MATLAB Central Newsreader - Interesting problem with image alpha data

Interesting problem with image alpha data

Michael — Fri, 19 Jun 2009 08:44:02 -0400

I have come across an interesting problem with setting the alpha data for an image, which I can't quite get my head around.

To explain, consider the following example:

h1 = figure(1);
h1a = axes('DataAspectRatio', [1 1 1], 'DrawMode', 'fast', 'Parent', h1);
h1b = image('Parent', h1a);

All works as expect, then:

set(h1b, 'CData', imread('Peppers.png')); % load a test image.

This works just as would be expected. So, I then try:

set(h1b, 'AlphaData', 0.75);

Which has the right effect. However, this image isn't positioned very nicely within the axes, so I use:

set(h1a, 'XLim', [1 512], 'YLim', [1 384], 'YDir', 'reverse');

Which makes everything look nice. But now if I set the alpha data to anything other than 1, the image just dissappears completely. Can anyone tell me what is going wrong here?

I'm using Matlab R2009a with version 6.3 of the Image Processing toolbox.

Many thanks!

Re: Interesting problem with image alpha data

Bill Winter — Tue, 22 Sep 2009 03:05:13 -0400

"Michael " <michael.bennett@lineone.net> wrote in message <h1fj4i$7ln$1@fred.mathworks.com>...
> I have come across an interesting problem with setting the alpha data for an image, which I can't quite get my head around.
>
> To explain, consider the following example:
>
> h1 = figure(1);
> h1a = axes('DataAspectRatio', [1 1 1], 'DrawMode', 'fast', 'Parent', h1);
> h1b = image('Parent', h1a);
>
> All works as expect, then:
>
> set(h1b, 'CData', imread('Peppers.png')); % load a test image.
>
> This works just as would be expected. So, I then try:
>
> set(h1b, 'AlphaData', 0.75);
>
> Which has the right effect. However, this image isn't positioned very nicely within the axes, so I use:
>
> set(h1a, 'XLim', [1 512], 'YLim', [1 384], 'YDir', 'reverse');
>
> Which makes everything look nice. But now if I set the alpha data to anything other than 1, the image just dissappears completely. Can anyone tell me what is going wrong here?
>
> I'm using Matlab R2009a with version 6.3 of the Image Processing toolbox.
>
> Many thanks!

Has anyone else seen this problem? More importantly, has anyone identified a viable solution?

Re: Interesting problem with image alpha data

Loren Shure — Tue, 22 Sep 2009 11:49:30 -0400

In article <h99et9$5as$1@fred.mathworks.com>,
whichpart.wwinter.isnotanemail@uci.edu says...
> "Michael " <michael.bennett@lineone.net> wrote in message <h1fj4i$7ln$1@fred.mathworks.com>...
> > I have come across an interesting problem with setting the alpha data for an image, which I can't quite get my head around.
> >
> > To explain, consider the following example:
> >
> > h1 = figure(1);
> > h1a = axes('DataAspectRatio', [1 1 1], 'DrawMode', 'fast', 'Parent', h1);
> > h1b = image('Parent', h1a);
> >
> > All works as expect, then:
> >
> > set(h1b, 'CData', imread('Peppers.png')); % load a test image.
> >
> > This works just as would be expected. So, I then try:
> >
> > set(h1b, 'AlphaData', 0.75);
> >
> > Which has the right effect. However, this image isn't positioned very nicely within the axes, so I use:
> >
> > set(h1a, 'XLim', [1 512], 'YLim', [1 384], 'YDir', 'reverse');
> >
> > Which makes everything look nice. But now if I set the alpha data to anything other than 1, the image just dissappears completely. Can anyone tell me what is going wrong here?
> >
> > I'm using Matlab R2009a with version 6.3 of the Image Processing toolbox.
> >
> > Many thanks!
>
> Has anyone else seen this problem? More importantly, has anyone identified a viable solution?
>

It works for me in both R2009a and R2009b.

--
Loren
http://blogs.mathworks.com/loren

Re: Interesting problem with image alpha data

Carlos — Tue, 18 May 2010 17:01:24 -0400

"Michael " <michael.bennett@lineone.net> wrote in message <h1fj4i$7ln$1@fred.mathworks.com>...
> I have come across an interesting problem with setting the alpha data for an image, which I can't quite get my head around.
>
> To explain, consider the following example:
>
> h1 = figure(1);
> h1a = axes('DataAspectRatio', [1 1 1], 'DrawMode', 'fast', 'Parent', h1);
> h1b = image('Parent', h1a);
>
> All works as expect, then:
>
> set(h1b, 'CData', imread('Peppers.png')); % load a test image.
>
> This works just as would be expected. So, I then try:
>
> set(h1b, 'AlphaData', 0.75);
>
> Which has the right effect. However, this image isn't positioned very nicely within the axes, so I use:
>
> set(h1a, 'XLim', [1 512], 'YLim', [1 384], 'YDir', 'reverse');
>
> Which makes everything look nice. But now if I set the alpha data to anything other than 1, the image just dissappears completely. Can anyone tell me what is going wrong here?
>
> I'm using Matlab R2009a with version 6.3 of the Image Processing toolbox.
>
> Many thanks!

I had a very similar problem and it turned out to be a bug with OpenGL. I tried it on another computer and it just worked! Right now I am trying to update my OpenGL driver.

Re: Interesting problem with image alpha data

Rene v B — Wed, 02 Jun 2010 14:40:08 -0400

Worked out for me:
rotate the picture a little bit...

"Michael " <michael.bennett@lineone.net> wrote in message <h1fj4i$7ln$1@fred.mathworks.com>...
> I have come across an interesting problem with setting the alpha data for an image, which I can't quite get my head around.
>
> To explain, consider the following example:
>
> h1 = figure(1);
> h1a = axes('DataAspectRatio', [1 1 1], 'DrawMode', 'fast', 'Parent', h1);
> h1b = image('Parent', h1a);
>
> All works as expect, then:
>
> set(h1b, 'CData', imread('Peppers.png')); % load a test image.
>
> This works just as would be expected. So, I then try:
>
> set(h1b, 'AlphaData', 0.75);
>
> Which has the right effect. However, this image isn't positioned very nicely within the axes, so I use:
>
> set(h1a, 'XLim', [1 512], 'YLim', [1 384], 'YDir', 'reverse');
>
> Which makes everything look nice. But now if I set the alpha data to anything other than 1, the image just dissappears completely. Can anyone tell me what is going wrong here?
>
> I'm using Matlab R2009a with version 6.3 of the Image Processing toolbox.
>
> Many thanks!