libvips/doc/Using-vipsthumbnail.md

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
```