images / alpha-blending

images / alpha-blending

[David Reitter]

I am trying to use a few new icons for the tool-bar and after a bit  
of fruitless research, I have the impression that alpha-blending is  
not supported. I can only use bitmasks, even with image types such as  
PNG that have an alpha channel. Is that correct?

(I have only tried this with the Carbon port, but I understand that  
X11 currently doesn't support alpha-blending anyways.)

Re: images / alpha-blending

[YAMAMOTO Mitsuharu]

>>>>> On Sat, 18 Feb 2006 10:00:33 +0000, David Reitter <david.reitter@gmail.com> said:

> I am trying to use a few new icons for the tool-bar and after a bit
> of fruitless research, I have the impression that alpha-blending is
> not supported. I can only use bitmasks, even with image types such
> as PNG that have an alpha channel. Is that correct?

It would work as expected only when :background is given explicitly.
Otherwise, transparent parts are merged with the frame background
color:

  http://lists.gnu.org/archive/html/emacs-devel/2004-04/msg00623.html

> (I have only tried this with the Carbon port, but I understand that
> X11 currently doesn't support alpha-blending anyways.)

I have some experimental code of alpha channel support for the Carbon
port using Quartz 2D, but I have the impression that full alpha
channel support is a post-release issue from the above post.  ("22.1"
didn't mean the next release there.)

				     YAMAMOTO Mitsuharu
				mituharu@math.s.chiba-u.ac.jp

Re: images / alpha-blending

[Richard M. Stallman]

What is alpha-blending?  What does the feature do?

We don't want Emacs to support features that are limited to a non-free
system; that would betray the goal we are working for.  What would be
necessary to support alpha-blending in a system-independent fashion
(after the release of course)?

Re: images / alpha-blending

[David Reitter]

On 19 Feb 2006, at 22:47, Richard M. Stallman wrote:

> What is alpha-blending?  What does the feature do?
>
> We don't want Emacs to support features that are limited to a non-free
> system; that would betray the goal we are working for.

An "alpha layer" is a form of "graded transparency layer". With basic  
transparency, you define a bitmask where one bit corresponds to one  
pixel, indicating whether or not the background should be displayed  
in that pixel (rather than a pixel belonging to the actual image). An  
alpha layer assignes a value of higher resolution to each pixel. When  
the image is displayed, the background color for that pixel blends in  
with the image pixel. This produces smooth edges.

Supporting this is not merely a matter for the application (Emacs),  
but also for the graphics device and layers in between. So of course  
it wouldn't make sense to produce an alternative icon set if the  
images required something that isn't and won't be equally supported  
on the free platforms. Hence my question.

Re: images / alpha-blending

[David Reitter]

On 19 Feb 2006, at 07:22, YAMAMOTO Mitsuharu wrote:
>  It would work as expected only when :background is given explicitly.
> Otherwise, transparent parts are merged with the frame background
> color:

Thanks, that's helpful information - I could make it work. Do other  
ports (in particular: X11, Windows) implement it like that as well?

Cheers
Dave