opengl reuse texture unit

Question

according to the https://www.opengl.org/wiki/Texture, "if two different GLSL samplers have different texture types, but are associated with the same texture image unit, then rendering will fail. Give each sampler a different texture image unit."

but the glactivetexture use enum to iterate texture unit. How to make sure the texture unit is associate with correct target when i want to reuse the unit. for example use unit 2 for 2D first, and then want reuse it for 3D. I have tried glBindTexture(GL_TEXTURE_1D/2D/3D, 0); but seems not work. Should i use glenable?

Answer 1

You have misinterpreted what this statement on the OpenGL Wiki means.

It is referring to sampler uniforms in GLSL. It is an error to have a sampler2D and samplerCube, for instance, that both reference the same texture image unit. Since this situation cannot be determined at compile- / link-time, there is no error state that will be generated. Instead, you will create undefined behavior at shader run-time if you try to use two different types of samplers that refer to the same texture image unit.

Regarding enabling GL_TEXTURE_1D, etc. That is for the fixed-function pipeline. It does nothing in shader-based OpenGL, textures are effectively "enabled" or "disabled" completely programatically. If you do not sample anything from a certain texture image unit during the execution of your shader, then you can think of it as "disabled." However, ultimately such thinking is not productive in the programmable pipeline. You should simply forget that those states ever existed.