Open GL Rendering : Alpha channel is always recorded in Premultiplied mode, even if you save your image in PNG #34276

New Issue

Francois Grassard · 2013-02-16T14:01:05+01:00

Francois Grassard commented

2013-02-16 14:01:05 +01:00

%%%--- Operating System, Graphics card ---

Windows 7 Pro 64 bits - nVidia GTX 460

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

Tested on Blender 2.66 TestBuild 1

- Short description of error ---

I use Open GL Rendering a lot, even for broadcast projects.

Since the Alpha has been refactored, the Alpha channel is always recorded in "a kind of" Premultiplied mode when we use the OpenGL render, even if you save your image in PNG. Not in Straight mode. I say "a kind of" because if I interpret the rendered image in After Effects as a premultiplied footage, anti-aliased pixels are wrong too ...

So, when you save your Open GL render in PNG files, it's totally impossible to reuse it in any compositing software ... even Blender.

And ... just a little feature request about OpenGL rendering : It could be so cool to directly catch the OpenGL render in the Compositor by using a Render Layer node ... even if only RGBA channels (and maybe Z channel) are relevants here. For now, I have to render my images on my hard drive ... then reload them in the compositor. :o) Thanks a lot !

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

Create a sphere
Set Alpha to Transparent
Render using "OpenGL Render Active Viewport"
Save as PNG
Inspect the RGB channels to see there are pre-mulitplied and not straight

Thanks a lot for your work !%%%

%%%--- Operating System, Graphics card --- Windows 7 Pro 64 bits - nVidia GTX 460 - Blender version with error, and version that worked --- Tested on Blender 2.66 TestBuild 1 - Short description of error --- I use Open GL Rendering a lot, even for broadcast projects. Since the Alpha has been refactored, the Alpha channel is always recorded in "a kind of" Premultiplied mode when we use the OpenGL render, even if you save your image in PNG. Not in Straight mode. I say "a kind of" because if I interpret the rendered image in After Effects as a premultiplied footage, anti-aliased pixels are wrong too ... So, when you save your Open GL render in PNG files, it's totally impossible to reuse it in any compositing software ... even Blender. And ... just a little feature request about OpenGL rendering : It could be so cool to directly catch the OpenGL render in the Compositor by using a Render Layer node ... even if only RGBA channels (and maybe Z channel) are relevants here. For now, I have to render my images on my hard drive ... then reload them in the compositor. :o) Thanks a lot ! - Steps for others to reproduce the error (preferably based on attached .blend file) --- - Create a sphere - Set Alpha to Transparent - Render using "OpenGL Render Active Viewport" - Save as PNG - Inspect the RGB channels to see there are pre-mulitplied and not straight Thanks a lot for your work !%%%

Francois Grassard commented

2013-02-16 14:01:05 +01:00

Changed status to: 'Open'

Sergey Sharybin commented

2013-02-18 12:17:01 +01:00

%%%I couldn't see an issue here. Semi-transparent cube rendered with OpenGL viewport renderer seems to be correct after saving it to png in gwenview and gimp. Also, it seems in exactly the same way as OpenGL Render Result image after opening this png in blender.

Please, reset to factory settings, try to reproduce issue and if it'll reproduce attach sample .blend file, steps to make alpha fail and describe what's exactly you consider behaving wrong there.%%%

%%%I couldn't see an issue here. Semi-transparent cube rendered with OpenGL viewport renderer seems to be correct after saving it to png in gwenview and gimp. Also, it seems in exactly the same way as OpenGL Render Result image after opening this png in blender. Please, reset to factory settings, try to reproduce issue and if it'll reproduce attach sample .blend file, steps to make alpha fail and describe what's exactly you consider behaving wrong there.%%%

Brecht Van Lommel commented

2013-02-18 15:04:21 +01:00

%%%I can confirm the issue here and have committed a fix, it's more clear on edges than transparency materials. The sRGB to linear color space conversion wasn't taking alpha into account.%%%

Brecht Van Lommel commented

2013-02-18 15:04:21 +01:00

Changed status from 'Open' to: 'Resolved'

Brecht Van Lommel closed this issue

2013-02-18 15:04:21 +01:00

Francois Grassard commented

2013-02-21 01:57:50 +01:00

%%%Sorry for my late reply ... i'm really busy those days ... :o)

Thank you Brecht ... I investigated the issue a little bit more and in fact, I discovered that the same problem was already there in Blender 2.65a.

Here is two screenshots I made about that :

The first one show the RGB channels only ... for an image coming from the BI (on the left side) and for the same scene, an image coming from the OpenGL Renderer (on the right side) :

http://www.coyhot.com/no_CoyHot/SideBySide_Compare.jpg

The second image show those two renders saved in PNG and opened in Photoshop. As you can see, the png saved from the OpenGL Renderer (on the right side) is showing some ugly black artefacts on the antialiased pixels ... who is not surprising when we saw the first screenshot :

http://www.coyhot.com/no_CoyHot/SideBySide_SavedAndImported.jpg

Here is the really simple scene I used (a sphere with a "shadeless" red shader) :

http://www.coyhot.com/no_CoyHot/SimpleSphere.blend

I tried to switch to cycles renderer to acces the "OpenGL Alpha" parameter (in "Film" tab) to switch it to "Transparent" ... but the issue remain the same.

I don't know if the commit from brecht already fixed it ... if not, I hope those informations will help you to solve the issue.

And once again ... if we can grab the OpenGl render in a compositing node ... it could be really interesting. As I previously said, I've rendered a lot of billboards and jingles for Broadcast TVs in France by using only OpenGl renderer and some Matcaps mapped in "Normal" mode (using GLSL renderer). It's a really good tricks to produce good looking animations in HD when you only have 1 hour or 2 to create a 15 seconds animation. :o)

Thanks again for your wonderfull work !%%%

%%%Sorry for my late reply ... i'm really busy those days ... :o) Thank you Brecht ... I investigated the issue a little bit more and in fact, I discovered that the same problem was already there in Blender 2.65a. Here is two screenshots I made about that : The first one show the RGB channels only ... for an image coming from the BI (on the left side) and for the same scene, an image coming from the OpenGL Renderer (on the right side) : http://www.coyhot.com/no_CoyHot/SideBySide_Compare.jpg The second image show those two renders saved in PNG and opened in Photoshop. As you can see, the png saved from the OpenGL Renderer (on the right side) is showing some ugly black artefacts on the antialiased pixels ... who is not surprising when we saw the first screenshot : http://www.coyhot.com/no_CoyHot/SideBySide_SavedAndImported.jpg Here is the really simple scene I used (a sphere with a "shadeless" red shader) : http://www.coyhot.com/no_CoyHot/SimpleSphere.blend I tried to switch to cycles renderer to acces the "OpenGL Alpha" parameter (in "Film" tab) to switch it to "Transparent" ... but the issue remain the same. I don't know if the commit from brecht already fixed it ... if not, I hope those informations will help you to solve the issue. And once again ... if we can grab the OpenGl render in a compositing node ... it could be really interesting. As I previously said, I've rendered a lot of billboards and jingles for Broadcast TVs in France by using only OpenGl renderer and some Matcaps mapped in "Normal" mode (using GLSL renderer). It's a really good tricks to produce good looking animations in HD when you only have 1 hour or 2 to create a 15 seconds animation. :o) Thanks again for your wonderfull work !%%%

Francois Grassard commented

2013-02-21 02:42:59 +01:00

%%%EDIT : I just tested the final 2.66 release and the issue has disapear ! Yeah !!!

Thank you so much guys ! :o)

So ... if you want to had an "OpenGl node" in the Compositing ... it will be my pleasure to test it everyday ... :o)

Thanks again !%%%

%%%EDIT : I just tested the final 2.66 release and the issue has disapear ! Yeah !!! Thank you so much guys ! :o) So ... if you want to had an "OpenGl node" in the Compositing ... it will be my pleasure to test it everyday ... :o) Thanks again !%%%

Luiz Paulo Iazzetti Santos commented

2016-07-23 06:18:38 +02:00

Added subscriber: @adapmal

Luiz Paulo Iazzetti Santos commented

2016-07-23 06:18:38 +02:00

Hi! Did the issue get back on Blender 2.77? Always get premultiplied alpha on Open GL render... My workaround is to render twice: One with the alpha objects and other without them - then compose both... But that's a bit annoying, agreed?

Brecht Van Lommel commented

2016-07-23 13:08:21 +02:00

@adapmal, if you think you found a bug, open a new report with precise instructions to redo the problem and details about your operating system, graphics card, etc.

Luiz Paulo Iazzetti Santos commented

2016-07-23 16:38:24 +02:00

In #34276#383081, @brecht wrote:
@adapmal, if you think you found a bug, open a new report with precise instructions to redo the problem and details about your operating system, graphics card, etc.

Well, I really don't know if that's a bug. But I cannot get straight alpha from OpenGl Render. So, my details:

- Operating System, Graphics card ---

MacOS 10.11.5 - AMD Radeon R9 M295X 4096 MB

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

Blender 2.77 (2016-03-19)

- Short description of error ---

In my 3d world I use a plane with a "png with alpha" texture, composed with 3d objects made in Blender. Alpha "Straight" is the one that works for me, and I can see it composing beautifully with my 3d objects, in my 3d Viewport (Texture shaders mode). When then i hit "Open Gl Render", to render a photo or animation it only renders in "Premultiplied mode". I can even choose witch premultiplied type I want, in |Info|Render>OpenGL Render Options>Alpha Mode (Sky/Transparent), but I cannot choose "Straight".

Thanks guys. You have changed my life.

> In #34276#383081, @brecht wrote: > @adapmal, if you think you found a bug, open a new report with precise instructions to redo the problem and details about your operating system, graphics card, etc. Well, I really don't know if that's a bug. But I cannot get straight alpha from OpenGl Render. So, my details: - Operating System, Graphics card --- MacOS 10.11.5 - AMD Radeon R9 M295X 4096 MB - Blender version with error, and version that worked --- Blender 2.77 (2016-03-19) - Short description of error --- In my 3d world I use a plane with a "png with alpha" texture, composed with 3d objects made in Blender. Alpha "Straight" is the one that works for me, and I can see it composing beautifully with my 3d objects, in my 3d Viewport (Texture shaders mode). When then i hit "Open Gl Render", to render a photo or animation it only renders in "Premultiplied mode". I can even choose witch premultiplied type I want, in |Info|Render>OpenGL Render Options>Alpha Mode (Sky/Transparent), but I cannot choose "Straight". Thanks guys. You have changed my life.

Brecht Van Lommel commented

2016-07-23 18:07:34 +02:00

@adapmal, opening a new report is still the best thing to do even if you aren't certain it's a bug.

Rendering indeed always outputs premultiplied alpha by design, since Blender 2.66. Only on file save do we convert to the appropriate alpha convention for the specific file type. Compositing with premultiplied alpha is the standard workflow in animation / VFX, and I don't think it imposes any significant limitations.

The specific thing you're trying to do is most likely still possible in another way, or maybe there is a bug or limitation in Blender, but there's not enough information for me to tell.

@adapmal, opening a new report is still the best thing to do even if you aren't certain it's a bug. Rendering indeed always outputs premultiplied alpha by design, since Blender 2.66. Only on file save do we convert to the appropriate alpha convention for the specific file type. Compositing with premultiplied alpha is the standard workflow in animation / VFX, and I don't think it imposes any significant limitations. The specific thing you're trying to do is most likely still possible in another way, or maybe there is a bug or limitation in Blender, but there's not enough information for me to tell.

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Open GL Rendering : Alpha channel is always recorded in Premultiplied mode, even if you save your image in PNG #34276