OpenGL ES 2.0 drawing more than one texture

Question

My question is quite trivial I believe, I'm using OpenGL ES 2.0 to draw a simple 2D scene.
I have a background texture that stretches the whole screen and another texture of a flower (or shel I say sprite?) that drawn at a specific location on screen.

So the trivial why i can think of doing it is to call glDrawArrays twice, one with the vertices of the background texture, and another one with the vertices of the flower texture.

Is that the right way? if so, is that mean that for 10 flowers i'll need to call glDrawArrays 10 times?

And what about blending? what if i want to blend the flower with the background, i need both the background and flower pixel colors and that may be a problem with two draws no?

Or is it possible to do it in one draw? if so how can I create a shader that knows if it now processing the background texture vertex or the flower texture vertex?

Or is it possible to do it in one draw?

The problem with one draw is that the shader needs to know if the current vertex is a background vertex (than use the background texture color) or a flower vertex( than use the flower texture color), and I don't know how to do it.

Here is how I use one draw call to draw the background image stretches the whole screen and the flower is half size centered.

- (void)renderOnce {
    //... set program, clear color..

    glActiveTexture(GL_TEXTURE2);
    glBindTexture(GL_TEXTURE_2D, backgroundTexture);
    glUniform1i(backgroundTextureUniform, 2);

    glActiveTexture(GL_TEXTURE3);
    glBindTexture(GL_TEXTURE_2D, flowerTexture);
    glUniform1i(flowerTextureUniform, 3);

    static const GLfloat allVertices[] = {
        -1.0f, -1.0f, // background texture coordinates
        1.0f, -1.0f,  // to draw in whole screen
        -1.0f,  1.0f, //
        1.0f,  1.0f,

        -0.5f, -0.5f, // flower texture coordinates
        0.5f, -0.5f,  // to draw half screen size
        -0.5f,  0.5f, // and centered
        0.5f,  0.5f,  //
    };

    // both background and flower texture coords use the whole texture
    static const GLfloat backgroundTextureCoordinates[] = {
        0.0f, 0.0f,
        1.0f, 0.0f,
        0.0f, 1.0f,
        1.0f, 1.0f,
    };

    static const GLfloat flowerTextureCoordinates[] = {
        0.0f, 0.0f,
        1.0f, 0.0f,
        0.0f, 1.0f,
        1.0f, 1.0f,
    };

    glVertexAttribPointer(positionAttribute, 2, GL_FLOAT, 0, 0, allVertices);
    glVertexAttribPointer(backgroundTextureCoordinateAttribute, 2, GL_FLOAT, 0, 0, backgroundTextureCoordinates);
    glVertexAttribPointer(flowerTextureCoordinateAttribute, 2, GL_FLOAT, 0, 0, flowerTextureCoordinates);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}

SteveL SteveL · Accepted Answer · 2013-09-30T10:54:53

You have two choices:

Call glDrawArrays for every texture you want to draw, this will be slow if you have more than 10-20 textures, to speed it up thought you can use hardware vbo
Batch the vertices(vertices,texture coords,color) of all the sprites you want to draw in one array and use a texture atlas(a texture that has all of the pictures you want to draw in it) and draw all this with one glDrawArrays

The second way is obviously the better and the right one.To get an idea of how to do it ,look at my awnser here

OpenGL ES 2.0 drawing more than one texture

1 Answers