Thanks for the PR Daniel. A quick thought: could setting a bitdepth of 7 result in 129 palette entries (128+transparency)?

Hi @lovell. Good question!
If a bitdepth of 7 is specified with this branch we will create a colour palette with a maximum of 128 entries and set the transparency index to the number of colours:

In this case, the valid indices are 0-127 for the image data and 128 is the transparency index.
cgif supports transparency indices that are outside of the active colour palette, but the highest possible index is 255 of course.

As far as I can tell, gifsicle and ImageMagick handle this the same way and allow the transparency index to be exactly one position outside of the active colour palette.

libvips chooses the transparency index and provides the transparent areas (so which areas are identical to the frame before) to cgif as CGIF_FRAME_ATTR_HAS_SET_TRANS is set (CGIF_FRAME_GEN_USE_TRANSPARENCY is ignored by cgif as libvips provides the transparency information). It is necessary that libvips provides the transparency information to support the interframe-maxerror parameter.

With this branch I see:

vips copy earth.gif[n=-1] 7.gif[bitdepth=7,reoptimise=1]

earth.gif

bitdepth 8	bitdepth 7	bitdepth 1

bitdepth 1 might look unexpected at first because sometimes one can see 3 colours. The colours per frame are limited to 2, but the transparency might cause pixels to shine through.

Thanks @dloebl !