How to figure out “Color of a specific pixel” in a

Question

I have an image which is inside an Image View. I want to find the color of a specific pixel (say x=10, y=20) inside the image. Working code would really help me out.

Answer 1

That depends very much on the type of image :)

If you have the byte data, and you know how it is arranged, for example PNG uses RGBA then you've got a leg up.

Liberal use of all the CGImageFunctions like CGImageGetBitsPerComponent and CGImageGetColorSpace will be your guide.

To get the actual byte data, CGImageDestinationCreateWithData Creates an image destination that writes to a Core Foundation mutable data object (NSMutableData* / CFMutableDataRef)

If all of this is gibberish, start with the Quart2D Programming Guide.

Answer 2

Here is very simple sceleton of class to work with image as bitmap. When you create this object with

ImageBitmap * imageBitmap = [[ImageBitmap alloc] initWithImage:myImageView.image  bitmapInfo:kCGImageAlphaNoneSkipLast];

You can access any pixel at (x, y) as

Byte * pixel = [imageBitmap pixelAtX:x Y:y];

RGB components at 0, 1, 2 byte so

Byte red = pixel[0]; etc.

you can both read or write pixels, for example set remove green component from pixel:

pixel[1] = 0;

if use kCGImageAlphaPremultipliedLast pixel[3] is alpha

//ImageBitmap.h
@interface ImageBitmap : NSObject {
  int height, width;
  void * contextData;
  CGContextRef context;
  CGBitmapInfo bitmapInfo;
}

@property(assign)       int height;
@property(assign)       int width;
@property(readonly)     CGContextRef    context;
@property(readonly)     void * contextData;

-(id)initWithSize:(CGSize)size bitmapInfo:(CGBitmapInfo)bmInfo;
-(id)initWithImage:(UIImage *)image  bitmapInfo:(CGBitmapInfo)bmInfo;
-(CGContextRef)createBitmapContextWithData:(void *)data;
- (UIImage *)imageFromContext;
-(Byte *)pixelAtX:(NSInteger)pixelX Y:(NSInteger)pixelY;

//ImageBitmap.m
#import "ImageBitmap.h"


@implementation ImageBitmap

@synthesize width, height;
@synthesize context, contextData;
-(id)initWithSize:(CGSize)size bitmapInfo:(CGBitmapInfo)bmInfo{
     if (self = [super init]) {
        height = size.height;
        width = size.width;
        contextData = malloc(width * height * 4);
        bitmapInfo = bmInfo;
        context = [self createBitmapContextWithData:contextData];
    }
    return self;
}

-(id)initWithImage:(UIImage *)image  bitmapInfo:(CGBitmapInfo)bmInfo{
    [self initWithSize:image.size bitmapInfo:bmInfo];
    CGContextDrawImage(context, CGRectMake(0, 0, width, height),image.CGImage);
    return self;
} 

-(CGContextRef)  createBitmapContextWithData:(void *)data{
    CGContextRef    ctx = NULL;
    int bitmapBytesPerRow   = (width * 4); 
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    if (data == NULL){
        return NULL;
    }
    ctx = CGBitmapContextCreate (data,
                                    width,
                                    height,
                                    8,      // bits per component
                                    bitmapBytesPerRow,
                                    colorSpace,
                                    bitmapInfo); //kCGImageAlphaNoneSkipLast or kCGImageAlphaPremultipliedLast

    CGColorSpaceRelease( colorSpace );
    return ctx;
}

- (UIImage *)imageFromContext{
    CGImageRef cgImage = CGBitmapContextCreateImage(context);
    return [UIImage imageWithCGImage:cgImage];
}

-(Byte *)pixelAtX:(NSInteger)pixelX Y:(NSInteger)pixelY{
    return (Byte *)contextData + (width * pixelY + pixelX)*4;
}

@end