maybe use alpha in place of the 'touched' array?

[PerBothner]

I don't know if this would be a worthwhile change, but it seems like you could get rid of the touched array by just checking if the 32-bit color value has a non-zero alpha component. Or, simpler and faster: just checking if the color value is non-zero.

[jerch]

Yes great idea, should speedup toImageData. 👍

(Didnt do it in the first place since I was not sure whether alpha is supported at all in sixel, but seems it is bound to 3 channels only).

[jerch]

Just removed the touched array - both parsing and creating final image is way faster. Sweet.

There is still a logical glitch with the fill color as it should not fill the whole target.

[PerBothner]

Great!