user interface - fastest way to draw a screen buffer on the iphone

Question

I have a "software renderer" that I am porting from PC to the iPhone. what is the fastest way to manually update the screen with a buffer of pixels on the iphone? for instance in windows the fastest function I have found is SetDIBitsToDevice.

I don't know much about the iphone, or the libraries, and there seem to be so many layers and different types of UI elements, so I might need a lot of explanation...

for now I'm just going to constantly update a texture in opengl and render that to the screen, I very much doubt that this is going to be the best way to do it.

UPDATE:

I have tried the openGL screen sized texture method:

I got 17fps...

I used a 512x512 texture (because it needs to be a power of two)

just the call of

glTexSubImage2D(GL_TEXTURE_2D,0,0,0,512,512,GL_RGBA,GL_UNSIGNED_BYTE, baseWindowGUI->GetBuffer());

seemed pretty much responsible for ALL the slow down.

commenting it out, and leaving in all my software rendering GUI code, and the rendering of the now non updating texture, resulted in 60fps, 30% renderer usage, and no notable spikes from the cpu.

note that GetBuffer() simply returns a pointer to the software backbuffer of the GUI system, there is no re-gigging or resizing of the buffer in anyway, it is properly sized and formatted for the texture, so I am fairly certain the slowdown has nothing to do with the software renderer, which is the good news, it looks like if I can find a way to update the screen at 60, software rendering should work for the time being.

I tried doing the update texture call with 512,320 rather than 512,512 this was oddly even slower... running at 10fps, also it says the render utilization is only like 5%, and all the time is being wasted in a call to Untwiddle32bpp inside openGLES.

I can change my software render to natively render to any pixle format, if it would result in a more direct blit.

fyi, tested on a 2.2.1 ipod touch G2 (so like an Iphone 3G on steroids)

UPDATE 2:

I have just finished writting the CoreAnimation/Graphics method, it looks good, but I am a little worried about how it updates the screen each frame, basically ditching the old CGImage, creating a brand new one... check it out in 'someRandomFunction' below: is this the quickest way to update the image? any help would be greatly appreciated.

//
//  catestAppDelegate.m
//  catest
//
//  Created by User on 3/14/10.
//  Copyright __MyCompanyName__ 2010. All rights reserved.
//




#import "catestAppDelegate.h"
#import "catestViewController.h"
#import "QuartzCore/QuartzCore.h"

const void* GetBytePointer(void* info)
{
    // this is currently only called once
    return info; // info is a pointer to the buffer
}

void ReleaseBytePointer(void*info, const void* pointer)
{
    // don't care, just using the one static buffer at the moment
}


size_t GetBytesAtPosition(void* info, void* buffer, off_t position, size_t count)
{
    // I don't think this ever gets called
    memcpy(buffer, ((char*)info) + position, count);
    return count;
}

CGDataProviderDirectCallbacks providerCallbacks =
{ 0, GetBytePointer, ReleaseBytePointer, GetBytesAtPosition, 0 };


static CGImageRef cgIm;

static CGDataProviderRef dataProvider;
unsigned char* imageData;
 const size_t imageDataSize = 320 * 480 * 4;
NSTimer *animationTimer;
NSTimeInterval animationInterval= 1.0f/60.0f;


@implementation catestAppDelegate

@synthesize window;
@synthesize viewController;


- (void)applicationDidFinishLaunching:(UIApplication *)application {    


    [window makeKeyAndVisible];


    const size_t byteRowSize = 320 * 4;
    imageData = malloc(imageDataSize);

    for(int i=0;i<imageDataSize/4;i++)
            ((unsigned int*)imageData)[i] = 0xFFFF00FF; // just set it to some random init color, currently yellow


    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    dataProvider =
    CGDataProviderCreateDirect(imageData, imageDataSize,
                               &providerCallbacks);  // currently global

    cgIm = CGImageCreate
    (320, 480,
     8, 32, 320*4, colorSpace,
     kCGImageAlphaNone | kCGBitmapByteOrder32Little,
     dataProvider, 0, false, kCGRenderingIntentDefault);  // also global, probably doesn't need to be

    self.window.layer.contents = cgIm; // set the UIWindow's CALayer's contents to the image, yay works!

   // CGImageRelease(cgIm);  // we should do this at some stage...
   // CGDataProviderRelease(dataProvider);

    animationTimer = [NSTimer scheduledTimerWithTimeInterval:animationInterval target:self selector:@selector(someRandomFunction) userInfo:nil repeats:YES];
    // set up a timer in the attempt to update the image

}
float col = 0;

-(void)someRandomFunction
{
    // update the original buffer
    for(int i=0;i<imageDataSize;i++)
        imageData[i] = (unsigned char)(int)col;

    col+=256.0f/60.0f;

    // and currently the only way I know how to apply that buffer update to the screen is to
    // create a new image and bind it to the layer...???
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    cgIm = CGImageCreate
    (320, 480,
     8, 32, 320*4, colorSpace,
     kCGImageAlphaNone | kCGBitmapByteOrder32Little,
     dataProvider, 0, false, kCGRenderingIntentDefault);

    CGColorSpaceRelease(colorSpace);

    self.window.layer.contents = cgIm;

    // and that currently works, updating the screen, but i don't know how well it runs...
}


- (void)dealloc {
    [viewController release];
    [window release];
    [super dealloc];
}


@end

Answer 1

The fastest App Store approved way to do CPU-only 2D graphics is to create a CGImage backed by a buffer using CGDataProviderCreateDirect and assign that to a CALayer's contents property.

For best results use the kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little or kCGImageAlphaNone | kCGBitmapByteOrder32Little bitmap types and double buffer so that the display is never in an inconsistent state.

edit: this should be faster than drawing to an OpenGL texture in theory, but as always, profile to be sure.

edit2: CADisplayLink is a useful class no matter which compositing method you use.