Question

I’m trying to draw two objects using two different textures with one shader program in OpenGL ES 2.0 for Android. The first object should have texture0 and the second sould have texture1.

In fragment shader I have:

uniform sampler2D tex;

and in java code:

int tiu0 = 0;
int tiu1 = 1;
int texLoc = glGetUniformLocation(program, "tex");

glUseProgram(program);

// bind texture0 to texture image unit 0
glActiveTexture(GL_TEXTURE0 + tiu0);
glBindTexture(GL_TEXTURE_2D, texture0);

// bind texture1 to texture image unit 1
glActiveTexture(GL_TEXTURE0 + tiu1);
glBindTexture(GL_TEXTURE_2D, texture1);

glUniform1i(texLoc, tiu0);
// success: glGetError returns GL_NO_ERROR, glGetUniformiv returns 0 for texLoc

drawFirstObject(); // should have texture0

glUniform1i(texLoc, tiu1);
// success: glGetError returns GL_NO_ERROR, glGetUniformiv returns 1 for texLoc

drawSecondObject(); // should have texture1

Running on Samsung Galaxy Ace with Android 2.3.3 both objects have the same texture0. Similar code runs correctly in OpenGL 2.0 on my desktop computer.

If I remove drawFirstObject, the second object will have texture1.

If I remove drawSecondObject, the first object will have texture0.

If somewhere between drawFirstObject and drawSecondObject I change the program for a while:

glUseProgram(0); // can be also any valid program other than the program from the next call
glUseProgram(program);

then both objects will have texture1.

Values of uniforms different from sampler2D are always set correctly.

I know I can draw the two objects with different textures using only one texture image unit and binding appropriate texture to that texture image unit before drawing the object, but I also want to know what’s going on here.

Is something wrong with my code? Is it possible in OpenGL ES 2.0 to draw the objects with different textures by only switching between texture image units as I shown in the code? If it’s impossible, is that difference between OpenGL 2.0 (where it’s possible) and OpenGL ES 2.0 documented anywhere? I can’t find it.

Answer 1

After hours of further research I’ve found out that this problem is specific to Adreno 200 GPU that is utilized in my Samsung Galaxy Ace (GT-S5830). It seems like Adreno 200 driver assigns the texture to the sampler in the first call to a drawing function and after that it ignores any changes to the sampler value (glUniform1i(samplerLocation, textureImageUnit)) until one of the two occurs:

• glUseProgram is called with a different shader program,
• a different texture is bound to any texture image unit used by the shader program.

There’s a thread in the forums of the manufacturer of Adreno 200 GPU describing the very same problem.

So if you call drawing functions several times with the same shader program and with different textures bound before, there are two workarounds to the described problem:

1. Call glUseProgram(0); glUseProgram(yourDrawingProgram); before every drawing function.

2. Before every drawing call, bind different texture to at least one texture image unit used by your shader program. This solution can be difficult to maintain, because if you bind the same texture that is already bound to the texture image unit, the problem will remain. So in this case the easiest solution is to simply not change sampler values and bind textures of all texture image units used by the shader program before every drawing call.