如何检查CGContext是否包含点？
基本上我有触摸移动时绘制的图像。 当我在一个名为橡皮擦的按钮上切换时，我希望它检测我所绘制的上下文在触摸位置处是否为黑色。 有没有办法做到这一点？
- (id)initWithFrame:(CGRect)frame { self = [super initWithFrame:frame]; if (self) { hue = 0.0; [self initContext:frame.size]; framsize = frame.size; } return self; } -(void)clear{ cacheContext = nil; cacheBitmap = nil; [self initContext:framsize]; [self setNeedsDisplay]; } - (BOOL) initContext:(CGSize)size { float scaleFactor = [[UIScreen mainScreen] scale]; int bitmapByteCount; int bitmapBytesPerRow; // Declare the number of bytes per row. Each pixel in the bitmap in this // example is represented by 4 bytes; 8 bits each of red, green, blue, and // alpha. bitmapBytesPerRow = (size.width * 4); bitmapByteCount = (bitmapBytesPerRow * size.height)*scaleFactor*scaleFactor; // Allocate memory for image data. This is the destination in memory // where any drawing to the bitmap context will be rendered. cacheBitmap = malloc( bitmapByteCount ); if (cacheBitmap == NULL){ return NO; } CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Big; CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); cacheContext = CGBitmapContextCreate (cacheBitmap, size.width*scaleFactor, size.height *scaleFactor, 8, bitmapBytesPerRow*scaleFactor, colorSpace, bitmapInfo); CGContextScaleCTM(cacheContext, scaleFactor, scaleFactor); CGColorSpaceRelease(colorSpace); CGContextSetRGBFillColor(cacheContext, 1, 0, 0, 0.0); CGContextFillRect(cacheContext, (CGRect){CGPointZero, CGSizeMake(size.height*scaleFactor, size.width*scaleFactor)}); return YES; } //-(float) alphaAtX:(int)xy:(int)y //get alpha component using the pointer 'pixelData' //{ // float scaleFactor = [[UIScreen mainScreen] scale]; // return pixelData[(y * 320*scaleFactor + x) *4 + 3]; //+0 for red, +1 for green, +2 for blue, +3 for alpha //} -(UIColor *) colorOfPoint:(CGPoint)point { unsigned char pixel[4] = {0}; CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB(); CGContextRef context = CGBitmapContextCreate(pixel, 1, 1, 8, 4, colorSpace, (CGBitmapInfo)kCGImageAlphaPremultipliedLast); CGContextTranslateCTM(context, -point.x, -point.y); [self.layer renderInContext:context]; CGContextRelease(context); CGColorSpaceRelease(colorSpace); 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; } - (void) drawToCache:(CGPoint)currentpos andLastPos:(CGPoint)pos Color:(UIColor *)colors Thickness:(float)thickness{ hue += 0.005; if(hue > 1.0) hue = 0.0; UIColor *color; if (!colors) { color = [UIColor colorWithHue:hue saturation:0.7 brightness:1.0 alpha:1.0]; } else { color = colors; } CGContextSetStrokeColorWithColor(cacheContext, [color CGColor]); CGContextSetLineCap(cacheContext, kCGLineCapRound); CGContextSetLineWidth(cacheContext, 6+thickness); CGPoint lastPoint = currentpos; CGPoint newPoint = lastPoint; CGContextMoveToPoint(cacheContext, lastPoint.x, lastPoint.y); CGContextAddLineToPoint(cacheContext, newPoint.x, newPoint.y); CGContextStrokePath(cacheContext); CGRect dirtyPoint1 = CGRectMake(lastPoint.x-10, lastPoint.y-10, 20, 20); CGRect dirtyPoint2 = CGRectMake(newPoint.x-10, newPoint.y-10, 20, 20); [self setNeedsDisplayInRect:CGRectUnion(dirtyPoint1, dirtyPoint2)]; } - (void) drawRect:(CGRect)rect { CGContextRef context = UIGraphicsGetCurrentContext(); CGImageRef cacheImage = CGBitmapContextCreateImage(cacheContext); CGContextDrawImage(context, self.bounds, cacheImage); CGImageRelease(cacheImage); } @end AFAIK没有简单的方法来获得CGContext中某些点的颜色。 但你可以使用这里描述的方法http://www.markj.net/iphone-uiimage-pixel-color/
我没有测试过这段代码，但我希望它有效或易于修复：
// ctxSize - is a size of context - (UIColor*) getPixelColorForContext:(CGContextRef)cgctx size:(CGSize)ctxSize atLocation:(CGPoint)point { UIColor* color = nil; // Create off screen bitmap context to draw the image into. Format ARGB is 4 bytes for each pixel: Alpa, Red, Green, Blue if (cgctx == NULL) { return nil; /* error */ } size_t w = ctxSize.width; // Now we can get a pointer to the image data associated with the bitmap // context. unsigned char* data = CGBitmapContextGetData (cgctx); if (data != NULL) { //offset locates the pixel in the data from x,y. //4 for 4 bytes of data per pixel, w is width of one row of data. int offset = 4*((w*round(point.y))+round(point.x)); int alpha = data[offset]; int red = data[offset+1]; int green = data[offset+2]; int blue = data[offset+3]; NSLog(@"offset: %i colors: RGB A %i %i %i %i",offset,red,green,blue,alpha); color = [UIColor colorWithRed:(red/255.0f) green:(green/255.0f) blue:(blue/255.0f) alpha:(alpha/255.0f)]; } // Free image data memory for the context if (data) { free(data); } return color; }