Bit Depth for reduced file size

[Anonymoose]

In Photoplus X6, it was possible to export an image to a gif at a bit depth of 1. This meant that I could put line drawings into my novels, and keep the file size to a minimum. I now have Affinity Photo, and I can't seem to find a way to do that. Does it still exist, or do I have to revert to PX6?

[John Rostron]

No you cannot. The workaround is to ensure that your image is in an 8-bit greyscale, and then apply a Threshold Adjustment Layer. Although it will technically still be an 8-bit image, it should compress down quite well when saved in a  png-8 format.1

John.

[R C-R]

Keep in mind that for raster images in any format, at a bit depth of 1 no anti-aliasing is possible so line drawings will have a jagged, stair-stepped appearance, a phenomenon known as "Jaggies." When this is unacceptable, the usual workaround is to increase the resolution (pixel dimensions) of the 1 bit image, so the stair steps are smaller, but this increases file size, often by considerably more than by using an 8 bit greyscale image instead with anti-aliasing applied.

[Fixx]

Sure you can save 1-bit gif from AP.

AP though dithers 1-bit output always which may not be desired. Threshold adjustment seems to behave oddly in my system so I could not get rid of antialiasing (which causes dither also). Also I got weird dither artifacts... but in principle it is doable.

[R C-R]

30 minutes ago, Fixx said:

Sure you can save 1-bit gif from AP.

AP though dithers 1-bit output always which may not be desired.

I am not so sure that is true. At least when I set the GIF export palette to either Black & White or to Automatic > 2 colors, export the document, & open it in Apple's Preview.app, the Preview 'Inspector" says the Color Model is RGB & the Depth is 8:

[Fixx]

You right in a way as seems AP does not export normal 1-bit gif. It is specced somehow else. Though in practice it seems to be close enough. Here is how PS interprets it: