much larger than RAM, even on 32-bit machines.
</para>
</refsect3>
<refsect3 id="data-sinks">
<title>Data sinks</title>
<para>
In a demand-driven system, something has to do the demanding. VIPS has a variety of data sinks that you can use to pull image data though a pipeline in various situations. There are sinks that will build a complete image in memory, sinks to draw to a display, sinks to loop over an image (useful for statistical operations, for example) and sinks to stream an image to disc.
</para>
<para>
The disc sink looks something like this:
</para>
<figure>
<mediaobject>
<imageobject>
<imagedata fileref="Sink.png" />
</imageobject>
<textobject><phrase></phrase></textobject>
</mediaobject>
</figure>
<para>
The sink keeps two buffers*, each as wide as the image. It starts threads as rapidly as it can up to the concurrency limit, filling each buffer with tiles** of calculated pixels, each thread calculating one tile at once. A separate background thread watches each buffer and, as soon as the last tile in a buffer finishes, writes that complete set of scanlines to disc using whatever image write library is appropriate. VIPS can write with libjpeg, libtiff, libpng and others. It then wipes the buffer and repositions it further down the image, ready for the next set of tiles to stream in.
</para>
<para>
These features in combination mean that, once a pipeline of image processing operations has been built, VIPS can run almost lock-free. This is very important for SMP scaling: you don’t want the synchronization overhead to scale with either the number of threads or the complexity of the pipeline of operations being performed. As a result, VIPS scales almost linearly with increasing numbers of threads:
</para>
<figure>
<mediaobject>
<imageobject>
<imagedata fileref="Vips-smp.png" />
</imageobject>
<textobject><phrase></phrase></textobject>
</mediaobject>
</figure>
<para>
Number of CPUs is on the horizontal axis, speedup is on the vertical axis. Taken from the [[Benchmarks]] page.
</para>
<para>
(* there can actually be more than one, it allocate enough buffers to ensure that there are at least two tiles for every thread)
</para>
<para>
(** tiles can be any shape and size, VIPS has a tile hint system that operations use to tell sinks what tile geometry they prefer)
</para>
</refsect3>
<refsect3 id="operation-cache">
<title>Operation cache</title>
<para>
Because VIPS operations are free of side-effects*, you can cache them. Every time you call an operation, VIPS searches the cache for a previous call to the same operation with the same arguments. If it finds a match, you get the previous result again. This can give a huge speedup.
</para>
<para>
By default, VIPS caches the last 1,000 operation calls. You can also control the cache size by memory use or by files opened.
</para>
<para>
(* Some vips operations DO have side effects, for example, <literal>vips_draw_circle()</literal> will draw a circle on an image. These operations emit an <quote>invalidate</quote> signal on the image they are called on and this signal makes all downstream operations and caches drop their contents.)
</para>
</refsect3>
<refsect3 id="operation-database-and-apis">
<title>Operation database and APIs</title>
<para>
VIPS has around 300 image processing operations written in this style. Each operation is a GObject class. You can use the standard GObject calls to walk the class hierarchy and discover operations, and libvips adds a small amount of extra introspection metadata to handle things like optional arguments.
</para>
<para>
The <ulink url="using-from-c.html">C API</ulink> is a set of simple wrappers which create class instances for you. The <ulink url="using-from-cpp.html">C++ API</ulink> is a little fancier and adds things like automatic object lifetime management. The <ulink url="using-cli.html">command-line interface</ulink> uses introspection to run any vips operation in the class hierarchy.
</para>
<para>
The <ulink url="using-from-python.html">Python API</ulink> is built on top of gobject-introspection. It is written in Python, so as long as you can get gobject-introspection working, you should be able to use vips. It supports python2 and python3 and works on Linux, OS X and Windows.
</para>
</refsect3>
<refsect3 id="snip">
<title>Snip</title>
<para>
The VIPS GUI, nip2, has its own scripting language called Snip. Snip is a lazy, higher-order, purely functional, object oriented language. Almost all of nip2’s menus are implemented in it, and nip2 workspaces are Snip programs.
</para>
<para>
VIPS operations listed in the operation database appear as Snip functions. For example, <literal>abs</literal> can be used from Snip as:
</para>
<programlisting>
// absolute value of image b
a = vips_call &quot;abs&quot; [b] [];
</programlisting>
<para>
However, <literal>abs</literal> won’t work on anything except the primitive vips image type. It can’t be used on any class, or list or number. Definitions in <literal>_stdenv.dev</literal> wrap each VIPS operation as a higher level Snip operation. For example:
</para>
<programlisting>
abs x
= oo_unary_function abs_op x, is_class x
= vips_call &quot;abs&quot; [x] [], is_image x
= abs_cmplx x, is_complex x
= abs_num x, is_real x
= abs_list x, is_real_list x
= abs_list (map abs_list x), is_matrix x
= error (_ &quot;bad arguments to &quot; ++ &quot;abs&quot;)
{
abs_op = Operator &quot;abs&quot; abs Operator_type.COMPOUND false;
abs_list l = (sum (map square l)) ** 0.5;
abs_num n
= n, n &gt;= 0
= -n;
abs_cmplx c = ((re c)**2 + (im c)**2) ** 0.5;
}
</programlisting>
<para>
This defines the behaviour of <literal>abs</literal> for the base Snip types (number, list, matrix, image and so on), then classes will use that to define operator behaviour on higher-level objects.
</para>
<para>
Now you can use:
</para>
<programlisting>
// absolute value of anything
a = abs b;
</programlisting>
<para>
and you ought to get sane behaviour for any object, including things like the <literal>Matrix</literal> class.
</para>
<para>
You can write Snip classes which present functions to the user as menu items. For example, <literal>Math.def</literal> has this:
</para>
<programlisting>
Math_arithmetic_item = class
Menupullright &quot;_Arithmetic&quot; &quot;basic arithmetic for objects&quot; {
Absolute_value_item = class
Menuaction &quot;A_bsolute Value&quot; &quot;absolute value of x&quot; {
action x = map_unary abs x;
}
}
</programlisting>
<para>
Now the user can select an object and click <literal>Math / Abs</literal> to find the absolute value of that object.
</para>
</refsect3>
</refentry>

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doc/Makefile.am
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@ -141,7 +141,9 @@ HTML_IMAGES = \
# Our markdown source files
markdown_content_files = \
How-it-works.md
How-it-works.md \
Using-vipsthumbnail.md \
Making-image-pyramids.md
# converted to xml in this dir by pandoc
markdown_content_files_docbook = $(markdown_content_files:.md=.xml)

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libvips includes `vips_dzsave()`, an operation that can build image pyramids
compatible with [DeepZoom](http://en.wikipedia.org/wiki/Deep_Zoom), Zoomify
and [Google Maps](https://developers.google.com/maps/) image viewers. It's
fast and can generate pyramids for large images using only a small amount
of memory.
The TIFF writer, `vips_tiffsave()` can also build tiled pyramidal TIFF images,
but that's very simple to use. This page concentrates on the DeepZoom builder.
Run dzsave with no arguments to see a summary:
```
$ vips dzsave
save image to deepzoom file
usage:
dzsave in filename
where:
in - Image to save, input VipsImage
filename - Filename to save to, input gchararray
optional arguments:
basename - Base name to save to, input gchararray
layout - Directory layout, input VipsForeignDzLayout
default: dz
allowed: dz, zoomify, google
suffix - Filename suffix for tiles, input gchararray
overlap - Tile overlap in pixels, input gint
default: 1
min: 0, max: 8192
tile-size - Tile size in pixels, input gint
default: 254
min: 1, max: 8192
centre - Center image in tile, input gboolean
default: false
depth - Pyramid depth, input VipsForeignDzDepth
default: onepixel
allowed: onepixel, onetile, one
angle - Rotate image during save, input VipsAngle
default: d0
allowed: d0, d90, d180, d270
container - Pyramid container type, input VipsForeignDzContainer
default: fs
allowed: fs, zip
properties - Write a properties file to the output directory, input
gboolean
default: false
compression - ZIP deflate compression level, input gint
default: 0
min: -1, max: 9
strip - Strip all metadata from image, input gboolean
default: false
background - Background value, input VipsArrayDouble
operation flags: sequential nocache
```
You can also call `vips_dzsave()` from any language with a libvips binding, or
by using `.dz` or `.szi` as an output file suffix.
### Writing [DeepZoom](http://en.wikipedia.org/wiki/Deep_Zoom) pyramids
The `--layout` option sets the basic mode of operation. With no
`--layout`, dzsave writes DeepZoom pyramids. For example:
```
$ vips dzsave huge.tif mydz
```
This will create a directory called `mydz_files` containing the image
tiles, and write a file called `mydz.dzi` containing the image
metadata. 
You can use the `--suffix` option to control how tiles are written. For
example:
```
$ vips dzsave huge.tif mydz --suffix .jpg[Q=90]
```
will write JPEG tiles with the quality factor set to 90. You can set any
format write options you like, see the API docs for `vips_jpegsave()`
for details.
### Writing Zoomify pyramids
Use `--layout zoomify` to put dzsave into zoomify mode. For example:
```
$ vips dzsave huge.tif myzoom --layout zoomify
```
This will create a directory called `myzoom` containing a file called
`ImageProperties.xml` with the image metadata in, and a series of
directories called `TileGroupn`, each containing 256 image tiles.
As with DeepZoom, you can use `--suffix` to set jpeg quality.
### Writing [Google Maps](https://developers.google.com/maps/) pyramids
Use `--layout google` to write Google maps-style pyramids. These are
compatible with the [NYU Pathology pyramid
builder](http://code.google.com/p/virtualmicroscope/wiki/SlideTiler).
For example:
```
$ vips dzsave wtc.tif gmapdir --layout google
```
Will create a directory called `gmapdir` containing `blank.png`, the
file to display for blank tiles, and a set of numbered directories, one
for each zoom level. The pyramid can be sparse (blank tiles are not
written).
As with DeepZoom, you can use `--suffix` to set jpeg quality.
Use `--background` to set the background colour. This is the colour
displayed for bits of the pyramid not in the image (image edges, for
example). By default, the image background is white.
Use `--centre` to add a border to the image large enough to centre the
image within the lowest resolution tile. By default, images are not
centred.
For example:
```
$ vips dzsave wtc.tif gmapdir --layout google --background 0 --centre
```
#### Other options
You can use `--tile-size` and `--overlap` to control how large the tiles
are and how they overlap (obviously). They default to the correct values
for the selected layout.
You can use `--depth` to control how deep the pyramid should be. Possible
values are `onepixel`, `onetile` and `one`. `onepixel` means the image
is shrunk until it fits within a single pixel. `onetile` means shrink
until it fits with a tile. `one` means only write one pyramid layer (the
highest resolution one). It defaults to the correct value for the selected
layout. `--depth one` is handy for slicing up a large image into tiles
(rather than a pyramid).
You can use `--angle` to do a 90, 180 or 270 degree rotate of an image
during pyramid write.
You can use `--container` to set the container type. Normally dzsave will
write a tree of directories, but with `--container zip` you'll get a zip file
instead. Use .zip as the directory suffix to turn on zip format automatically:
```
$ vips dzsave wtc.tif mypyr.zip
```
to write a zipfile containing the tiles. You can use `.szi` as a suffix to
enable zip output as well.
Use `--properties` to output an XML file called `vips-properties.xml`. This
contains a dump of all the metadata vips has about the image as a set of
name-value pairs. It's handy with openslide image sources.
#### Preprocessing images
You can use `.dz` as a filename suffix, meaning send the image to
`vips_dzsave()`. This means you can write the output of any vips operation to a
pyramid. For example:
```
$ vips extract_area huge.svs mypy.dz[layout=google] 100 100 10000 10000
```
The arguments to `extract_area` are image-in, image-out, left, top,
width, height. So this command will cut out a 10,000 by 10,000 pixel
area from near the top-left-hand corner of an Aperio slide image, then
build a pyramid in Google layout using just those pixels.
If you are working from OpenSlide images, you can use the shrink-on-load
feature of many of those formats. For example:
```
$ vips dzsave CMU-1.mrxs[level=1] x
```
Will pull out level 1 (the half-resolution level of an MRXS slide) and
make a pyramid from that.
#### Troubleshooting
If you are building vips from source you do need to check the summary at
the end of configure carefully. You must have the `libgsf-1-dev` package
for `vips_dzsave()` to work.

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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<refentry id="Making-image-pyramids.md">
<refmeta>
<refentrytitle>Making-image-pyramids.md</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>libvips</refmiscinfo>
</refmeta>
<refnamediv>
<refname>libvips</refname>
<refpurpose>Making-image-pyramids.md</refpurpose>
</refnamediv>
<para>
libvips includes <literal>vips_dzsave()</literal>, an operation that can build image pyramids compatible with <ulink url="http://en.wikipedia.org/wiki/Deep_Zoom">DeepZoom</ulink>, Zoomify and <ulink url="https://developers.google.com/maps/">Google Maps</ulink> image viewers. It’s fast and can generate pyramids for large images using only a small amount of memory.
</para>
<para>
The TIFF writer, <literal>vips_tiffsave()</literal> can also build tiled pyramidal TIFF images, but that’s very simple to use. This page concentrates on the DeepZoom builder.
</para>
<para>
Run dzsave with no arguments to see a summary:
</para>
<programlisting>
$ vips dzsave
save image to deepzoom file
usage:
dzsave in filename
where:
in - Image to save, input VipsImage
filename - Filename to save to, input gchararray
optional arguments:
basename - Base name to save to, input gchararray
layout - Directory layout, input VipsForeignDzLayout
default: dz
allowed: dz, zoomify, google
suffix - Filename suffix for tiles, input gchararray
overlap - Tile overlap in pixels, input gint
default: 1
min: 0, max: 8192
tile-size - Tile size in pixels, input gint
default: 254
min: 1, max: 8192
centre - Center image in tile, input gboolean
default: false
depth - Pyramid depth, input VipsForeignDzDepth
default: onepixel
allowed: onepixel, onetile, one
angle - Rotate image during save, input VipsAngle
default: d0
allowed: d0, d90, d180, d270
container - Pyramid container type, input VipsForeignDzContainer
default: fs
allowed: fs, zip
properties - Write a properties file to the output directory, input
gboolean
default: false
compression - ZIP deflate compression level, input gint
default: 0
min: -1, max: 9
strip - Strip all metadata from image, input gboolean
default: false
background - Background value, input VipsArrayDouble
operation flags: sequential nocache
</programlisting>
<para>
You can also call <literal>vips_dzsave()</literal> from any language with a libvips binding, or by using <literal>.dz</literal> or <literal>.szi</literal> as an output file suffix.
</para>
<refsect3 id="writing-deepzoom-pyramids">
<title>Writing <ulink url="http://en.wikipedia.org/wiki/Deep_Zoom">DeepZoom</ulink> pyramids</title>
<para>
The <literal>--layout</literal> option sets the basic mode of operation. With no <literal>--layout</literal>, dzsave writes DeepZoom pyramids. For example:
</para>
<programlisting>
$ vips dzsave huge.tif mydz
</programlisting>
<para>
This will create a directory called <literal>mydz_files</literal> containing the image tiles, and write a file called <literal>mydz.dzi</literal> containing the image metadata. 
</para>
<para>
You can use the <literal>--suffix</literal> option to control how tiles are written. For example:
</para>
<programlisting>
$ vips dzsave huge.tif mydz --suffix .jpg[Q=90]
</programlisting>
<para>
will write JPEG tiles with the quality factor set to 90. You can set any format write options you like, see the API docs for <literal>vips_jpegsave()</literal> for details.
</para>
</refsect3>
<refsect3 id="writing-zoomify-pyramids">
<title>Writing Zoomify pyramids</title>
<para>
Use <literal>--layout zoomify</literal> to put dzsave into zoomify mode. For example:
</para>
<programlisting>
$ vips dzsave huge.tif myzoom --layout zoomify
</programlisting>
<para>
This will create a directory called <literal>myzoom</literal> containing a file called <literal>ImageProperties.xml</literal> with the image metadata in, and a series of directories called <literal>TileGroupn</literal>, each containing 256 image tiles.
</para>
<para>
As with DeepZoom, you can use <literal>--suffix</literal> to set jpeg quality.
</para>
</refsect3>
<refsect3 id="writing-google-maps-pyramids">
<title>Writing <ulink url="https://developers.google.com/maps/">Google Maps</ulink> pyramids</title>
<para>
Use <literal>--layout google</literal> to write Google maps-style pyramids. These are compatible with the <ulink url="http://code.google.com/p/virtualmicroscope/wiki/SlideTiler">NYU Pathology pyramid builder</ulink>. For example:
</para>
<programlisting>
$ vips dzsave wtc.tif gmapdir --layout google
</programlisting>
<para>
Will create a directory called <literal>gmapdir</literal> containing <literal>blank.png</literal>, the file to display for blank tiles, and a set of numbered directories, one for each zoom level. The pyramid can be sparse (blank tiles are not written).
</para>
<para>
As with DeepZoom, you can use <literal>--suffix</literal> to set jpeg quality.
</para>
<para>
Use <literal>--background</literal> to set the background colour. This is the colour displayed for bits of the pyramid not in the image (image edges, for example). By default, the image background is white.
</para>
<para>
Use <literal>--centre</literal> to add a border to the image large enough to centre the image within the lowest resolution tile. By default, images are not centred.
</para>
<para>
For example:
</para>
<programlisting>
$ vips dzsave wtc.tif gmapdir --layout google --background 0 --centre
</programlisting>
<sect2 id="other-options">
<title>Other options</title>
<para>
You can use <literal>--tile-size</literal> and <literal>--overlap</literal> to control how large the tiles are and how they overlap (obviously). They default to the correct values for the selected layout.
</para>
<para>
You can use <literal>--depth</literal> to control how deep the pyramid should be. Possible values are <literal>onepixel</literal>, <literal>onetile</literal> and <literal>one</literal>. <literal>onepixel</literal> means the image is shrunk until it fits within a single pixel. <literal>onetile</literal> means shrink until it fits with a tile. <literal>one</literal> means only write one pyramid layer (the highest resolution one). It defaults to the correct value for the selected layout. <literal>--depth one</literal> is handy for slicing up a large image into tiles (rather than a pyramid).
</para>
<para>
You can use <literal>--angle</literal> to do a 90, 180 or 270 degree rotate of an image during pyramid write.
</para>
<para>
You can use <literal>--container</literal> to set the container type. Normally dzsave will write a tree of directories, but with <literal>--container zip</literal> you’ll get a zip file instead. Use .zip as the directory suffix to turn on zip format automatically:
</para>
<programlisting>
$ vips dzsave wtc.tif mypyr.zip
</programlisting>
<para>
to write a zipfile containing the tiles. You can use <literal>.szi</literal> as a suffix to enable zip output as well.
</para>
<para>
Use <literal>--properties</literal> to output an XML file called <literal>vips-properties.xml</literal>. This contains a dump of all the metadata vips has about the image as a set of name-value pairs. It’s handy with openslide image sources.
</para>
</sect2>
<sect2 id="preprocessing-images">
<title>Preprocessing images</title>
<para>
You can use <literal>.dz</literal> as a filename suffix, meaning send the image to <literal>vips_dzsave()</literal>. This means you can write the output of any vips operation to a pyramid. For example:
</para>
<programlisting>
$ vips extract_area huge.svs mypy.dz[layout=google] 100 100 10000 10000
</programlisting>
<para>
The arguments to <literal>extract_area</literal> are image-in, image-out, left, top, width, height. So this command will cut out a 10,000 by 10,000 pixel area from near the top-left-hand corner of an Aperio slide image, then build a pyramid in Google layout using just those pixels.
</para>
<para>
If you are working from OpenSlide images, you can use the shrink-on-load feature of many of those formats. For example:
</para>
<programlisting>
$ vips dzsave CMU-1.mrxs[level=1] x
</programlisting>
<para>
Will pull out level 1 (the half-resolution level of an MRXS slide) and make a pyramid from that.
</para>
</sect2>
<sect2 id="troubleshooting">
<title>Troubleshooting</title>
<para>
If you are building vips from source you do need to check the summary at the end of configure carefully. You must have the <literal>libgsf-1-dev</literal> package for <literal>vips_dzsave()</literal> to work.
</para>
</sect2>
</refsect3>
</refentry>

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@ -0,0 +1,342 @@
libvips has shipped with a handy thumbnail maker for a while now. I
thought a post of tips and tricks might be useful. Scroll all the way to
the bottom for a summary and recommended usage.
### Why use vipsthumbnail? 
It’s fast and uses little memory. For example, here’s ImageMagick with
`wtc.tif`, a 10,000 x 10,000 pixel RGB tiff image:
```
$ time convert wtc.tif -resize 128 tn_wtc.jpg
peak RSS: 705m
real 0m2.639s
user 0m4.036s
sys 0m0.516s
```
And here’s `vipsthumbnail`:
```
$ time vipsthumbnail wtc.tif
peak RSS: 52mb
real 0m0.239s
user 0m0.168s
sys 0m0.072s
```
So `vipsthumbnail`
is about 11 times faster and needs 1 / 13th of the memory.
`vipsthumbnail`
and `convert`
are using the same downsizing algorithm: a fast box filter for
large-scale reduction, and a high-quality lanczos3 interpolator for the
final 200%.
You see similar improvements with png images, but much less with jpeg.
This is because libjpeg includes support for shrink-during-load, so the
image processing system has much less effect.
```
$ time convert -define jpeg:size=256x256 wtc.jpg -resize 128
tn_wtc.jpg
peak rss: 19mb
real 0m0.259s
user 0m0.284s
sys 0m0.004s
$ time vipsthumbnail wtc.jpg
peak rss: 30mb
real 0m0.268s
user 0m0.256s
sys 0m0.016s
```
The `define` argument makes `convert`
load the image at twice the target size, then use a high-quality
downsampler to get to the exact output dimensions. If you don’t leave
this headroom you can get bad aliasing artifacts. `vipsthumbnail`
does exactly this automatically.
At larger output sizes you start to see a difference, since there are
actually some pixels being processed:
```
$ time convert -define jpeg:size=4000x4000 wtc.jpg -resize 2000
tn_wtc.jpg
peak rss: 285mb
real 0m1.126s
user 0m2.508s
sys 0m0.240s
$ time vipsthumbnail wtc.jpg -s 2000
peak rss: 47mb
real 0m0.499s
user 0m0.928s
sys 0m0.028s
```
### libvips options
`vipsthumbnail` supports the usual range of vips command-line options. A
few of them are useful:
`--vips-cache-trace` shows each operation as libvips starts it. It can be
handy to see exactly what operations `vipsthumbnail` is running for you.
`--vips-leak` turns on the libvips memory leak checker. As well as reporting
leaks (hopefully there are none) it also tracks and reports peak memory use.
`--vips-progress` runs a progress indicator during computation. It can be
useful to see where libvips is looping and how often.
`--vips-info` shows a higher level view of the operations that `vipsthumbnail`
is running. 
### Looping
vipsthumbnail can process many images in one operation. For example:
```
$ vipsthumbnail *.jpg
```
will make a thumbnail for every jpeg in the current directory.  See the
**Output directory** section below to see how to change where thumbnails
are written.
`vipsthumbnail` will process images one after the other. You can get a good
speedup by running several `vipsthumbnail`s in parallel, depending on how
much load you want to put on your system.
### Thumbnail size
You can set the bounding box of the generated thumbnail with the `--size`
option. For example:
```
$ vipsthumbnail shark.jpg --size 200x100
```
Use a single number to set a square bounding box. You can omit either number
but keep the x to mean resize just based on that axis, for example:
```
$ vipsthumbnail shark.jpg --size 200x
```
Will resize to 200 pixels across, no matter what the height of the input image
is.
You can append `<` or `>` to mean only resize if the image is smaller or larger
than the target.
### Cropping
`vipsthumbnail` normally shrinks images to fit within the box set by `--size`.
You can use the `--smartcrop` option to crop to fill the box instead. Excess
pixels are trimmed away using the strategy you set. For example:
```
$ vipsthumbnail owl.jpg --smartcrop attention -s 128
```
Where `owl.jpg` is an off-centre composition:
![](owl.jpg)
Gives this result:
![](tn_owl.jpg)
First it shrinks the image to get the vertical axis to 128 pixels, then crops
down to 128 pixels across using the `attention` strategy. This one searches
the image for features which might catch a human eye, see `vips_smartcrop()`
for details.
### Linear light
Shrinking images involves combining many pixels into one. Arithmetic
averaging really ought to be in terms of the number of photons, but (for
historical reasons) the values stored in image files are usually related
to the voltage that should be applied to a CRT electron gun.
`vipsthumbnail` has an option to perform image shrinking in linear space, that
is, a colourspace where values are proportional to photon numbers. For example:
```
$ vipsthumbnail fred.jpg --linear
```
The downside is that in linear mode, none of the very fast shrink-on-load
tricks that `vipsthumbnail` normally uses are possible, since the shrinking
done by the image libraries is done at encode time, and done in
terms of CRT voltage, not light. This can make linear light thumbnailing of
large images extremely slow.
### Output directory
You set the thumbnail write parameters with the `-o`
option. This is a pattern which the input filename is pasted into to
produce the output filename. For example:
```
$ vipsthumbnail fred.jpg jim.tif -o tn_%s.jpg
```
For each of the files to be thumbnailed, `vipsthumbnail`
will drop the extension (`.jpg` and `.tif`
in this case) and then substitute the name into the `-o`
option, replacing the `%s`
So this example will write thumbnails to `tn_fred.jpg` and `tn_jim.jpg`.
If the pattern given to `-o`
is an absolute path, any path components are dropped from the input
filenames. This lets you write all of your thumbnails to a specific
directory, if you want. For example:
```
$ vipsthumbnail fred.jpg ../jim.tif -o /mythumbs/tn_%s.jpg
```
Now both thumbnails will be written to `/mythumbs`,
even though the source images are in different directories.
Conversely, if `-o`
is set to a relative path, any path component from the input file is
prepended. For example:
```
$ vipsthumbnail fred.jpg ../jim.tif -o mythumbs/tn_%s.jpg
```
Now both input files will have thumbnails written to a subdirectory of
their current directory.
### Output format and options
You can use `-o`
to specify the thumbnail image format too. For example: 
```
$ vipsthumbnail fred.jpg ../jim.tif -o tn_%s.png
```
Will write thumbnails in PNG format.
You can give options to the image write operation as a list of
comma-separated arguments in square brackets. For example:
```
$ vipsthumbnail fred.jpg ../jim.tif -o > tn_%s.jpg[Q=90,optimize_coding]
```
will write jpeg images with quality 90, and will turn on the libjpeg
coding optimizer.
Check the image write operations to see all the possible options. For
example:
```
$ vips jpegsave
save image to jpeg file
usage:
jpegsave in filename
where:
in - Image to save, input VipsImage
filename - Filename to save to, input gchararray
optional arguments:
Q - Q factor, input gint
default: 75
min: 1, max: 100
profile - ICC profile to embed, input gchararray
optimize-coding - Compute optimal Huffman coding tables, input gboolean
default: false
interlace - Generate an interlaced (progressive) jpeg, input gboolean
default: false
no-subsample - Disable chroma subsample, input gboolean
default: false
trellis-quant - Apply trellis quantisation to each 8x8 block, input gboolean
default: false
overshoot-deringing - Apply overshooting to samples with extreme values, input gboolean
default: false
optimize-scans - Split the spectrum of DCT coefficients into separate scans, input gboolean
default: false
quant-table - Use predefined quantization table with given index, input gint
default: 0
min: 0, max: 8
strip - Strip all metadata from image, input gboolean
default: false
background - Background value, input VipsArrayDouble
```
The `strip` option is especially useful. Many image have very large IPCT, ICC or
XMP metadata items embedded in them, and removing these can give a large
saving.
For example:
```
$ vipsthumbnail 42-32157534.jpg
$ ls -l tn_42-32157534.jpg
-rw-r–r– 1 john john 6682 Nov 12 21:27 tn_42-32157534.jpg
```
`strip` almost halves the size of the thumbnail:
```
$ vipsthumbnail 42-32157534.jpg -o x.jpg[optimize_coding,strip]
$ ls -l x.jpg
-rw-r–r– 1 john john 3600 Nov 12 21:27 x.jpg
```
### Colour management
`vipsthumbnail` will optionally put images through LittleCMS for you. You can
use this to move all thumbnails to the same colour space. All web browsers
assume that images without an ICC profile are in sRGB colourspace, so if
you move your thumbnails to sRGB, you can strip all the embedded profiles.
This can save several kb per thumbnail.
For example:
```
$ vipsthumbnail shark.jpg
$ ls -l tn_shark.jpg
-rw-r–r– 1 john john 7295 Nov  9 14:33 tn_shark.jpg
```
Now encode with sRGB and delete any embedded profile:
```
$ vipsthumbnail shark.jpg --eprofile /usr/share/color/icc/sRGB.icc --delete
$ ls -l tn_shark.jpg
-rw-r–r– 1 john john 4229 Nov  9 14:33 tn_shark.jpg
```
It’ll look identical to a user, but be almost half the size.
You can also specify a fallback input profile to use if the image has no
embedded one, but this is less useful.
### Auto-rotate
Many JPEG files have a hint set in the header giving the image orientation. If
you strip out the metadata, this hint will be lost, and the image will appear
to be rotated.
If you use the `--rotate` option, `vipsthumbnail` examines the image header and
if there's an orientation tag, applies and removes it.
### Final suggestion
Putting all this together, I suggest this as a sensible set of options:
```
$ vipsthumbnail fred.jpg \
--size 128 \
-o tn_%s.jpg[optimize_coding,strip] \
--eprofile /usr/share/color/icc/sRGB.icc \
--rotate
```

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2
doc/libvips-docs.xml.in
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@ -40,6 +40,8 @@
<xi:include href="xml/file-format.xml"/>
<xi:include href="xml/using-threads.xml"/>
<xi:include href="xml/How-it-works.xml"/>
<xi:include href="xml/Making-image-pyramids.xml"/>
<xi:include href="xml/Using-vipsthumbnail.xml"/>
</chapter>
<chapter>

5
tools/vipsthumbnail.c
View File

@ -156,9 +156,10 @@ static GOptionEntry options[] = {
{ "linear", 'a', 0,
G_OPTION_ARG_NONE, &linear_processing,
N_( "process in linear space" ), NULL },
{ "smartcrop", 'c', 0,
{ "smartcrop", 'm', 0,
G_OPTION_ARG_STRING, &smartcrop_image,
N_( "crop exactly to SIZE" ), NULL },
N_( "shrink and crop to fill SIZE using STRATEGY" ),
N_( "STRATEGY" ) },
{ "rotate", 't', 0,
G_OPTION_ARG_NONE, &rotate_image,
N_( "auto-rotate" ), NULL },