300 lines
8.9 KiB
Markdown
300 lines
8.9 KiB
Markdown
<refmeta>
|
||
<refentrytitle>Using `vipsthumbnail`</refentrytitle>
|
||
<manvolnum>3</manvolnum>
|
||
<refmiscinfo>libvips</refmiscinfo>
|
||
</refmeta>
|
||
|
||
<refnamediv>
|
||
<refname>`vipsthumbnail`</refname>
|
||
<refpurpose>Introduction to `vipsthumbnail`, with examples</refpurpose>
|
||
</refnamediv>
|
||
|
||
libvips ships with a handy command-line image thumbnailer, `vipsthumbnail`.
|
||
This page introduces it, with some examples.
|
||
|
||
The thumbnailing functionality is implemented by `vips_thumbnail()` and
|
||
`vips_thumbnail_buffer()` (which thumbnails an image held as a string),
|
||
see the docs for details. You can use these functions from any language
|
||
with a libvips binding. For example, from PHP you could write:
|
||
|
||
```php?start_inline=1
|
||
$filename = "image.jpg";
|
||
$image = Vips\Image::thumbnail($filename, 200, ["height" => 200]);
|
||
$image->writeToFile("my-thumbnail.jpg");
|
||
```
|
||
|
||
# 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 command. For example:
|
||
|
||
```
|
||
$ vipsthumbnail *.jpg
|
||
```
|
||
|
||
will make a thumbnail for every jpeg in the current directory. See the
|
||
[Output directory](#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. For example:
|
||
|
||
```
|
||
$ parallel vipsthumbnail ::: *.jpg
|
||
```
|
||
|
||
# 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:
|
||
|
||
|
||
|
||
Gives this result:
|
||
|
||
|
||
|
||
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 the electron gun in a CRT display.
|
||
|
||
`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 is
|
||
done at encode time, not decode time, and is done in terms of CRT voltage, not
|
||
photons. This can make linear light thumbnailing of large images extremely slow.
|
||
|
||
For example, for a 10,000 x 10,000 pixel JPEG I see:
|
||
|
||
```
|
||
$ time vipsthumbnail wtc.jpg
|
||
real 0m0.317s
|
||
user 0m0.292s
|
||
sys 0m0.016s
|
||
$ time vipsthumbnail wtc.jpg --linear
|
||
real 0m4.660s
|
||
user 0m4.640s
|
||
sys 0m0.016s
|
||
```
|
||
|
||
# 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
|
||
```
|