iOS -- detect the color of a pixel?

Question

For example, suppose I want to detect the color of the pixel with screen coordinates (100, 200). Is there a way to do this?

EDIT -- I'm not worried about retina display issues for now.

score 9 · Accepted Answer · edited May 23 '17 at 12:17

9

This may not be the most direct route, but you could:

Use UIGraphicsBeginImageContextWithOptions to grab the screen (see the Apple Q&A QA1703 - "Screen Capture in UIKit Applications").
Then use CGImageCreateWithImageInRect to grab the portion of the resultant image you require.
Finally analyse the resultant image. It gets complicated at this point, but thankfully there's an existing question that should show you the way: How to get the RGB values for a pixel on an image on the iphone

Alternatively, there's the following blog article that has accompanying code: What Color is My Pixel? Image based color picker on iPhone

edited May 23 '17 at 12:17

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answered Jan 06 '11 at 16:16

John Parker

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@WilliamJockusch I've implemented this however I want to detect the color (of just 1 pixel on screen) regularly (every 0.04 seconds using a timer) creating a screenshot of the whole screen is causing this to lag my app... you didn't happen to find a more memory efficient way of doing this did you? And apologies for commenting on such an old thread! ;) – simonthumper Mar 23 '13 at 16:05
@simonthumper Sadly not. If you don't need to grab the screen you could probably make things faster. (It really depends on the source of the image data.) That said, I'd be tempted to create a fresh question (although mention that you've read/absorbed this one) if the problem you're attempting to solve is sufficiently different. – John Parker Mar 23 '13 at 17:16

score 5 · Answer 2 · answered Jan 25 '12 at 23:21

Here is how to do it

CGContextRef ctx;
CGImageRef imageRef = [image CGImage];
NSUInteger width = CGImageGetWidth(imageRef);
NSUInteger height = CGImageGetHeight(imageRef);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
unsigned char *rawData = malloc(height * width * 4);
NSUInteger bytesPerPixel = 4;
NSUInteger bytesPerRow = bytesPerPixel * width;
NSUInteger bitsPerComponent = 8;
CGContextRef context = CGBitmapContextCreate(rawData, width, height,
                                             bitsPerComponent, bytesPerRow, colorSpace,
                                             kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);

CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGContextRelease(context);

//GET PIXEL FROM POINT
int index = 4*((width*round(yCoor))+round(xCoor));

int R = rawData[index];
int G = rawData[index+1];
int B = rawData[index+2];

NSLog(@"%d   %d   %d", R, G, B);

//IF YOU WANT TO ALTER THE PIXELS

int byteIndex = 0;

for(int ii = 0 ; ii < width * height ; ++ii)
{ 
    rawData[byteIndex] = (char)(newPixelValue);
    rawData[byteIndex+1] = (char)(newPixelValue);
    rawData[byteIndex+2] = (char)(newPixelValue);

    byteIndex += 4;
}


ctx = CGBitmapContextCreate(rawData,
                            CGImageGetWidth( imageRef ),
                            CGImageGetHeight( imageRef ),
                            8,
                            CGImageGetBytesPerRow( imageRef ),
                            CGImageGetColorSpace( imageRef ),
                            kCGImageAlphaPremultipliedLast ); 

imageRef = CGBitmapContextCreateImage(ctx);
UIImage* rawImage = [UIImage imageWithCGImage:imageRef];  

CGContextRelease(ctx);  

image = rawImage;  

free(rawData);

Why do you multiply the index with 4? Because it's 4 bytes per pixel? — C0D3, May 16 '12 at 15:22

score 0 · Answer 3 · edited May 04 '14 at 12:46

Try This One Where "self.m_imgvwSource" is the uiview/uiimageview as per your need

- (UIColor *) GetCurrentPixelColorAtPoint:(CGPoint)point
{
    unsigned char pixel[4] = {0};

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    CGContextRef context = CGBitmapContextCreate(pixel, 1, 1, 8, 4, colorSpace, kCGBitmapAlphaInfoMask & kCGImageAlphaPremultipliedLast);

    CGContextTranslateCTM(context, -point.x, -point.y);

    [self.m_imgvwSource.layer renderInContext:context];

    CGContextRelease(context);

    CGColorSpaceRelease(colorSpace);

    NSLog(@"pixel: %d %d %d %d", pixel[0], pixel[1], pixel[2], pixel[3]);

    UIColor *color = [UIColor colorWithRed:pixel[0]/255.0 green:pixel[1]/255.0 blue:pixel[2]/255.0 alpha:pixel[3]/255.0];

    return color;
}

iOS -- detect the color of a pixel?

3 Answers3

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