From b2b95ca04593998cd5ce860cc9dcc496ab422d5d Mon Sep 17 00:00:00 2001 From: John Cupitt Date: Fri, 31 Mar 2017 18:11:39 +0100 Subject: [PATCH] finish up thumbnail docs --- doc/Makefile.am | 2 + doc/Using-vipsthumbnail.md | 91 ++---------- doc/Using-vipsthumbnail.xml | 287 ++++++++++++++++++++++++++++++++++++ 3 files changed, 301 insertions(+), 79 deletions(-) create mode 100644 doc/Using-vipsthumbnail.xml diff --git a/doc/Makefile.am b/doc/Makefile.am index 77c58cc8..1b3ae99e 100644 --- a/doc/Makefile.am +++ b/doc/Makefile.am @@ -126,6 +126,8 @@ IGNORE_HFILES = $(IGNORE_VIPS_INCLUDE) $(IGNORE_VIPS_C) # Images to copy into HTML directory. # e.g. HTML_IMAGES=$(top_srcdir)/gtk/stock-icons/stock_about_24.png HTML_IMAGES = \ + $(top_srcdir)/doc/images/owl.jpg \ + $(top_srcdir)/doc/images/tn_owl.jpg \ $(top_srcdir)/doc/images/interconvert.png \ $(top_srcdir)/doc/images/Combine.png \ $(top_srcdir)/doc/images/Memtrace.png \ diff --git a/doc/Using-vipsthumbnail.md b/doc/Using-vipsthumbnail.md index 91391338..cdcb33ec 100644 --- a/doc/Using-vipsthumbnail.md +++ b/doc/Using-vipsthumbnail.md @@ -1,81 +1,10 @@ +libvips ships with a handy command-line image thumbnailer, `vipsthumbnail`. +This page introduces it with examples. -libvips has shipped with a handy thumbnail maker for a while now. I -thought a post of tips and tricks might be useful. Scroll all the way to -the bottom for a summary and recommended usage. - -### Why use vipsthumbnail?  - -It’s fast and uses little memory. For example, here’s ImageMagick with -`wtc.tif`, a 10,000 x 10,000 pixel RGB tiff image: - -``` -$ 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 -``` +The thumbnailing functionality is implemeted by +`vips_thumbnail()` and +`vips_thumbnail_buffer()`, see the docs for details. You can use these +functions from any language with a libvips binding. ### libvips options @@ -108,7 +37,11 @@ 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. +much load you want to put on your system. For example: + +``` +$ parallel vipsthumbnail ::: *.jpg +``` ### Thumbnail size @@ -130,7 +63,7 @@ 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. +than the target. ### Cropping diff --git a/doc/Using-vipsthumbnail.xml b/doc/Using-vipsthumbnail.xml new file mode 100644 index 00000000..563112c0 --- /dev/null +++ b/doc/Using-vipsthumbnail.xml @@ -0,0 +1,287 @@ + + + + + + Using-vipsthumbnail.md + 3 + libvips + + + + libvips + Using-vipsthumbnail.md + + + + + libvips ships with a handy command-line image thumbnailer, vipsthumbnail. This page introduces it with examples. + + + The thumbnailing functionality is implemeted by vips_thumbnail() and vips_thumbnail_buffer(), see the docs for details. You can use these functions from any language with a libvips binding. + + + 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 vipsthumbnails 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 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 + + + + +