Its unclear if something similar can be done in vips well still using the
very efficient sequential read and not creating a giant temporary file.
There is a -a option to vipsthumbnail that could maybe be used.

In my test, the -a option slowed things down quite a bit, and increased memory usage to beyond 400mb. :(

jcupitt wrote:

Some benchmarks:

$ time vipsthumbnail -a wtc.jpg --vips-leak
memory: high-water mark 65.98 MB
real 0m4.039s
user 0m4.003s
sys 0m0.038s
$ time vipsthumbnail wtc.jpg --vips-leak
memory: high-water mark 6.22 MB
real 0m0.307s
user 0m0.289s
sys 0m0.020s

wtc.jpg is a 10,000 x 10,000 pixel RGB image.

The main source of the slowdown is that you can't use libjpeg's very fast shrink-on-load feature, since that's calculated in Y, which is not linear. Instead, you have to decode the entire image, convert to a linear space (vipsthumbnail uses XYZ), and then shrink that.

-a is much more competitive if you use a format that doesn't support shrink-on-load, such as vanilla tif:

$ time vipsthumbnail wtc.tif --vips-leak
memory: high-water mark 44.31 MB
real 0m2.662s
user 0m0.513s
sys 0m0.119s
$ time vipsthumbnail -a wtc.tif --vips-leak
memory: high-water mark 66.62 MB
real 0m3.623s
user 0m3.547s
sys 0m0.078s

memuse should not be huge, the whole process can still be streamed.

Some image formats allow decoding in a lower resolution, for example PNG with enabled interlacing adam 7. JPEG files can probably decoded in a lower resolution by simply ignoring the DCT coefficients of the higher frequencies (simple example: use only DC values).
I assume that in the PNG case the lower-resolution result corresponds to a subsampled image, and in the JPEG case it corresponds to a linearly-downscaled image without gamma-correction.

Maybe, as a tradeoff between quality and computation time, the image could be decoded to two times the target resolution(*) (if the decoder allows it) and then gamma-correctly downscaled to the target resolution.

(*) (to be more precise: a resolution which is at least two times as big in width and height, but not bigger than the input image resolution)

The situation in vips is not much better. We use vips directly, which means that configuring it to work in linear space is not easy and probably requires multiple commands and more temporary files. vipsthumbnail has a command-line switch, but may cause other problems (I haven't tested it).