从图片中的一点取色

http://www.devdiv.com/thread-67521-1-1.html

最近看这个比较多人用到,以前有使用过,现在,粘过来给大伙看看.
注意:UIImageView的大小只能跟图片一样大.要不然取色不对.

- (UIColor *) getPixelColorAtLocation:(CGPoint)point {
        UIColor* color = nil;
        CGImageRef inImage = self.image.CGImage;
        // Create off screen bitmap context to draw the image into. Format ARGB is 4 bytes for each pixel: Alpa, Red, Green, Blue
        CGContextRef cgctx = [self createARGBBitmapContextFromImage:inImage];
        if (cgctx == NULL) {
                return nil; /* error */ 
        }
        
    size_t w = CGImageGetWidth(inImage);
        size_t h = CGImageGetHeight(inImage);
        CGRect rect = {{0,0},{w,h}}; 
        
        // Draw the image to the bitmap context. Once we draw, the memory 
        // allocated for the context for rendering will then contain the 
        // raw image data in the specified color space.
        CGContextDrawImage(cgctx, rect, inImage); 
        
        // Now we can get a pointer to the image data associated with the bitmap
        // context.
        unsigned char* data = CGBitmapContextGetData (cgctx);
        if (data != NULL) {
                //offset locates the pixel in the data from x,y. 
                //4 for 4 bytes of data per pixel, w is width of one row of data.
                int offset = 4*((w*round(point.y))+round(point.x));
                int alpha =  data[offset]; 
                int red = data[offset+1]; 
                int green = data[offset+2]; 
                int blue = data[offset+3]; 
                //NSLog(@"offset: %i colors: RGB A %i %i %i  %i",offset,red,green,blue,alpha);
                color = [UIColor colorWithRed:(red/255.0f) green:(green/255.0f) blue:(blue/255.0f) alpha:(alpha/255.0f)];
                
        }
        
        // When finished, release the context
        CGContextRelease(cgctx); 
        // Free image data memory for the context
        if (data) { free(data); }
        return color;
}

- (CGContextRef) createARGBBitmapContextFromImage:(CGImageRef) inImage {
        
        CGContextRef    context = NULL;
        CGColorSpaceRef colorSpace;
        void *          bitmapData;
        int             bitmapByteCount;
        int             bitmapBytesPerRow;
        
        // Get image width, height. We'll use the entire image.
        size_t pixelsWide = CGImageGetWidth(inImage);
        size_t pixelsHigh = CGImageGetHeight(inImage);
        
        // Declare the number of bytes per row. Each pixel in the bitmap in this
        // example is represented by 4 bytes; 8 bits each of red, green, blue, and
        // alpha.
        bitmapBytesPerRow   = (pixelsWide * 4);
        bitmapByteCount     = (bitmapBytesPerRow * pixelsHigh);
        
        // Use the generic RGB color space.
        colorSpace = CGColorSpaceCreateDeviceRGB();

        if (colorSpace == NULL)
        {
                fprintf(stderr, "Error allocating color space\n");
                return NULL;
        }
        
        // Allocate memory for image data. This is the destination in memory
        // where any drawing to the bitmap context will be rendered.
        bitmapData = malloc( bitmapByteCount );
        if (bitmapData == NULL) 
        {
                fprintf (stderr, "Memory not allocated!");
                CGColorSpaceRelease( colorSpace );
                return NULL;
        }
        
        // Create the bitmap context. We want pre-multiplied ARGB, 8-bits 
        // per component. Regardless of what the source image format is 
        // (CMYK, Grayscale, and so on) it will be converted over to the format
        // specified here by CGBitmapContextCreate.
        context = CGBitmapContextCreate (bitmapData,
                                                                         pixelsWide,
                                                                         pixelsHigh,
                                                                         8,      // bits per component
                                                                         bitmapBytesPerRow,
                                                                         colorSpace,
                                                                         kCGImageAlphaPremultipliedFirst);
        if (context == NULL)
        {
                free (bitmapData);
                fprintf (stderr, "Context not created!");
        }
        
        // Make sure and release colorspace before returning
        CGColorSpaceRelease( colorSpace );
        
        return context;
}