# GL\_ARB\_texture\_env\_combine MBX limitation ? **URL:** \ **Category:** PowerVR Insider\ **Created:** [December 3, 2008, 6:28pm UTC](https://forums.imgtec.com/t/gl-arb-texture-env-combine-mbx-limitation/198 "2008-12-03T18:28:26Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![xav](https://avatars.discourse-cdn.com/v4/letter/x/f1d935/32.png) [@xav](https://forums.imgtec.com/u/xav)\ **Post date:** [December 3, 2008, 6:28pm UTC](https://forums.imgtec.com/t/gl-arb-texture-env-combine-mbx-limitation/198/1 "2008-12-03T18:28:26Z")
Hi! I am experiencing difficulties when using combiners GL\_INTERPOLATE with OpenGL ES 1.1 / MBX hardware, while same code works fine on a regular OpenGL/x86 hardware. When I used the emulation x86/Linux layer from the PowerVR SDK, it says: “OGLESPCViewer: _WARNING_ GLES implemenation has limited support for GL\_INTERPOLATE, which OGLESPCViewer will fail to represent.” (Emulation and real native execution of this code fails to represent the same way in fact) I can’t find any explanation about such limitation in the avalaible documentation. Please, could you precise me what are the limitations in the OpenGLES/MBX implementation for this extension? (My code doesn’t use enhanced behaviour authorized by the crossbar extension) Thanks! Sample code follows: … glColor4f(texture\_back\_color.r,     texture\_back\_color.g,     texture\_back\_color.b,     texture\_back\_color.a); // TEX UNIT 0 // Simple Modulation glActiveTexture(GL\_TEXTURE0); glEnable(GL\_TEXTURE\_2D); glBindTexture(GL\_TEXTURE\_2D, texture\_id\_back); glClientActiveTexture(GL\_TEXTURE0); glEnableClientState(GL\_TEXTURE\_COORD\_ARRAY); glTexEnvf(GL\_TEXTURE\_ENV, GL\_TEXTURE\_ENV\_MODE, GL\_MODULATE); glTexCoordPointer(2, GL\_FLOAT, 0, texture\_coordinates);  // TEX UNIT 1 // Texture combiner : INTERPOLATE (RGB & ALPHA) glActiveTexture(GL\_TEXTURE1); glEnable(GL\_TEXTURE\_2D); glBindTexture(GL\_TEXTURE\_2D, texture\_id\_front); glClientActiveTexture(GL\_TEXTURE1); glEnableClientState(GL\_TEXTURE\_COORD\_ARRAY); glTexCoordPointer(2, GL\_FLOAT, 0, texture\_coordinates); glTexEnvf(GL\_TEXTURE\_ENV, GL\_TEXTURE\_ENV\_MODE, GL\_COMBINE); glTexEnvf(GL\_TEXTURE\_ENV, GL\_COMBINE\_RGB, GL\_INTERPOLATE); glTexEnvf(GL\_TEXTURE\_ENV, GL\_COMBINE\_ALPHA, GL\_INTERPOLATE); // RGB : interpolate -\> s0 \* s2 + s1 \* (1 - s2) // -\> texture\_front\_color \* texture\_front + texture\_back \* (1 - texture\_front) // Source 0 glTexEnvfv(GL\_TEXTURE\_ENV, GL\_TEXTURE\_ENV\_COLOR, texture\_front\_color); glTexEnvf(GL\_TEXTURE\_ENV, GL\_SRC0\_RGB, GL\_CONSTANT); glTexEnvf(GL\_TEXTURE\_ENV, GL\_OPERAND0\_RGB, GL\_SRC\_COLOR); // Source 1 glTexEnvf(GL\_TEXTURE\_ENV, GL\_SRC1\_RGB, GL\_PREVIOUS); glTexEnvf(GL\_TEXTURE\_ENV, GL\_OPERAND1\_RGB, GL\_SRC\_COLOR); // Source 2 glTexEnvf(GL\_TEXTURE\_ENV, GL\_SRC2\_RGB, GL\_TEXTURE); glTexEnvf(GL\_TEXTURE\_ENV, GL\_OPERAND2\_RGB, GL\_SRC\_COLOR); // Alpha : interpolate // interpolate -\> s0 \* s2 + s1 \* (1 - s2) // -\> texture\_front\_color.a \* texture\_front.a + (texture\_back \* texture\_back\_color).a \* //    (1 - texture\_front.a) // Source 0 glTexEnvf(GL\_TEXTURE\_ENV, GL\_SRC0\_ALPHA, GL\_CONSTANT); glTexEnvf(GL\_TEXTURE\_ENV, GL\_OPERAND0\_ALPHA, GL\_SRC\_ALPHA); // Source 1 glTexEnvf(GL\_TEXTURE\_ENV, GL\_SRC1\_ALPHA, GL\_PREVIOUS); glTexEnvf(GL\_TEXTURE\_ENV, GL\_OPERAND1\_ALPHA, GL\_SRC\_ALPHA); // Source 2 glTexEnvf(GL\_TEXTURE\_ENV, GL\_SRC2\_ALPHA, GL\_TEXTURE); glTexEnvf(GL\_TEXTURE\_ENV, GL\_OPERAND2\_ALPHA, GL\_SRC\_ALPHA); … ---
**Author:** ![xmas](https://avatars.discourse-cdn.com/v4/letter/x/958977/32.png) [@xmas](https://forums.imgtec.com/u/xmas)\ **Post date:** [December 4, 2008, 10:09am UTC](https://forums.imgtec.com/t/gl-arb-texture-env-combine-mbx-limitation/198/2 "2008-12-04T10:09:42Z")
Could you give us more details on how the results are different, and what platform you are targeting? I’m not aware of any reason the above code should fail. ---
**Author:** ![xav](https://avatars.discourse-cdn.com/v4/letter/x/f1d935/32.png) [@xav](https://forums.imgtec.com/u/xav)\ **Post date:** [December 4, 2008, 1:20pm UTC](https://forums.imgtec.com/t/gl-arb-texture-env-combine-mbx-limitation/198/3 "2008-12-04T13:20:06Z")
I have resolved my problem, let me explain what happened : The initial code was different from the sample code I have posted here, it was using GL\_TEXTURE1 for Source alpha & color for interpolation not GL\_TEXTURE as I wrote, I changed it while I was searching a solution ( when I suspected the crossbar extension ). My first platform was on  x86/OpenGL2.0, all the things looks right. Then I tried it on the target platform, an i.MX31/MBX OpenGLES 1.1. When I first tested my code directly on i.MX31/MBX the code fails to represent correctly (it seems like alpha values wasn’t treated correctly: all was opaque) due to GL\_TEXTURE1 in fact. After that, I tried it through the PowerVR SDK Emulation Layer on x86/Linux and it warns about a “GLES limitation support for GL\_INTERPOLATE”. So I tried to find the reasons for this and I replaced GL\_TEXTURE1 by GL\_TEXTURE as it was the same thing for texture unit 1… But the Emulation layer always fails to represent it and always claims about this limitation, so I did not retried it on i.MX and I thought there was a limitation for GLES as the ‘OGLESPCViewer’ says… So the ‘OGLESPCViewer’ warning message confused me, because it says that “_WARNING_ GLES implemenation has limited support for GL\_INTERPOLATE” even without using GL\_TEXTURE1… Thanks for your support. ---
**Author:** ![xmas](https://avatars.discourse-cdn.com/v4/letter/x/958977/32.png) [@xmas](https://forums.imgtec.com/u/xmas)\ **Post date:** [December 4, 2008, 1:38pm UTC](https://forums.imgtec.com/t/gl-arb-texture-env-combine-mbx-limitation/198/4 "2008-12-04T13:38:45Z")
Glad to hear you solved the problem. It’s always a good idea to check glGetError when you get unexpected results, it helps especially with those hard-to-spot invalid enums.