A Fast way to display compressed images?

Question

Hi,
I currently working on iPhone and trying to display (home-made) compressed images in an UIImageView : it is updated as soon as I have finished building the new one. I already do it with raw (BMP) images but I use them at 2D GLTextures that I found faster than the use of an UIImageView.
To update the view with compressed images, I do:
//member in .h
UIImage * myImage;
UIImageView * MyImgView;
// In .m
-(void)UpdateImageWithData :(NSData *) p_Data
myImage = [UIImage imageWithData:p_Data];
MyImgView.image = myImage;
This works quite well, but I encounter a major drawback for me : the frame rate of this is never better than 10 fps. I build my images faster that 25 per second, but cannot display them that fast.
I tried to change compression quality and compression methods (PNG interlaced or not, JPEG (411, 421, 444, varying quality from 10 to 100%), etc.) but I always have the same frame rate. So, I am assuming that the max frame rate you can have with an UIImageView is limiter whatever the format is.
Now, I am wondering a few things :
- Am I right about this limitation or do you see any flaw in my discussion?
- Is there some way to 'accelerate' the rendering with an UIMageView?
- Is there another faster way to display compressed images (or at least any kind of JPEG images)
- Is there a (fast) way with UIKit (or any compatible lib) to retrieve the raw image so that I can use my first method and display it as a 2D OpenGL Texture?
- Since from one image to the other only a few changes occur, do you see any way to update only what actually changed in order to save some CPU?
- Do you think that the problem about UIImageView frame refreshement rate is linked with the iPhone Video Processor or with its CPU? (thus the new 3GS may help me solve this problem)
Thank your for reading this untill the end.

Answer 1

Hey, guess what, I had pretty much this same exact problem. Client wanted full screen 30 fps animation. Here's what I found experimenting with full screen sizes:
- Whether you use UIImageView conveniences, or load and draw yourself in a UIView subclass, makes no apparent difference.
- If you're doing basic Quartz drawing, never a problem keeping 30 fps up.
- But as soon as you change the image in a UIImageView, or draw it with CGContextDrawImage() yourself in a UIView, then the frame rate plummets to about 10 fps on an iPod touch 1G, or 20-25 on an iPhone 3GS.
For my purposes most of the full screen animation could be put into movies, and the motion clipped down to manageable sections in the interactive portions (and anyway, the fps doesn't really matter for those background bits). If it's not for you, then I guess you need to go down to OpenGL and see how that works out.