Basic Drawing of Pixels From Unsigned Short * Using OpenGLES on iOS - iphone

Question

For my non-app store app, I've been using the private framework Core Surface to draw directly to the screen of the iPhone. However, this can be rather slow on older devices because it heavily uses the CPU to do its drawing. To fix this, I've decided to try to use OpenGLES to render the pixels to the screen.
Currently (and I have no way of changing this), I have a reference to an unsigned short * variable called BaseAddress, and essential 3rd party code accesses BaseAddress and updates it with the new pixel data.
I've set up a GLKViewController, and implemented the viewDidLoad as follows:
- (void)viewDidLoad {
[super viewDidLoad];
self.context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
if (!self.context) {
NSLog(#"Failed to create ES context");
}
[EAGLContext setCurrentContext:self.context];
GLKView *view = (GLKView *)self.view;
view.context = self.context;
glGenBuffers(1, &screenBuffer);
glBindBuffer(GL_ARRAY_BUFFER, screenBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(BaseAddress), BaseAddress, GL_DYNAMIC_DRAW);
}
where screenBuffer is an instance variable. In the glkView:drawInRect: method I have the following:
- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect {
glDrawElements(GL_ARRAY_BUFFER, sizeof(BaseAddress)/sizeof(BaseAddress[0]), GL_UNSIGNED_SHORT, BaseAddress);
}
Unfortunately, only a black screen appears when I run the app. If I go back to using Core Surface, the app works fine. So basically, how can I draw the pixels to the screen using OpenGLES?

Answer 1

I think that it might be best to use a texture and for your case I'd try to find some older ES1 template for iOS devices. Basically what you need is a frame buffer and a color buffer made from your UIView layer:
glGenFramebuffers(1, &viewFramebuffer);
glBindFramebuffer(GL_FRAMEBUFFER, viewFramebuffer);
glGenRenderbuffers(1, &viewColorBuffer);
glBindRenderbuffer(GL_RENDERBUFFER, viewColorBuffer);
[context renderbufferStorage:GL_RENDERBUFFER fromDrawable:(CAEAGLLayer*)self.layer];
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, viewColorBuffer);
glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &backingWidth);
glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, & backingHeight);
As for projection matrix I suggest you use glOrthof(.0f, backingWidth, backingHeight, .0f, -1.0f, 1.0f); that will make your GL coordinates same as your view coordinates.
Next on some initialization generate a texture, bind it and give it dimensions of power of 2 (textureWidth = 1; while(textureWidth < backingWidth) textureWidth = textureWidth << 1;) and pass it NULL for data pointer (all in function "glTexImage2D")
Then generate vertex array for a square same as texture, from (0,0) to (textureWidth, textureHeight) and texture coordinates from (0,0) to (1,1)
When you get the data to your pointer and are ready to be pushed to texture use glTexSubImage2D to update it: You can update only a segment of a texture if you get data for it or to update a whole screen use rect (0,0,screenWidth, screenHeight)
Now just draw those 2 triangles with your texture..
Note that there is a limit on texture size: most active iOS devices 1<<11 (2048) iPad3 1<<12
Do not forget to set texture parameters when creating it: glTexParameteri
Do check for retina display and set content scale of CAEAGLLayer to 2 if needed