Question

I have very simple OpenGL ES example similar to Hehe’s example : http://nehe.gamedev.net/tutorial/ios_lesson_02__first_triangle/50001/

As shown above triangle filled with three colors – red, blue, green.
Instead in my app i always get triangle almost completely filled with black color, only small area around top vertex filled with green and small area around right bottom filled with red … and there is no blue at all.

The first question is : why do the colors not interpolate in the middle of my triangle and why does blue color is not visible at all?

Any changes in my colors array affect nothing, e.g. when i try to make triangle white the colors do not change anyway … in the meantime if i change Z coordinate in positions array then i can see the blue color.

The second question is : why any changes in colors do nothing and changes in positions change the color instead?

Seems like somewhere here i made one stupid mistake but i can’t catch it.

This is Vertex / Color arrays :

const float colors[] = { // this does not work, triangle still black-green-red
    1.0, 1.0, 1.0, 1.0,
    1.0, 1.0, 1.0, 1.0,
    1.0, 1.0, 1.0, 1.0
};

const float positions[] = { // if i change 3rd index to 1.0 then i will see blue color
    -0.5, -0.5, 0.0, 1.0,
     0.0,  0.5, 0.0, 1.0,
     0.5, -0.5, 0.0, 1.0
};

This is VBO :

- (BOOL)setupVBO 
{
    BOOL success = YES;

    glGenBuffers(1, &_positionBuffer);
    glBindBuffer(GL_ARRAY_BUFFER, _positionBuffer);
    glBufferData(
        GL_ARRAY_BUFFER, 
        sizeof(positions) * sizeof(float), 
        &positions[0], 
        GL_STATIC_DRAW);

    glGenBuffers(1, &_colorBuffer);
    glBindBuffer(GL_ARRAY_BUFFER, _colorBuffer);
    glBufferData(
        GL_ARRAY_BUFFER, 
        sizeof(colors) * sizeof(float), 
        &colors[0], 
        GL_STATIC_DRAW);

    return success;
}

Render :

- (void)render:(CADisplayLink*)displayLink
{
    glClearColor(0.5, 0.5, 0.5, 1.0);
    glClear(GL_COLOR_BUFFER_BIT);

    glBindBuffer(GL_ARRAY_BUFFER, _positionBuffer);
    glVertexAttribPointer(_positionSlot, 4, GL_FLOAT, GL_FALSE, 0, NULL);

    glBindBuffer(GL_ARRAY_BUFFER, _colorRenderBuffer);
    glVertexAttribPointer(_colorSlot, 4, GL_FLOAT, GL_FALSE, 0, NULL);

    glDrawArrays(GL_TRIANGLES, 0, 3);

    [_glContext presentRenderbuffer:GL_RENDERBUFFER];
}

Thanks for any advice …

Answer 1

Ok, i found where was the issue 🙂

As far as i am beginner in Open GL i just copypasted the code from example and renamed some variables … and did not catch that i renamed ColorBuffer (VBO color, i.e. actual color data of drawing object) variable to ColorRenderBuffer variable (place in memory where GL processes actual color data)

Stupid mistake and i hope nobody will do the same 🙂