iPhone : How to change color of particular pixel of a UIImage?

Question

I have a UIImage, I want to change colour of its particular pixel, say 224 x 200th pixel. How can I do this? Is it even possible to do? Thanks in advance.

Answer 1

After searching a lot for a complete day I got the exact solution, though I found many but none of them worked. Here is what I have got, it works flawlessly -:

// This method creates an image by changing individual pixels of an image. Color of pixel has been taken from an array of colours('avgRGBsOfPixel')
-(void)createTexture{

self.sampleImage = [[UIImageView alloc]initWithImage:[UIImage imageNamed:@"whitebg.jpg"]];
CGRect imageRect = CGRectMake(0, 0, self.sampleImage.image.size.width, self.sampleImage.image.size.height);

UIGraphicsBeginImageContext(sampleImage.image.size);
CGContextRef context = UIGraphicsGetCurrentContext();

//Save current status of graphics context
CGContextSaveGState(context);
CGContextDrawImage(context, imageRect, sampleImage.image.CGImage);
//    And then just draw a point on it wherever you want like this:

//    CGContextFillRect(context, CGRectMake(x,y,1,1));
//Fix error according to @gsempe's comment
int pixelCount =0;
for(int i = 0 ; i<sampleImage.image.size.width ; i++){
    for(int j = 0 ; j<sampleImage.image.size.height ; j++){
        pixelCount++;
        int index = pixelCount/pixelCountForCalculatingAvgColor;
        //            NSLog(@"Index : %d", index);
        if(index >= [avgRGBsOfPixel count]){
            index = [avgRGBsOfPixel count] -1;
            NSLog(@"Bad Index");
        }
        //            if(pixelCount >9999){
        //                pixelCount = 9999;
        //                NSLog(@"Bad Index");
        //            }
        UIColor *color = [avgRGBsOfPixel objectAtIndex:index];
        float red ,green, blue ,alpha ;
        [color getRed:&red green:&green blue:&blue alpha:&alpha];
        CGContextSetRGBFillColor(context, red , green, blue , 1);
        CGContextFillRect(context, CGRectMake(i,j,1,1));
    }
}

//    Then just save it to UIImage again:

CGContextRestoreGState(context);
UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
[self.iv setImage:img];

}

Answer 2

CGContextRef does all of this. You can do all sorts of magic with this.

Here are some slides that go over basic usage.

Answer 3

It is possible with core graphics. The link has code to convert your image to grey scale. You can modify it for your need.