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SRGB with FBO problem


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#1 Ponkx   Members   -  Reputation: 113

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Posted 08 February 2013 - 08:42 AM

Hello,

 

I was trying to write linear color values to FBO with glEnable(GL_FRAMEBUFFER_SRGB) enabled, but conversion from linear -> SRGB space seems not working.

My test program was doing something like this:

 

1. Load some test texture with GL_SRGB8 format

2. Initialize FBO with texture with GL_SRGB8_ALPHA8 format

3. Bind that FBO

4. glEnable(GL_FRAMEBUFFER_SRGB)

5. render test texture to FBO

  -> now FBO should contain very the same color values as input test texture (conversion SRGB -> linear of test texture during sampling, and then conversion linear -> SRGB during write to FBO).

6. glDisable(GL_FRAMEBUFFER_SRGB)

7. Unbind FBO and render FBO texture to back buffer.

 

I was expecting, that output color will be the same as input texture, but result is darker, that means, that conversion is not working. Strange is, that rendering that test GL_SRGB8 texture directly to the back buffer (no FBO) when GL_FRAMEBUFFER_SRGB was enabled is working (output is very the same as input texture).

 

I was trying to initialize FBO just as GL_RGBA8 format, but results were still dark.

 

So, I want to ask, shoud SRGB conversion work even for FBO buffers? (as I was reading the spec of that extension, it should work). Or is it just some driver bug? (I'm using AMD card), or did I overlook something?

 

Thanks.



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#2 Vilem Otte   Crossbones+   -  Reputation: 1468

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Posted 09 February 2013 - 05:51 AM

No, this isn't how this extension works (according to specs) - see http://www.g-truc.net/post-0263.html or the specs http://www.opengl.org/registry/specs/ARB/framebuffer_sRGB.txt

 

As stated, GL_FRAMEBUFFER_SRGB only performs automatic conversions at blending stage.


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#3 Hodgman   Moderators   -  Reputation: 31122

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Posted 09 February 2013 - 07:15 AM

The spec that you linked to says that if blending is enabled, the the framebuffer value is decoded, blended with the linear shader output, then re-encoded. It also says that if blending is disabled, the shader output will be encoded before being written. (all this also depending on the FBO state)


@Ponkx, maybe see the bits in the spec about FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING and the interactions with EXT_texture_sRGB.

#4 Ponkx   Members   -  Reputation: 113

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Posted 09 February 2013 - 02:42 PM

When I check value return value from FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING, it says GL_SRGB (if I create FBO texture with GL_SRGB. It return GL_LINEAR when I create it with RGBA). Extension also says:

 

"If FRAMEBUFFER_SRGB is enabled and the value of
    FRAMEBUFFER_ATTACHMENT_COLOR_ENCODING for the framebuffer attachment
    corresponding to the destination buffer is SRGB (see section 6.1.3),
    the R, G, and B values after blending are converted into the non-linear
    sRGB color space by some approximation of the following:

             {  0.0,                          0         <= cl
             {  12.92 * c,                    0         <  cl < 0.0031308
        cs = {  1.055 * cl^0.41666 - 0.055,   0.0031308 <= cl < 1
             {  1.0,                                       cl >= 1

    where cl is the R, G, or B element and cs is the result
    (effectively converted into an sRGB color space).

so I even tried to enable blending, but still no effect. :( EXT_texture_sRGB is supported on my system.



#5 Hodgman   Moderators   -  Reputation: 31122

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Posted 09 February 2013 - 07:59 PM

Have you tried updating your drivers?
Which drivers are you using?
Which GPU are you using?

#6 Ponkx   Members   -  Reputation: 113

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Posted 10 February 2013 - 01:43 PM

I Have ATI 5650 mobility, drivers 12.10, quite recent.



#7 Ponkx   Members   -  Reputation: 113

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Posted 11 February 2013 - 05:18 AM

Just tried to set GL_TEXTURE_SRGB_DECODE_EXT texture paramter to GL_SKIP_DECODE_EXT - it should turn off sRGB->linear conversion. But again it doesn't working. Results are still dark, as if conversion would be performed (GL_EXT_texture_sRGB_decode is supported on my system).

So it seems to me, that all sRGB conversion stuff is somehow strange, at least on my system...






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