323 lines
9.5 KiB
Markdown
323 lines
9.5 KiB
Markdown
<refmeta>
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<refentrytitle>Using `vipsthumbnail`</refentrytitle>
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<manvolnum>3</manvolnum>
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<refmiscinfo>libvips</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>`vipsthumbnail`</refname>
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<refpurpose>Introduction to `vipsthumbnail`, with examples</refpurpose>
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</refnamediv>
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libvips ships with a handy command-line image thumbnailer, `vipsthumbnail`.
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This page introduces it, with some examples.
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The thumbnailing functionality is implemented by `vips_thumbnail()` and
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`vips_thumbnail_buffer()` (which thumbnails an image held as a string),
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see the docs for details. You can use these functions from any language
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with a libvips binding. For example, from PHP you could write:
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```php?start_inline=1
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$filename = "image.jpg";
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$image = Vips\Image::thumbnail($filename, 200, ["height" => 200]);
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$image->writeToFile("my-thumbnail.jpg");
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```
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# libvips options
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`vipsthumbnail` supports the usual range of vips command-line options. A
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few of them are useful:
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`--vips-cache-trace` shows each operation as libvips starts it. It can be
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handy to see exactly what operations `vipsthumbnail` is running for you.
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`--vips-leak` turns on the libvips memory leak checker. As well as reporting
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leaks (hopefully there are none) it also tracks and reports peak memory use.
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`--vips-progress` runs a progress indicator during computation. It can be
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useful to see where libvips is looping and how often.
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`--vips-info` shows a higher level view of the operations that `vipsthumbnail`
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is running.
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# Looping
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`vipsthumbnail` can process many images in one command. For example:
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```
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$ vipsthumbnail *.jpg
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```
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will make a thumbnail for every jpeg in the current directory. See the
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[Output directory](#output-directory) section below to see how to change
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where thumbnails are written.
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`vipsthumbnail` will process images one after the other. You can get a good
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speedup by running several `vipsthumbnail`s in parallel, depending on how
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much load you want to put on your system. For example:
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```
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$ parallel vipsthumbnail ::: *.jpg
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```
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# Thumbnail size
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You can set the bounding box of the generated thumbnail with the `--size`
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option. For example:
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```
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$ vipsthumbnail shark.jpg --size 200x100
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```
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Use a single number to set a square bounding box. You can omit either number
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but keep the x to mean resize just based on that axis, for example:
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```
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$ vipsthumbnail shark.jpg --size 200x
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```
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Will resize to 200 pixels across, no matter what the height of the input image
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is.
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You can append `<` or `>` to mean only resize if the image is smaller or larger
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than the target.
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You can append `!` to force a resize to the exact target size, breaking
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the aspect ratio.
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# Cropping
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`vipsthumbnail` normally shrinks images to fit within the box set by `--size`.
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You can use the `--smartcrop` option to crop to fill the box instead. Excess
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pixels are trimmed away using the strategy you set. For example:
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```
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$ vipsthumbnail owl.jpg --smartcrop attention -s 128
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```
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Where `owl.jpg` is an off-centre composition:
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Gives this result:
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First it shrinks the image to get the vertical axis to 128 pixels, then crops
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down to 128 pixels across using the `attention` strategy. This one searches
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the image for features which might catch a human eye, see `vips_smartcrop()`
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for details.
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# Linear light
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Shrinking images involves combining many pixels into one. Arithmetic
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averaging really ought to be in terms of the number of photons, but (for
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historical reasons) the values stored in image files are usually related
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to the voltage that should be applied to the electron gun in a CRT display.
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`vipsthumbnail` has an option to perform image shrinking in linear space, that
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is, a colourspace where values are proportional to photon numbers. For example:
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```
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$ vipsthumbnail fred.jpg --linear
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```
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The downside is that in linear mode, none of the very fast shrink-on-load
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tricks that `vipsthumbnail` normally uses are possible, since the shrinking is
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done at encode time, not decode time, and is done in terms of CRT voltage, not
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photons. This can make linear light thumbnailing of large images extremely slow.
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For example, for a 10,000 x 10,000 pixel JPEG I see:
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```
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$ time vipsthumbnail wtc.jpg
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real 0m0.317s
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user 0m0.292s
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sys 0m0.016s
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$ time vipsthumbnail wtc.jpg --linear
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real 0m4.660s
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user 0m4.640s
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sys 0m0.016s
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```
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# Output directory
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You set the thumbnail write parameters with the `-o`
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option. This is a pattern which the input filename is pasted into to
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produce the output filename. For example:
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```
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$ vipsthumbnail fred.jpg jim.tif -o tn_%s.jpg
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```
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For each of the files to be thumbnailed, `vipsthumbnail` will drop the
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extension (`.jpg` and `.tif` in this case) and then substitute the name into
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the `-o` option, replacing the `%s` So this example will write thumbnails to
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`tn_fred.jpg` and `tn_jim.jpg`.
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If the pattern given to `-o` is an absolute path, any path components are
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dropped from the input filenames. This lets you write all of your thumbnails
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to a specific directory, if you want. For example:
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```
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$ vipsthumbnail fred.jpg ../jim.tif -o /mythumbs/tn_%s.jpg
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```
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Now both thumbnails will be written to `/mythumbs`, even though the source
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images are in different directories.
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Conversely, if `-o` is set to a relative path, any path component from the
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input file is prepended. For example:
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```
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$ vipsthumbnail fred.jpg ../jim.tif -o mythumbs/tn_%s.jpg
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```
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Now both input files will have thumbnails written to a subdirectory of
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their current directory.
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# Output format and options
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You can use `-o` to specify the thumbnail image format too. For example:
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```
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$ vipsthumbnail fred.jpg ../jim.tif -o tn_%s.png
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```
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Will write thumbnails in PNG format.
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You can give options to the image write operation as a list of comma-separated
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arguments in square brackets. For example:
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```
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$ vipsthumbnail fred.jpg ../jim.tif -o tn_%s.jpg[Q=90,optimize_coding]
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```
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will write jpeg images with quality 90, and will turn on the libjpeg coding
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optimizer.
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Check the image write operations to see all the possible options. For example:
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```
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$ vips jpegsave
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save image to jpeg file
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usage:
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jpegsave in filename
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where:
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in - Image to save, input VipsImage
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filename - Filename to save to, input gchararray
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optional arguments:
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Q - Q factor, input gint
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default: 75
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min: 1, max: 100
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profile - ICC profile to embed, input gchararray
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optimize-coding - Compute optimal Huffman coding tables, input gboolean
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default: false
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interlace - Generate an interlaced (progressive) jpeg, input gboolean
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default: false
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no-subsample - Disable chroma subsample, input gboolean
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default: false
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trellis-quant - Apply trellis quantisation to each 8x8 block, input gboolean
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default: false
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overshoot-deringing - Apply overshooting to samples with extreme values, input gboolean
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default: false
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optimize-scans - Split the spectrum of DCT coefficients into separate scans, input gboolean
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default: false
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quant-table - Use predefined quantization table with given index, input gint
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default: 0
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min: 0, max: 8
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strip - Strip all metadata from image, input gboolean
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default: false
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background - Background value, input VipsArrayDouble
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```
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The `strip` option is especially useful. Many image have very large IPCT, ICC or
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XMP metadata items embedded in them, and removing these can give a large
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saving.
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For example:
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```
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$ vipsthumbnail 42-32157534.jpg
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$ ls -l tn_42-32157534.jpg
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-rw-r–r– 1 john john 6682 Nov 12 21:27 tn_42-32157534.jpg
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```
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`strip` almost halves the size of the thumbnail:
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```
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$ vipsthumbnail 42-32157534.jpg -o x.jpg[optimize_coding,strip]
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$ ls -l x.jpg
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-rw-r–r– 1 john john 3600 Nov 12 21:27 x.jpg
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```
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# Colour management
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`vipsthumbnail` will optionally put images through LittleCMS for you. You can
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use this to move all thumbnails to the same colour space. All web browsers
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assume that images without an ICC profile are in sRGB colourspace, so if
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you move your thumbnails to sRGB, you can strip all the embedded profiles.
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This can save several kb per thumbnail.
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For example:
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```
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$ vipsthumbnail shark.jpg
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$ ls -l tn_shark.jpg
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-rw-r–r– 1 john john 7295 Nov 9 14:33 tn_shark.jpg
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```
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Now encode with sRGB and delete any embedded profile:
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```
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$ vipsthumbnail shark.jpg --eprofile /usr/share/color/icc/sRGB.icc --delete
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$ ls -l tn_shark.jpg
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-rw-r–r– 1 john john 4229 Nov 9 14:33 tn_shark.jpg
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```
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It’ll look identical to a user, but be almost half the size.
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You can also specify a fallback input profile to use if the image has no
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embedded one. This can often happen with CMYK images, producing an error
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message like:
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```
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$ vipsthumbnail kgdev.jpg
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vipsthumbnail: unable to thumbnail kgdev.jpg
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vips_colourspace: no known route from 'cmyk' to 'srgb'
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```
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If you supply a CMYK profile, it will be able to convert the image,
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for example:
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```
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$ vipsthumbnail kgdev.jpg --iprofile /usr/share/color/icc/colord/FOGRA28L_webcoated.icc
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```
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I've had good results with this profile:
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https://github.com/libvips/nip2/blob/master/share/nip2/data/cmyk.icm
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It makes nice-looking images from most CMYK files, and is completely free.
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# Auto-rotate
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Many JPEG files have a hint set in the header giving the image orientation. If
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you strip out the metadata, this hint will be lost, and the image will appear
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to be rotated.
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If you use the `--rotate` option, `vipsthumbnail` examines the image header and
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if there's an orientation tag, applies and removes it.
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# Final suggestion
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Putting all this together, I suggest this as a sensible set of options:
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```
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$ vipsthumbnail fred.jpg \
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--size 128 \
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-o tn_%s.jpg[optimize_coding,strip] \
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--eprofile /usr/share/color/icc/sRGB.icc \
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--rotate
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```
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