Alpha channel effecting other channels with a TGA image. #36252

New Issue

B Y · 2013-07-22T20:00:37+02:00

B Y commented

2013-07-22 20:00:37 +02:00

%%%--- Operating System, Graphics card ---
Windows 7 sp1 and Nvidia Quadro FX 580
Windows XP sp3 and Nvidia Geforce 9800 GT

- Blender version with error, and version that worked ---

2.68 - broken
2.65 - works
2.63 - works

- Short description of error ---

I have a TGA with two circles, one red and one blue. The alpha channel is set to display part of the red circle. Using the separate channels node to remove the alpha and reveal the blue circle no longer works.

- Steps for others to reproduce the error (preferably based on attached .blend file) ---

Using a "image node" select the "circles.tga" image.
Connect it to a "Separate RGBA node"
Connect the Blue channel to either a "viewer node" or a "composite node."

in 2.63 - 2.65 there will be a white circle as the alpha channel was properly removed revealing the blue channel.
screen shot:
http://imagebin.org/265268

in 2.8 the image will (incorrectly) be black. It seems as if the alpha channel is removing color data from the source image .
screen shot:
http://imagebin.org/265270

P.S. While this is similar to an earlier issue I had reported earlier. I believe this is a simpler example and more accurate description of the problem I'm experiencing. I have not yet found a solution to this bug other than to separate the alpha channel in another application first.%%%

%%%--- Operating System, Graphics card --- Windows 7 sp1 and Nvidia Quadro FX 580 Windows XP sp3 and Nvidia Geforce 9800 GT - Blender version with error, and version that worked --- 2.68 - broken 2.65 - works 2.63 - works - Short description of error --- I have a TGA with two circles, one red and one blue. The alpha channel is set to display part of the red circle. Using the separate channels node to remove the alpha and reveal the blue circle no longer works. - Steps for others to reproduce the error (preferably based on attached .blend file) --- 1. Using a "image node" select the "circles.tga" image. 2. Connect it to a "Separate RGBA node" 3. Connect the Blue channel to either a "viewer node" or a "composite node." in 2.63 - 2.65 there will be a white circle as the alpha channel was properly removed revealing the blue channel. screen shot: http://imagebin.org/265268 in 2.8 the image will (incorrectly) be black. It seems as if the alpha channel is removing color data from the source image . screen shot: http://imagebin.org/265270 P.S. While this is similar to an earlier issue I had reported earlier. I believe this is a simpler example and more accurate description of the problem I'm experiencing. I have not yet found a solution to this bug other than to separate the alpha channel in another application first.%%%

B Y commented

2013-07-22 20:00:37 +02:00

Changed status to: 'Open'

Lukas Tönne commented

2013-08-08 11:15:49 +02:00

%%%The problem is caused by the new compositor using the rect_float values of the image buffer (ImBuf) instead of the rect byte array (see BaseImageOperation::getImBuf(), calling IMB_float_from_rect(ibuf)). This works fine in general, but rect_float is always premultiplied with alpha, which causes the color channels to lose all information in 0-alpha areas (i.e. the blue circle in this case is gone). RGBA splitting becomes pretty much useless for such masking purposes.%%%

%%%The problem is caused by the new compositor using the rect_float values of the image buffer (ImBuf) instead of the rect byte array (see BaseImageOperation::getImBuf(), calling IMB_float_from_rect(ibuf)). This works fine in general, but rect_float is *always* premultiplied with alpha, which causes the color channels to lose all information in 0-alpha areas (i.e. the blue circle in this case is gone). RGBA splitting becomes pretty much useless for such masking purposes.%%%

Brecht Van Lommel commented

2013-08-08 19:29:46 +02:00

%%%This is the intended behavior now, we did a major refactor of the alpha handling system in 2.66:
http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.66/Image_Transparency

The compositor now always uses premultiplied alpha rather than the user having to set this manually. That reduces some flexibility for cases where you want to use the alpha channel in strange ways where it's not actually the transparency in the image, but overall also helps us get rid of artifacts and errors .

The workaround for a case like this is indeed to load the image twice, once with Use Alpha disabled in the image settings.%%%

%%%This is the intended behavior now, we did a major refactor of the alpha handling system in 2.66: http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.66/Image_Transparency The compositor now always uses premultiplied alpha rather than the user having to set this manually. That reduces some flexibility for cases where you want to use the alpha channel in strange ways where it's not actually the transparency in the image, but overall also helps us get rid of artifacts and errors . The workaround for a case like this is indeed to load the image twice, once with Use Alpha disabled in the image settings.%%%

Brecht Van Lommel commented

2013-08-08 19:29:46 +02:00

Changed status from 'Open' to: 'Archived'

Brecht Van Lommel closed this issue

2013-08-08 19:29:46 +02:00

B Y commented

2013-08-15 18:23:04 +02:00

%%%Thanks for the feedback and potential workaround, but there still is a problem, and I'm not sure if this is windows only, but in Blender 2.68a when I select the image node there is no checkbox for "Use Alpha" it just shows the alpha type below the colorspace. If that checkbox were there this would be an acceptable workaround.%%%

Brecht Van Lommel commented

2013-08-15 20:44:25 +02:00

%%%The node only shows a subset of all image options, you can find all of them when selecting the image in the image editor.%%%

Lukas Tönne commented

2013-08-16 00:29:22 +02:00

%%%I've recently added extended image options in the node editor sidebar (n key), so you don't actually have to open the image editor to access all the details of the image data block. But this is not in 2.68a yet, so unless you use a recent build (e.g. from builder.blender.org) you'll have to use the image editor, until next release.%%%

B Y commented

2013-08-16 05:19:45 +02:00

%%%Found it. Indeed this resolves the issue It will be nice to have that in the node options in the future. %%%

Sign in to join this conversation.

No Label

Download

What's New

Blender Studio

Manual

Developers Blog

Documentation

Benchmark

Blender Conference

Development Fund

One-time Donations

Alpha channel effecting other channels with a TGA image. #36252