How can I get the underlying pixel data from a UIImage or CGImage? - objective-c

Question

I've tried numerous 'solutions' around the net, all of those I found have errors and thus don't work. I need to know the color of a pixel in a UIImage. How can i get this information?

Answer 1

Getting the raw data
From Apple's Technical Q&A QA1509 it says this will get the raw image data in it's original format by getting it from the Data Provider.
CFDataRef CopyImagePixels(CGImageRef inImage)
{
return CGDataProviderCopyData(CGImageGetDataProvider(inImage));
}
Needed in a different format or color-space
If you want to get the data color-matched and in a specific format you can use something similar to the following code sample:
void ManipulateImagePixelData(CGImageRef inImage)
{
// Create the bitmap context
CGContextRef cgctx = CreateARGBBitmapContext(inImage);
if (cgctx == NULL)
{
// error creating context
return;
}
// Get image width, height. We'll use the entire image.
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.
void *data = CGBitmapContextGetData (cgctx);
if (data != NULL)
{
// **** You have a pointer to the image data ****
// **** Do stuff with the data here ****
}
// When finished, release the context
CGContextRelease(cgctx);
// Free image data memory for the context
if (data)
{
free(data);
}
}
CGContextRef CreateARGBBitmapContext (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 = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);
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;
}
Color of a particular pixel
Assuming RGB, once you have the data in a format you like finding the color is a matter of moving through the array of data and getting the RGB value at a particular pixel location.

Answer 2

If you're looking to just get a single pixel or a few ones you can look to do a little different approach. Create a 1x1 bitmap context and draw the image over it with an offset so you just get the pixel you want.
CGImageRef image = uiimage.CGImage;
NSUInteger width = CGImageGetWidth(image);
NSUInteger height = CGImageGetHeight(image);
// Setup 1x1 pixel context to draw into
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
unsigned char rawData[4];
int bytesPerPixel = 4;
int bytesPerRow = bytesPerPixel;
NSUInteger bitsPerComponent = 8;
CGContextRef context = CGBitmapContextCreate(rawData,
1,
1,
bitsPerComponent,
bytesPerRow,
colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);
CGContextSetBlendMode(context, kCGBlendModeCopy);
// Draw the image
CGContextDrawImage(context,
CGRectMake(-offset.x, offset.y-height, width, height),
image);
// Done
CGContextRelease(context);
// Get the pixel information
unsigned char red = rawData[0];
unsigned char green = rawData[1];
unsigned char blue = rawData[2];
unsigned char alpha = rawData[3];