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    <p>What exactly takes place when you use this function? Is the function returning an invalid image, or is the display not showing it correctly? </p> <p>This is the method I use to convert to greyscale.</p> <pre><code>- (UIImage *) convertToGreyscale:(UIImage *)i { int kRed = 1; int kGreen = 2; int kBlue = 4; int colors = kGreen | kBlue | kRed; int m_width = i.size.width; int m_height = i.size.height; uint32_t *rgbImage = (uint32_t *) malloc(m_width * m_height * sizeof(uint32_t)); CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); CGContextRef context = CGBitmapContextCreate(rgbImage, m_width, m_height, 8, m_width * 4, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaNoneSkipLast); CGContextSetInterpolationQuality(context, kCGInterpolationHigh); CGContextSetShouldAntialias(context, NO); CGContextDrawImage(context, CGRectMake(0, 0, m_width, m_height), [i CGImage]); CGContextRelease(context); CGColorSpaceRelease(colorSpace); // now convert to grayscale uint8_t *m_imageData = (uint8_t *) malloc(m_width * m_height); for(int y = 0; y &lt; m_height; y++) { for(int x = 0; x &lt; m_width; x++) { uint32_t rgbPixel=rgbImage[y*m_width+x]; uint32_t sum=0,count=0; if (colors &amp; kRed) {sum += (rgbPixel&gt;&gt;24)&amp;255; count++;} if (colors &amp; kGreen) {sum += (rgbPixel&gt;&gt;16)&amp;255; count++;} if (colors &amp; kBlue) {sum += (rgbPixel&gt;&gt;8)&amp;255; count++;} m_imageData[y*m_width+x]=sum/count; } } free(rgbImage); // convert from a gray scale image back into a UIImage uint8_t *result = (uint8_t *) calloc(m_width * m_height *sizeof(uint32_t), 1); // process the image back to rgb for(int i = 0; i &lt; m_height * m_width; i++) { result[i*4]=0; int val=m_imageData[i]; result[i*4+1]=val; result[i*4+2]=val; result[i*4+3]=val; } // create a UIImage colorSpace = CGColorSpaceCreateDeviceRGB(); context = CGBitmapContextCreate(result, m_width, m_height, 8, m_width * sizeof(uint32_t), colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaNoneSkipLast); CGImageRef image = CGBitmapContextCreateImage(context); CGContextRelease(context); CGColorSpaceRelease(colorSpace); UIImage *resultUIImage = [UIImage imageWithCGImage:image]; CGImageRelease(image); free(m_imageData); // make sure the data will be released by giving it to an autoreleased NSData [NSData dataWithBytesNoCopy:result length:m_width * m_height]; return resultUIImage; } </code></pre>
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    1. POConvert image to grayscale
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    1. POConvert image to grayscale
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    1. COThanks, that is working, but I get an image that is rotated by 90 degrees from the original position. How can I fix it?
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      1. USIlya Suzdalnitski
    2. COCheck out http://www.cocoadev.com/index.pl?UIImage (or just google "How to rotate a UIImage"
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    3. COI notice that this function doesn't respect the original image's alpha transparency mask.
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