This option selects a different mode of operation from crunch's usual texture file conversion role. It causes the tool to crawl through a directory and load every .PNG file there. It will then randomly select a percentage of the 4x4 pixel blocks from the image and append the results into one or more 4096x4096 output images. These output images can then be used as test vectors to compare different block encoders.
crunch -corpus_gen -deep .035 -width 4096 -height 4096 -in J:\dev\test_images\*.png
You can specify multiple -in arguments, and -in @file.txt loads a textual listing file of files/directories to load or scan.
The -corpus_test option can be used to compare the different DXT encoders supported by crunch, using images generated using -corpus_gen.
Here's a very zoomed in example from the test vector generator:
Notice how the blocks are sorted by the sum of R, G's, and B's standard deviation as a key.