Converting a C array to a UIImage (for iOS)

Question

I currently have some image data in a C array, which contains RGBA data.

float array[length][4]

I am trying to get this to a UIImage, which it looks like these are initialized with files, NSData, and URLs. Since the other two methods are slow, I am most interested in the NSData approach.

I can get all of these values into an NSArray like so:

for (i=0; i<image.size.width * image.size.height; i++){
    replace = [UIColor colorWithRed:array[i][0] green:array[i][1] blue:array[i][2] alpha:array[i][3]];
    [output replaceObjectAtIndex:i withObject:replace];
}

So, I have a NSArray full of objects that are a UIColor. I have tried many methods, but how do I convert this to a UIImage?

I think it would be straight forward. A function sorta like imageWithData:data R:0 B:1 G:2 A:3 length:length width:width length:length would be nice, but there is no function as far as I can tell.

omz · Accepted Answer · 2012-10-13 15:51:41Z

6

imageWithData: is meant for image data in a standard image file format, e.g. a PNG or JPEG file that you have in memory. It's not suitable for creating images from raw data.

For that, you would typically create a bitmap graphics context, passing your array, pixel format, size, etc. to the CGBitmapContextCreate function. When you've created a bitmap context, you can create an image from it using CGBitmapContextCreateImage, which gives you a CGImageRef that you can pass to the UIImage method imageWithCGImage:.

Here's a basic example that creates a tiny 1×2 pixel image with one red pixel and one green pixel. It just uses hard-coded pixel values that are meant to show the order of the color components, normally, you would get this data from somewhere else of course:

size_t width = 2;
size_t height = 1;
size_t bytesPerPixel = 4;
//4 bytes per pixel (R, G, B, A) = 8 bytes for a 1x2 pixel image:
unsigned char rawData[8] = {255, 0, 0, 255,  //red
                            0, 255, 0, 255}; //green

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
size_t bytesPerRow = bytesPerPixel * width;
size_t bitsPerComponent = 8;
CGContextRef context = CGBitmapContextCreate(rawData, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast);

CGImageRef cgImage = CGBitmapContextCreateImage(context);
//This is your image:
UIImage *image = [UIImage imageWithCGImage:cgImage];
//Don't forget to clean up:
CGImageRelease(cgImage);
CGColorSpaceRelease(colorSpace);
CGContextRelease(context);

edited Oct 13, 2012 at 15:51

answered Oct 13, 2012 at 0:32

omz

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3 Comments

Scott Over a year ago

Could you give me some more details on that? An example would be extremely helpful, if you want to. From what I understand, I make a CGBitmapContextRef with the CGBitmapContextCreate function, but can not find CGBitmapContextRef anywhere. I then pass that creation to CGBitmapContextCreateImage then pass that to imageWithCGImage. Correct?

omz Over a year ago

Sorry, CGBitmapContextRef was incorrect, the CGBitmapContextCreate function actually just returns a regular CGContextRef. I'll see if I can find a simple example...

omz Over a year ago

I've added a basic example to the answer.

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