How can I display multiple separate textures (not multi-texturing) with OpenGL ES 2.0?

Question

My iOS 4 app uses OpenGL ES 2.0 and renders elements with a single texture. I would like to draw elements using multiple different textures and am having problems getting things to work.

I added a variable to my vertex shader to indicate which texture to apply:

...
attribute float TextureIn;
varying float TextureOut;

void main(void)
{ 
    ...
    TextureOut = TextureIn;
}

I use that value in the fragment shader to select the texture:

...
varying lowp float TextureOut;

uniform sampler2D Texture0;
uniform sampler2D Texture1;

void main(void)
{   
    if (TextureOut == 1.0) 
    {
        gl_FragColor = texture2D(Texture1, TexCoordOut);
    }
    else // 0
    {
        gl_FragColor = texture2D(Texture0, TexCoordOut);
    }
}

Compile shaders:

...
_texture = glGetAttribLocation(programHandle, "TextureIn");
glEnableVertexAttribArray(_texture);
_textureUniform0 = glGetUniformLocation(programHandle, "Texture0");
_textureUniform1 = glGetUniformLocation(programHandle, "Texture1");

Init/Setup:

...
GLuint _texture;
GLuint _textureUniform0;
GLuint _textureUniform1;

...
glActiveTexture(GL_TEXTURE0);
glEnable(GL_TEXTURE_2D); // ?
glBindTexture(GL_TEXTURE_2D, _textureUniform0);
glUniform1i(_textureUniform0, 0);

glActiveTexture(GL_TEXTURE1);
glEnable(GL_TEXTURE_2D); // ?
glBindTexture(GL_TEXTURE_2D, _textureUniform1);
glUniform1i(_textureUniform1, 1);

glActiveTexture(GL_TEXTURE0);

Render:

...
glVertexAttribPointer(_texture, 1, GL_FLOAT, GL_FALSE, sizeof(Vertex), (GLvoid*) (sizeof(float) * 13));    

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, _textureUniform0);
glUniform1i(_textureUniform0, 0);

glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, _textureUniform1);
glUniform1i(_textureUniform1, 1);

glActiveTexture(GL_TEXTURE0);

glDrawElements(GL_TRIANGLES, indicesCountA, GL_UNSIGNED_SHORT, (GLvoid*) (sizeof(GLushort) * 0));
glDrawElements(GL_TRIANGLES, indicesCountB, GL_UNSIGNED_SHORT, (GLvoid*) (sizeof(GLushort) * indicesCountA));
glDrawElements(GL_TRIANGLES, indicesCountC, GL_UNSIGNED_SHORT, (GLvoid*) (sizeof(GLushort) * (indicesCountA + indicesCountB)));

My hope was to dynamically apply the texture associated with a vertex but it seems to only recognize GL_TEXTURE0.

The only way I have been able to change textures is to associated each texture with GL_TEXTURE0 and then draw:

glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, _textureUniformX);
glUniform1i(_textureUniformX, 0);
glDrawElements(GL_TRIANGLES, indicesCountA, GL_UNSIGNED_SHORT, (GLvoid*) (sizeof(GLushort) * 0));
...

In order to render all the textures, I would need a separate glDrawElements() call for each texture, and I have read that glDrawElements() calls are a big hit to performance and the number of calls should be minimized. Thats why I was trying to dynamically specifiy which texture to use for each vertex.

It's entirely possible that my understanding is wrong or I am missing something important. I'm still new to OpenGL and the more I learn the more I feel I have more to learn.

It must be possible to use textures other than just GL_TEXTURE0 but I have yet to figure out how.

Any guidance or direction would be greatly appreciated.

Christian Rau Christian Rau · Accepted Answer · 2012-09-18T08:28:41

Can it be you're just experiencing floating point rounding issues? There shouldn't be any (except if a single privimitve shares vertices with different textures), but just to be sure replace this TextureOut == 1.0 with a TextureOut > 0.5 or something the like.

As a general advice, you are correct in that the number of draw calls should be reduced as much a possible, but your approach is quite odd. You are buying draw call reduction with fragment shader branching. Your approach also doesn't scale well with the overall number of textures, since you always need all textures in separate texture units.

The usual approach to reduce texture switches is to put all the textures into a single large texture, a so-called texture atlas, and use the texture coordinates to select the appropriate subregion in this texture. This also has some pitfalls (which are an entirely different question), but nothing comes for free.

EDIT: Oh wait, I see what you're actually doing wrong

glBindTexture(GL_TEXTURE_2D, _textureUniform0);

You're binding a texture to the current texture unit, but instead of the texture object you give this function a uniform location, which is complete rubbish (but might even work in some weird circumstances, since both uniform locations and texture objects are themselves just integers). Of course you have to bind the actual texture.

How can I display multiple separate textures (not multi-texturing) with OpenGL ES 2.0?

1 Answers