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OpenGL Simple multiple textures

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I did D3D for a "long" time now and am somewhat new to OpenGL (still at the basics)


What i am trying to do is:

... Initialize

> Load texture1, Load texture2

... Render

> Bind texture1 to texture unit 0, render some geometry

> Bind texture2 to texture unit 0, render some geometry


In short it doenst work as expected. The result is, that both geometry is rendered with texture2 on it. So i looked up the docs and they were stating:

"glTexSubImage2D specifies a two-dimensional subtexture for the current texture unit, specified with glActiveTexture."


So I spent some time on trial and erroring around and finally I activated texture unit 1 before loading texture 2 which yields the correct result if I bind at rendering texture1 to tex unit 0 and texture2 to tex uint 1. Binding both textures to tex unit 0 will render the geometry only with texture1.

Actually this feels like the instand I call glTexImage* when having bound a texture to a certain tex uint (texture2 at 0) the the data I loaded before that (texture1 at 0) gets discarded?


I'm not quite sure what that would mean. Is it ture, that I need to spend a tex uint for each texture??? I cannot believe that, this would suggest that my graphics card has a 96 (GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS) textures max limit (no way). 


so here some snippets on how I (initially inteded to) do the loading and rendereing (both geometry gets rendered with texture2 on it):

glGenTextures (1, &tex);
glBindTexture (cfg.GetBindFlag (), tex);
glTexStorage2D(tex, cfg.GetMipLevel (), cfg.GetTextureFormat (), cfg.GetWidth (), cfg.GetHeight ());
glTexImage2D (cfg.GetBindFlag (), 
	cfg.GetMipLevel (), 
	cfg.GetTextureFormat (), 
	cfg.GetWidth (), cfg.GetHeight (), 0, 
	cfg.GetDataFormat (), 
	cfg.GetDataType (), initdata);
glActiveTexture (_TexUnit); // _TexUnit is asserted as GL_TEXTURE0 + _TexUnitIndex
glBindTexture (resource->GetBindFlag (), resource->GetGLID ()); // resource::_GLID is corresponds to names of texture1 and texture 2
glUniform1i (_Location, _TexUnitIndex); // _Location is the uniform sampler bindpoint in the shader, _TexUnitIndex is 0 in both rendering events
_Sampler->Bind (_TexUnitIndex);

I actually conclude that I have to have missed something, the question is what? Hopefully some of you can point me a diretion.

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Ok, so I publically denounced myself... it was a "variable not initialized error" which took me 1 and a half day to track down and of cource I posted here 2 hours befor i finally found the error....


But anyways, i would be glad of some explanation to the documentation. What does that "specifies a two-dimensional subtexture for the current texture unit" mean? Why is the texture unit relevant in the context of uploading texture data?

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But anyways, i would be glad of some explanation to the documentation. What does that "specifies a two-dimensional subtexture for the current texture unit" mean?

glTexSubImage, and equivalent for the other Sub-variants, modifies as sub-rectangle of an existing texture. Say you have a 128x128-pixel texture consisting of a 4x4 grid of 32x32-pixel tiles. You can use it to update/replace a single tile without having to upload an entirely new texture image.

Why is the texture unit relevant in the context of uploading texture data?

OpenGL has a bind-to-edit model which means you have to bind the texture object to make it the currently active object. Subsequent calls to glTexImage and such then operates on the currently bound texture. As you may have noticed, you never tell glTexImage which texture object to populate with texture data; that is given from the previous call to glBindTexture.


Since multitexturing allows for multiple textures to be bound at the same time, the texture bound to the currently active texture unit is the target of glTexImage.

Edited by Brother Bob

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I'm sorry to say that but you just pointed out the obvisous for me :P


you never tell glTexImage which texture object to populate with texture data


Yes of course. What I was actually wondering, is why the docs refer to the texture unit and especially glActiveTexture.

I thought glActiveTexture, should have nothing to do with what is stored in the texture but instead is a mechanism to access the texture via glsl.

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The glTexImage function doesn't have a parameter specifying which texture unit to store the data to, so the documentation tells you it will store the data to the current texture unit. The way to specify the current texture unit is glActiveTexture.


To get the texture in GLSL, you need to use a sampler and make it available to your shader program.


Edit: this is the same thing Brother Bob said, but hopefully if a few people tell you the same thing in different ways it will click.

Edited by richardurich

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