Easiest, fastest way to render UIImage/PNG using OpenGL on iPhone?

Question

I've written an iPhone application that takes input from a user and updates a bitmap context based on the input. After the update, it then updates the display of a UIView. I am accomplishing such by implementing drawRect in a UIView subclass.

Currently I am using Quartz 2D. Given that the updates are occurring multiple times per second, I have noticed that while app performs well in the emulator it is much slower (albeit unusable) on an actual device. I would like to rewrite drawRect to use OpenGL instead (as I have heard this is much more performant?). If you have other ideas, I am all ears.

I have browsed around the web for samples/tutorials on how to simple display a PNG using OpenGL, but most are overkill for what I need. I am looking for a short and sweet sample for quickly taking a PNG file or UIImage and then rendering this within a UIView using OpenGL (accomplishable? does this even conceptually even make sense?) - or a pointer to a documentation for speeding up image rendering using the existing Quartz API's.

Before you decide to reply with a RTFM response - please don't waste my time or yours. I'm not looking for a shortcut, I don't mind doing the homework. I'm just looking for a simple solution/guidance that fits my needs and which has been hard to come by. I am sure someone who is well versed in OpenGL could probably explain what needs to be done and crank this code in 5 minutes. If that's you, it would make my day :).

Any help, guidance, etc. is always appreciated. Thanks.

Answer 1

A faster way to display an updated bitmap may be to just assign the bitmap context to the CALayer of a UIView (a subview sized and/or transformed to your desired destination rect if required). No drawRect or OpenGL frame render needed.

Answer 2

Create a bitmap context with memory you have allocated yourself

void* data = malloc(width*height*4);

CGContextRef ctx = CGBitmapContextCreate( . . . );

and render whatever you want into that bitmap context. Next create a texture from the data

glGenTextures(1, &texref);
glBindTexture(GL_TEXTURE_2D, texref);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);

Now just follow any tutorial on rendering this texture with GL, using Apple's example code on how to render GL into a UIView. Do be careful that you use an RGBA byte order in the bitmap context(or swap the GL call to ARGB). Also, remember that GL only likes textures with dimensions 2^n x 2^n.

With the iPhone's shared memory architecture, data isn't being sent to the graphics card each cycle, so this doesn't incur much of performance penalty.