NSBitmapImageRep to RGB with high efficency

Question

I am working on my first mac osx cocoa app for 10.5+ where I have a CVImageBufferRef (captured using QTKit), I need to transfer this image over TCP Socket to the client app. The client app needs RGB values. So here is what I am currently doing (my current solution works as needed but uses a lot of CPU)

CVImageBufferRef --> NSBitmapImageRep --> NSData --> then transmit NSData over TCP Socket to client app and at client side I have the following code to get the RGB:

UInt8 r,g,b
int width=320;
int height=240;

NSData *data; //read from TCP Socket
NSBitmapImageRep *bitmap=[[NSBitmapImageRep alloc] initWithData:data];

for (y=1;y<=height;y++) {
    for(x=1;x<=width;x++){


        NSColor *color=[bitmap colorAtX:x y:y];
        // ^^ this line is actually a culprit and uses a lot of CPU


        r=[color redComponent]*100;
        g=[color greenComponent]*100;
        b=[color blueComponent]*100;
        NSLog(@"r:%d g:%d b:%d",r,g,b);
    }
}

[bitmap release];

If CVImageBufferRef can be converted to an array of RGB values that would be perfect otherwise I need an efficient solution to convert NSBitmapImageRep to RGB values.

Answer 1

If going straight from a CVImageBufferRef, you can use CVPixelBuffer as it is derived from CVImageBuffer. Use CVPixelBufferLockBaseAddress() and then CVPixelBufferGetBaseAddress() to get a pointer to the first pixel. There is also many other CVPixelBufferGet* methods (http://developer.apple.com/library/mac/#documentation/QuartzCore/Reference/CVPixelBufferRef/Reference/reference.html) to enquire width, height etc for knowledge of the size of data to be transmitted. And of course CVPixelBufferUnlockBaseAddress() once done with the data.

If using the NSBitmapImageRep, I agree with Justin. using [bitmap bitmapData] will return the pointer to the first element. Even with no prior knowledge, you can send the raw data that will be:

unsigned char *data = [bitmap bitmapData];
unsigned long size = [bitmap pixelsWide]*[bitmap pixelsHigh]*[bitmap samplesPerPixel]*sizeof(unsigned char);

This data will then be RGB successive values i.e. 1stRedComponent = *(data + 0), 1stGreenComponent = *(data + 1) 1stBlueComponent = *(data + 2) will give you the first RGB component in the data if you need pixel specific access outside of the NSBitmapImageRep

Answer 2

You can just get a hold of the bitmap data using bitmapData and take the pixel values you need from it. That should make many cases more than 100x faster.

(the log should go too)