如何获得ios中的图像像素

Question

我有一个彩色图像。我想要在图像触摸时返回像素值。 所以，我想下面的代码：

- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{ 

float x, y; 

for (UITouch *touch in touches) {   
    x = [touch locationInView:imageView].x; 
    y = [touch locationInView:imageView].y; 
    if ((x >= 0) && (x <= imageView.frame.size.width) && (y >= 0) && (y <= imageView.frame.size.height)) { 
     [self getRGBAFromImage:imageView.image atX:x andY:y]; 
    } 
    } 
} 

- (void)getRGBAFromImage:(UIImage *)image atX:(int)xx andY:(int)yy { 

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); 


int byteIndex = (bytesPerRow * yy) + xx * bytesPerPixel; 


CGFloat red = (rawData[byteIndex]  * 1.0) ; 
CGFloat green = (rawData[byteIndex + 1] * 1.0) ; 
CGFloat blue = (rawData[byteIndex + 2] * 1.0) ; 
CGFloat alpha = (rawData[byteIndex + 3] * 1.0)/255.0; 

self.textView.textColor=[UIColor colorWithRed:red/255.0 green:green/255.0 blue:blue/255.0 alpha:alpha]; 
free(rawData); 
} 

这将返回像素但不正确的！例如：ImageView中的触摸颜色是红色的，但我的textView文本不是红色。

请帮我确认一下我的代码。

谢谢！

Answer 1

这里是UIImage的类别，我使用的，当我需要得到一个特定的像素颜色的图像：

@implementation UIImage (ColorImage) 

/* 
Returns the color of the image pixel at point. Returns nil if point lies outside of the image bounds. 
If the point coordinates contains a decimal part, it will be truncated (trunc). 

This method retreives the pixel data of the image by drawing the specific pixel onto a bitmap. 
To query pixel colors of the same image many times, it is best to cache the drawn image for better performance. 
*/ 
- (UIColor *)colorAtPixel:(CGPoint)point { 
    // Cancel if the point is outside of the image coordinates 
    if (!CGRectContainsPoint(CGRectMake(0.0f, 0.0f, self.size.width, self.size.height), point)) { 
     return nil; 
    } 

    // Create a 1x1 pixel byte array and bitmap context to draw the pixel onto. 
    // Reference: http://stackoverflow.com/questions/1042830/retrieving-a-pixel-alpha-value-for-a-uiimage 
    NSInteger pointX = trunc(point.x); 
    NSInteger pointY = trunc(point.y); 

    CGImageRef cgImage = self.CGImage; 

    NSUInteger width = CGImageGetWidth(cgImage); 
    NSUInteger height = CGImageGetHeight(cgImage); 

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); 

    int bytesPerPixel = 4; 
    int bytesPerRow = bytesPerPixel * 1; 
    NSUInteger bitsPerComponent = 8; 

    unsigned char pixelData[4] = { 0, 0, 0, 0 }; 

    CGContextRef context = CGBitmapContextCreate(pixelData, 1, 1, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big); 

    CGColorSpaceRelease(colorSpace); 

    CGContextSetBlendMode(context, kCGBlendModeCopy); 

    // Draw the pixel we are interested in 
    CGContextTranslateCTM(context, -pointX, -pointY); 
    CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), cgImage); 
    CGContextRelease(context); 

    // Convert the color values [0...255] to floats [0.0f...1.0f] 
    CGFloat r = (CGFloat)pixelData[0]/255.0f; 
    CGFloat g = (CGFloat)pixelData[1]/255.0f; 
    CGFloat b = (CGFloat)pixelData[2]/255.0f; 
    CGFloat a = (CGFloat)pixelData[3]/255.0f; 

    return [UIColor colorWithRed:r green:g blue:b alpha:a]; 
} 
@end 

您可以在UIImage直接调用-colorAtPixel，它将返回的颜色一个UIColor。