I was trying to tab complete rgrep, but got this command instead. From its man(1) page:

rgb2ycbcr converts RGB color, greyscale, or bi-level TIFF images to YCbCr images by transforming and sampling pixel data. If multiple files are specified on the command line each source file is converted to a separate directory in the destination file.

This would have really helped me a few years ago, because the Gimp didn’t do CMYK. I’m not sure if it does now.

Regarding rgrep, I’ve aliased it to grep -R. I haven’t checked if ack has compatible options to alias to that.