如何获得特定点击点的UIImage的RGBA值

Question

我正在开发一个iPad应用程序，该应用程序在viewcontoller中具有多个UIImageViews。 每个图像都有一些透明的部分。 当用户单击图像时，我要测试他单击的图像区域是否不透明，那么我想执行一些操作

搜索后得出的结论是，我必须访问图像的原始数据并检查用户单击的点的alpha值

我使用这里找到的解决方案，它很有帮助。 我修改了代码，以便如果用户单击的点是透明的（alpha <1），则prent 0否则显示1。但是，结果在运行时不准确。 我有时会在点击的点不是透明和viseversa的地方得到0。 我认为byteIndex值有问题，我不确定它是否在用户单击时返回颜色数据

这是我的代码

   CGPoint touchPoint;
    - (void)viewDidLoad
{
    [super viewDidLoad];
    [logo addGestureRecognizer:[[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(handleSingleTap:)]];
    }

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

    UITouch *touch = [[event allTouches] anyObject];
     touchPoint = [touch locationInView:self.view];

}
- (void)handleSingleTap:(UITapGestureRecognizer *)recognizer
{

    int x = touchPoint.x;
    int y = touchPoint.y;

    [self getRGBAsFromImage:img atX:x andY:y];
}

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

    // First get the image into your data buffer
    CGImageRef imageRef = [image CGImage];
    NSUInteger width = CGImageGetWidth(imageRef);
    NSUInteger height = CGImageGetHeight(imageRef);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    unsigned char *rawData = (unsigned char*) calloc(height * width * 4, sizeof(unsigned char));
    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);

    // Now your rawData contains the image data in the RGBA8888 pixel format.
    int byteIndex = (bytesPerRow * yy) + xx * bytesPerPixel;


        CGFloat alpha = (rawData[byteIndex + 3] * 1.0) / 255.0;
        byteIndex += 4;
        if (alpha < 1) {
            NSLog(@"0");
// here I should add the action I want

        }
        else NSLog(@"1");



    free(rawData);

}

谢谢你

Answer 1

该方法不好，因为它每次都会绘制完整图像，并且还依赖于确切的字节布局。

我建议只画一个1x1的上下文并获取其中的alpha

---

- (void)handleSingleTap:(UITapGestureRecognizer *)recognizer
{
int x = touchPoint.x;
int y = touchPoint.y;

CGFloat alpha = [self getAlphaFromImage:img atX:x andY:y];
if(alpha<1)
   …
}

- (CGFloat)getAlphaFromImage:(UIImage*)image atX:(NSInteger)xx andY:(NSInteger)yy {
// Cancel if point is outside image coordinates
if (!CGRectContainsPoint(CGRectMake(0.0f, 0.0f, image.size.width, image.size.height), CGPointMake(xx,yy))) {
    return 0;
}

// Create a 1x1 pixel byte array and bitmap context to draw the pixel into.
// Reference: http://stackoverflow.com/questions/1042830/retrieving-a-pixel-alpha-value-for-a-uiimage
NSInteger pointX = xx;
NSInteger pointY = yy;
CGImageRef cgImage = self.CGImage;
NSUInteger width = self.size.width;
NSUInteger height = self.size.height;
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 onto the bitmap context
CGContextTranslateCTM(context, -pointX, pointY-(CGFloat)height);
CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), cgImage);
CGContextRelease(context);

CGFloat alpha = (CGFloat)pixelData[3] / 255.0f;
return alpha;
}