Setting a matte colour for anti-aliasing to transparent

[RobS]

Hi,

I've been trying to create pngs and gifs via the web export function, where the background of the graphic is transparent.

The transparency options are to:

1) Not dither transparency
2) Set transparency to colour
3) Set partial transparency to colour
4) Dither transparency

None of these are really what is needed. 1 & 4 (threshold method) get closest, but seem to assume that the transparency will be sitting on a white background, and so gives small halo effect when the resulting graphics get placed on darker colours.

#2 doesn't work because the whole transparent area becomes blocked in with the single colour, and #3 does similar with semi transparent pixels and the end result is jaggies around the edge of transparency.

What is needed is something like the photoshop ability to set a matte colour for the transition from non-transparent to transparent to be anti-aliased against.

I've tried setting the background colour in the main toolbar to something suitable before trying the web export, but that doesn't seem to affect the effective background colour for anti-aliasing against... it remains white (or some very pale colour).

I hope that's clear enough. It seems at the moment that semi-transparent pixels are calculated assuming that the background colour is white. What is needed is some method of changing the assumed background colour such that semi-transaparency can be calculated against a different, non-white background... This is very different to setting semi-transparent pixels to a specific colour.

Cheers


Rob

[Hoogo]

I created a test picture:
-backgroundlayer yellow.
-foreground is a transition from black, 100% transparency, to black, no transparency. That's one of the built-in radial gradients. So I have something like a yellow sun with a wide area of ray around it. In the color-reduce-dialog for transparency I have chosen red as assumed background color (the color-box there)

#1 does not touch the transparency. If there is semi-transparency in the picture, you can not save it as gif this way.
#2 with color red assumes that the file format can not store any transparency and will be put onto a red background. My sun turns red.
#3 and #4 assume a file format that can store transparency, but only 1Bit-Transparency, as for GIF. #3 does the same as #2, but I keep a yellow sun in the middle - that's where full transparency is kept.
#4 dithers the transparency channel. It does not care about the coresponding color channels.

I'm not sure if I understand what you need, I don't have any white jaggies around.