How do I do pixel perfect collisions with cocos2d iphone

I am making a space invaders clone on iPhone using Cocos2D v0.99. I am getting there with the framework but there is still a lot I don't know. The step I am stuck on comes down to collisions. I need to do two things:

1) I need to do a pixel perfect collision with the fleet of CCSprites that are animated using a spritesheet. 2) I need to do a similar pixelperfect collision with the bases but I also need to delete some pixels for the base.

I have found a few resources (http://groups.google.com/group/cocos2d-iphone-discuss/browse_thread/thread/f6e734e00d863f5e/41768952a1bcca0e?lnk=gst&q=image+mask) and tried to use CCMutableTexture2D but it doesn't work very well with this cocos2d version.

My approach at the moment is to try and fix CCMutableTexture2D but I am struggling to get the sprite texture data into the MutableTexture.

Is this the best way to be doing this or am I missing some other technique?

Thanks.

----- UPDATE -----

I managed to get this to work in the simulator using the color blend method here: http://www.cocos2d-iphone.org/forum/topic/18522

I also managed to use CCRenderTexture and a blend function to do the pixel deletion thing via the RenderMask class (rm below).

But... the collision detection fails on device. I think the masking works ok though. My best guess is that there is something that I am using that is not OpenGL ES (or I have not set something up correctly. My code is:

rt = [CCRenderTexture renderTextureWithWidth:480  height:320];
[rt beginWithClear:0 g:0 b:0 a:0];

[rm visit];

// Read pixels
ccColor4B *buffer = malloc( sizeof(ccColor4B) * numPixels );

glReadPixels(x, y, w, h, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
 [_rt end];

// loop through pixels
for(unsigned int i=0; i<numPixels; i++)
{
    ccColor4B color = buffer[i];

    if ((float)color.a >0) {
        [self doSomeStuff];
    }
}

So, 开发者_如何学JAVAis glReadPixels() supported in OpenGL ES? Or, is there some better way to do this?

// RenderMask.h

//
//  RenderMask.h
//

#import <UIKit/UIKit.h>
#import <stdlib.h>
#import "cocos2d.h"

@interface RenderMask : CCRenderTexture {
@public
NSMutableArray *sprites;
NSMutableArray *spriteMasks;
int currentSprite;
float frameInterval;    //How fast this animates
}

-(void) initArrays;
-(void) drawCurrent;
-(void) addSpriteMask: (CCSprite*)spriteMask;
-(void) addSprite: (CCSprite*)sprite;

@property(nonatomic, retain) NSMutableArray *sprites;
@property(nonatomic, retain) NSMutableArray *spriteMasks;
@property(readwrite, assign) int currentSprite;
@property(readwrite, assign) float frameInterval;

@end

// RenderMask.m

//
//  RenderMask.m
//

#import "RenderMask.h"
#import "CCActionInterval.h"

@implementation RenderMask

@synthesize sprites, spriteMasks, currentSprite, frameInterval;

-(void) initArrays {
    sprites = [[NSMutableArray alloc] init];
    spriteMasks = [[NSMutableArray alloc] init];
    currentSprite = 0;
    frameInterval = 0.1f;
}

-(void) drawCurrent {
    [self clear:0.0f g:0.0f b:0.0f a:0.0f];

    [self begin];
    //Limit drawing to the alpha channel

    NSValue *spriteValue = [sprites objectAtIndex:0];
    CCSprite *sprite = (CCSprite*)[spriteValue pointerValue];
    [sprite setOpacityModifyRGB:NO];
    [sprite visit];

    glColorMask(1.0f, 1.0f, 1.0f, 1.0f);

    NSLog(@"spriteMasks size: %u", [spriteMasks count]);

    for(NSValue *value in spriteMasks){
        CCSprite *spriteMask = (CCSprite*)[value pointerValue];
        [sprite setOpacityModifyRGB:NO];
        [spriteMask visit];
    }

    glColorMask(1.0f, 1.0f, 1.0f, 1.0f);
    [self end];
}

-(void) addSpriteMask: (CCSprite*)spriteMask {
    NSLog(@"Adding spriteMask!!!");
    [spriteMask retain];
    [spriteMasks addObject:[NSValue valueWithPointer:spriteMask] ];
}

-(void) addSprite: (CCSprite*)sprite {
    NSLog(@"Adding sprite!!!");
    [sprite retain];
    [sprites addObject:[NSValue valueWithPointer:sprite] ];
}

-(void) runAnimation {
    NSLog(@"running animation");
    [self drawCurrent];
    currentSprite = currentSprite + 1;
    if(currentSprite >= [sprites count]){
        currentSprite = 0;
    }

    CCActionInterval * runAction = [CCCallFunc actionWithTarget:self selector:@selector(runAnimation)];
    CCActionInterval * delayAndRunAction = [CCSequence actions:[CCDelayTime actionWithDuration:frameInterval], runAction, nil];
    [self runAction:delayAndRunAction];
}

@end

Also, there is a pretty serious performance issue with this code but I can probably work through that.

Thanks.

----- UPDATE (AGAIN) ------

I have a process partly working now where a data structure is build on base initialisation and used as a lookup for a collision. It was working 50%ish of the time but then crashing with an malloc EXC_BAD_ACCESS error until I turned on Zombies and all the various malloc debugging options in XCode. Now, I get a consistent crash on the line

CGContextClearRect(imageContext, CGRectMake(0, 0, contextWidth, contextHeight));

I get to this by doing (with a few lines missing for clarity):

CGImageRef image = [UIImage imageNamed:@"aspeaker.png"].CGImage;
imageContext = CGBitmapContextCreate(theData, width, height, 8, 4 * width,  CGColorSpaceCreateDeviceRGB(), kCGImageAlphaNoneSkipLast | kCGBitmapByteOrder32Big);
NSLog(@"imageContext: %@",imageContext);
CGContextClearRect(imageContext, CGRectMake(0, 0, width, height));
CGContextTranslateCTM(imageContext, 0, height - imageSize.height);
CGContextDrawImage(imageContext, CGRectMake(0, 0, CGImageGetWidth(image), CGImageGetHeight(image)), image);
CGContextRelease(imageContext);

The debug console says:

2011-09-12 10:49:07.753 Invaders[1341:c803] imageContext: <CGContext 0x7f798fa0>
Invaders(1341,0xad06c2c0) malloc: protecting edges
Invaders(1341,0xad06c2c0) malloc: recording malloc stacks to disk using standard recorder
Invaders(1341,0xad06c2c0) malloc: enabling scribbling to detect mods to free blocks

I have read a few things about being on the correct thread but it all looks like it is on thread 1.

Any ideas or a link to a good resource on how to debug malloc errors would be very gratefully received.

Thanks,

Martin

----- ANOTHER UPDATE -----

One thing that came out of Googling this is that there is some complexity in hooking up the OpenGL context properly (http://stackoverflow.com/questions/4459415/problem-with-opengles-to-show-an-image) I am just using the cocos2d 0.99 base so could this be the problem? If so how the *&%@£$ do I fix it?

I would suggest letting a physics engine handle the collisions, cocos2d integrates well with chipmunk and box2D, this engines can handle the collisions for you with callbacks.