alpha channel of imported raster images gets altered

[Hadmut Danisch]

Hi,

I tried to use inkscape to draw annotations (pointers, texts etc.) onto given raster images, which basically works as wanted and expected. Just start inkscape with the given PNG file, and inkscape automatically opens a drawing with the appropriate size and imports the image. Save as .svg and call inkscape with -e to create a modified PNG. Great. :-)

But when the given PNG image has parts with a alpha channel < 1, e.g. for a drop shadow, these parts are dropped by inkscape.

Try the attached Image (which I found somewhere in the Internet as a good example).

Open it with inkscape, draw something, save it as .svg and call inkscape -e ... . Shadow of the image is lost.

[bbyak (buliabyak)]

works for me, see attached svg where it is imported, background objects clearly show through the shadow

[Hadmut Danisch (hadmut)]

Please try to leave the background transparent, just paint something onto the graphics. And then render to png with inkscape -e shadow-ok.png shadow-ok.svg.

[bbyak (buliabyak)]

Tried that, works as expected. Please attach your source/result files if you're seeing any misbehavior.

[ScislaC (scislac)]

Hadmut,

When reporting bugs, please also provide us with the information of what version of Inkscape you are using and on what Operating System.

[Hadmut Danisch (hadmut)]

Problem occurs both with 0.46 on ubuntu 9.04 and 0.47-pre1 on ubuntu 9.10 beta.

Example output attached. Note that the shadow is gone.

regards

[Pablo Trabajos (pajarico)]

I opened your last attachment and there is indeed transparency on the dropshadow, so I don't understand very well what are you talking about. Are you by chance using internet explorer to view the resulting PNG? I see nothing wrong with the file.

[Hadmut Danisch (hadmut)]

Compare the orignal png with the modified. The original has a dark shadow where the modified hasn't and is just transparent.
Display them both and see the difference.

And no, I am using ubuntu and eog to view pictures.

regards

[su_v (suv-lp)]

…the two above attached PNGs from Hadmut look very different on OS X when compared side-by-side in the default OS X viewer.

[su_v (suv-lp)]

…however using his initial PNG imported into Inkscape 0.46+devel r22401 on OS X 10.5.8 and exported as PNG using the GUI keeps the shadow intact. Using the commandline produces the same correct result.

[Pablo Trabajos (pajarico)]

> Compare the orignal png with the modified. The original has a dark shadow where the modified hasn't and is just transparent.
Well, as a matter of fact, is not transparent, but, yes I see now what you mean: the dropshadow is not as intense on the exported PNG.

[Guillermo Espertino (Gez) (gespertino-gmail)]

Try with a black transparent background instead of a white transparent background.
I guess that the difference is the key color.
Another possibility is that the problem is related to how was the original image created in the first place, if the alpha channel is premultiplied or not.
I remember having some similar alpha problems some time ago (and iirc I reported them, so this report might be a duplicate)

[Guillermo Espertino (Gez) (gespertino-gmail)]

Hmmm. Forget about the background color.
In bug #232239 Niko Kiirala comments about inkscape using white as alpha matte and gimp using black.
In bug #336332 there are also some aspects apparently related to this.
Those 2 previous bugs and this seem to be caused almost by the same issue, the alpha channel premultiplication. Maybe it's a good idea to join these reports in a single one about "alpha channel management issues" or something like that.

Anyway, i tried to reproduce the bug in Inkscape pre3 and I cannot spot any differences between the original and the exported.

[Pablo Trabajos (pajarico)]

Related bug #284147?

[Kris (kris-degussem)]

Is this problem still present in recent Inkscape versions or fixed along with bug #284147?

[Kris (kris-degussem)]

Closing report by lack of feedback.

[su_v (suv-lp)]

Issue as originally reported had never been reproduced on other systems.