Keying off Stephen's idea of drawing noise quickly, I came up with a solution that I can use for my own purposes. It seems like splitting up large image manipulations for execution on different processor cores could be a good introduction to GCD and blocks. I read the intro to GCD docs but am still a little in the dark about the whole topic. For example, say I'm using the following method to generate 10,000 x 10,000 or 50,000 x 50,000 pixel images, how would I go about "blockifying" it for GCD?
#define RGB_BYTES_PER_PIXEL 3
- (CGImageRef) noiseImageRefWithBaseRed:(float) inRed
green:(float) inGreen
blue:(float) inBlue
width:(NSUInteger) inWidth
height:(NSUInteger) inHeight
{
NSUInteger resultSize = RGB_BYTES_PER_PIXEL * inWidth
* inHeight;
// used to preserve the basic hue of the input color
float clipPercent = 4.0/10;
// convert input color floats to 8 bit ints
unsigned char *buffer = (unsigned char *) malloc(resultSize),
*pr = buffer,
*pg = pr + 1,
*pb = pg + 1,
*e = pr +
resultSize,
r = (inRed * 255),
g = (inGreen *
255),
b = (inBlue *
255),
redOffset,
greenOffset,
blueOffset,
noise;
// seed the random function
srandom(time(0));
// To preserve the basic hue of the input color,
// clip offsets of non-major colors by a fixed amount
if ((r > g) && (r > b))
{
redOffset = 0;
greenOffset = (r - g) - (r - g) * clipPercent;
blueOffset = (r - b) - (r - b) * clipPercent;
}
else if ((g > r) && (g > b))
{
greenOffset = 0;
redOffset = (g - r) - (g - r) * clipPercent;
blueOffset = (g - b) - (g - b) * clipPercent;
}
else
{
blueOffset = 0;
redOffset = (b - r) - (b - r) * clipPercent;
greenOffset = (b - g) - (b - g) * clipPercent;
}
NSLog(@"starting noise loop");
while (pr < e)
{
noise = random();
*pr = r + (noise % redOffset); pr +=
RGB_BYTES_PER_PIXEL;
*pg = g + (noise % greenOffset); pg +=
RGB_BYTES_PER_PIXEL;
*pb = b + (noise & blueOffset); pb +=
RGB_BYTES_PER_PIXEL;
}
NSLog(@"noise loop complete");
// create data provider
CGDataProviderRef imgDataProvider =
CGDataProviderCreateWithData(NULL, buffer, resultSize, NULL);
CGColorSpaceRef imgColorSpace = CGColorSpaceCreateDeviceRGB();
CGImageRef img
= CGImageCreate (
inWidth,
inHeight,
8,
RGB_BYTES_PER_PIXEL * 8,
RGB_BYTES_PER_PIXEL * inWidth,
imgColorSpace,
kCGImageAlphaNone,
imgDataProvider,
NULL,
true,
kCGRenderingIntentDefault
);
CFRelease(imgDataProvider);
CFRelease(imgColorSpace);
free(buffer);
return img;
}_______________________________________________
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