Question

I have two arrays that I declared on the stack in a function, and verified that they both contain the exact same data:

/// Rasters the textured quad using the specified parameters.
- (void)privateConfigureRasterWithTexture:(GLuint)theTexture
                          bottomLeftX:(GLfloat)bottomLeftX
                          bottomLeftY:(GLfloat)bottomLeftY
                            topRightX:(GLfloat)topRightX
                            topRightY:(GLfloat)topRightY
{

    const GLfloat texices[] =
      { bottomLeftX, bottomLeftY,   // bottom left corner
        bottomLeftX,   topRightY,   // top left corner
          topRightX,   topRightY,   // top right corner
          topRightX, bottomLeftY }; // bottom right corner

    const GLfloat texices2[] =
      { 0.0f, 0.0f,
        0.0f, 1.0f,
        1.0f, 1.0f,
        1.0f, 0.0f };

    for(int x=0;x<8;x++)
        if(texices[x] != texices2[x])
        {
            NSLog(@"Mismatch!");
            abort();
        }

When I execute the following line of code in (the bottom of) that function

glVertexAttribPointer(_attributeTexturedTexCoords, 2, GL_FLOAT, GL_FALSE,
                      2*sizeof(GLfloat), texices2);

I get the expected result, however if I instead execute

glVertexAttribPointer(_attributeTexturedTexCoords, 2, GL_FLOAT, GL_FALSE,
                      2*sizeof(GLfloat), texices);

I get a bad result. I don’t get what the difference is.

Edit: Here is how I’m invoking this function.

[self privateConfigureRasterWithTexture:_atlasTexture1
                            bottomLeftX:0.0f
                            bottomLeftY:0.0f
                              topRightX:1.0f
                              topRightY:1.0f];

Answer 1

Nevermind, I think I’ve found it (probably at around the exact same time Martins did). It looks like glVertexAttribPointer doesn’t make a copy of the data until the call to glDrawElements. Since my glDrawElements is outside of that function, the stack memory was being released (yet for some reason texices2 remained valid).

I’ve rewritten the code a bit so that texices is heap allocated once when that class is allocated, and just reused over and over again and free’d when the application deallocs.