8.9 KiB
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:
$filename = ...;
$image = Vips\Image::thumbnail($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 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 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