Question

I’m starting to work with OpenGL in iOS. I have always learned to draw stuff in OpenGL using glBegin() and glEnd() so this is kind of new to me.

I am trying to draw a simple triangle. I can draw a white triangle nicely, I even can draw an entire colored triangle using glColor. But whenever I try to assign a color to each vertex using this code below, I get a EXC_BAD_ACCESS when drawing the array. I am using iOS 4.3 simulator for this. What am I doing wrong?

- (void) render:(CADisplayLink*)displayLink {
    glClearColor(0, 104.0/255.0, 55.0/255.0, 1.0);
    glClear(GL_COLOR_BUFFER_BIT);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    GLfloat vertices [] = {0,0,0, 0,100,0, 100,0,0};
    GLfloat colours [] = {1.0,1.0,1.0, 1.0,1.0,1.0, 1.0,1.0,1.0};
    glEnableClientState(GL_VERTEX_ARRAY);
    glEnableClientState(GL_COLOR_ARRAY);
    glVertexPointer(3, GL_FLOAT, 0, vertices);
    glColorPointer(3, GL_FLOAT, 0, colours);
    glDrawArrays(GL_TRIANGLES, 0, 3);   <-- CRASHES HERE
    glDisableClientState(GL_VERTEX_ARRAY);
    glDisableClientState(GL_COLOR_ARRAY);

    [self.context presentRenderbuffer:GL_RENDERBUFFER];
}    

Answer 1

The line glColorPointer(3, GL_FLOAT, 0, colours) generates the GL_INVALID_VALUE error (you can see that doing po glGetError right after executing that line, it will print 1281).

The reason is that OpenGL ES doesn’t support 3 color components, the documentation states:

GL_INVALID_VALUE is generated if size is not 4.

You code will be ok if you change the number of color components to 4 by adding alpha.