Thursday, December 13, 2012

[android-developers] Re: HelloEffects confusion

Thanks.  I'm still a little confused about why there is a glActiveTexture and a glBindTexture.  Seems like there ought to be one function for setting the current texture.  



On Thursday, December 13, 2012 4:14:52 AM UTC-6, a1 wrote:

 GLES20.glUniform1i(mTexSamplerHandle, 0);

I would think it should be:

 GLES20.glUniform1i(mTexSamplerHandle, texId);

But that doesn't work.

Can someone help me understand this line?

Each GPU has multiple texture units (at least 8 as required by GL ES 2.0 specification), sampler2D (and other samplers type) are used to reference texture unit in shader languages, therefore you set it to texture unit id ordinal (unitId - GL_TEXTURE0), texture is bound on client side by selecting texture unit, with call to glActiveTexture and then with glBindTexture, which binds texture name to given sample type (eg. sampler2D with GL_TEXTURE_2D) on currently selected texture unit.

So to pass texture to shader you need to execute following sequence:

GLES20.glActiveTexture(GL_TEXTURE0); // activate texture unit
GLES20.glBindTexture(GL_TEXTURE_2D, textureName); // bind texture to 2D sampler on texture unit 0
/* setup progam, states etc... */
GLES20.glUniform1i(uniformLocationForSampler, 0); // pass texture unit ordinal to shader

--
Bart

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