OpenGL ES on iPhone doesn't draw anything - iphone

Question

I'm an absolute beginner in OpenGL ES programming in iOS.
This is my first attempt to draw some simple 2D primitives with OpenGL ES onto a view.
Here is the class declaration:
#interface OGLGameCanvas : UIView <GameCanvas> {
EAGLContext* context;
Game* game;
GLuint framebuffer, renderbuffer, depthbuffer;
}
Here is my initialization code:
- (void)initialize {
// Get the layer and set properties
CAEAGLLayer* layer = (CAEAGLLayer*)self.layer;
layer.opaque = NO;
layer.drawableProperties = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:NO], kEAGLDrawablePropertyRetainedBacking, kEAGLColorFormatRGBA8, kEAGLDrawablePropertyColorFormat, nil];
// Set the context
context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES1];
if (!context || ![EAGLContext setCurrentContext:context])
DLog(#"Cannot create EAGLContext!");
// Create the color buffer and the render buffer
glGenFramebuffers(1, &framebuffer);
glGenRenderbuffers(1, &renderbuffer);
glBindFramebuffer(GL_FRAMEBUFFER, framebuffer);
glBindRenderbuffer(GL_RENDERBUFFER, renderbuffer);
[context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer*)layer];
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, renderbuffer);
GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
if (status != GL_FRAMEBUFFER_COMPLETE)
NSLog(#"Failed to make complete frame buffer: %x", status);
// Get width and height of the render buffer
GLint width, height;
glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &width);
glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &height);
// Create and start animation loop
CADisplayLink* displayLink = [CADisplayLink displayLinkWithTarget:self selector:#selector(drawFrame:)];
[displayLink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSDefaultRunLoopMode];
}
and my drawing code:
- (void)drawFrame:(CADisplayLink*)sender {
glLoadIdentity();
glClearColor(0.0, 0.0, 0.0, 1.0);
glClear(GL_COLOR_BUFFER_BIT);
glEnableClientState(GL_VERTEX_ARRAY);
GLfloat vertices[] = { -20.0f, 2.5f, 0.0f, 0.0f, 1.5f, 7.5f };
glVertexPointer(2, GL_FLOAT, 0, vertices);
glColor4f(1.0, 0.0, 0.0, 1.0);
glPointSize(5.0);
glDrawArrays(GL_POINTS, 0, 3);
glDisableClientState(GL_VERTEX_ARRAY);
[context presentRenderbuffer:GL_RENDERBUFFER];
}
The canvas gets cleared (in fact, it becomes black, or red, or whatever I set into glClearColor), but no points are drawn.
I'm pretty sure I'm forgetting something basic and essential.
Thanks for your help.

Answer 1

The problem is here:
GLfloat vertices[] = { -20.0f, 2.5f, 0.0f, 0.0f, 1.5f, 7.5f };
The normalized device coordinates lay in range [-1..1] so you're drawing them outside the visible area.
And here:
You have to do this before drawing.
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();