Iphone将彩色图像转换为2bit图像（黑白） [英] Iphone convert color image to 2bit image (black-white)

问题描述

我需要帮助才能将彩色图像转换为黑白，而不是灰度。

I need help to convert a color image to "black and white", not grayscale.

我用iPhone SDK和Core Graphics做了什么，我确信这是可能的。

I what to do this with the iPhone SDK and Core Graphics, as I'm convinced this is possible.

推荐答案

这是可能的，我之前尝试过两条路径：

This is possible and I previously tried two paths:

1. 转换为灰度，然后逐像素转换为B& W。
   
   - 问题，这是我不使用透明图像获得良好效果。

1. Convert to Grayscale then apply pixel by pixel conversion to B&W.
   --Problem with this is I don't get good results with images with transparency.

如果你不是很严格，给定RGBA图像，得到像素RGB平均值并转换为B& W提供阈值并保持透明度。从技术上讲，这仍然是RGBA，但更多是黑色，白色和透明度。

If you're not very strict, given an RGBA image, get the pixel RGB average and convert to B&W with a supplied threshold and retain its transparency. Technically this is still RGBA but more of Black, White and Transparency.

例如

  UIImage *originalImage = [UIImage imageNamed:@"option_bluetooth.png"];  

  unsigned char *pixelBuffer = [self getPixelData:originalImage.CGImage]; 
  size_t length = originalImage.size.width * originalImage.size.height * 4;
  CGFloat intensity;
  int bw;
  //50% threshold
  const CGFloat THRESHOLD = 0.5;
  for (int index = 0; index < length; index += 4)  
  {  
    intensity = (pixelBuffer[index] + pixelBuffer[index + 1] + pixelBuffer[index + 2]) / 3. / 255.;
    if (intensity > THRESHOLD) {
      bw = 255;
    } else {
      bw = 0;
    }
    pixelBuffer[index] = bw;


    pixelBuffer[index + 1] = bw;  
    pixelBuffer[index + 2] = bw;
  }

  CGColorSpaceRef colorSpace=CGColorSpaceCreateDeviceRGB();
  CGContextRef bitmapContext=CGBitmapContextCreate(pixelBuffer, originalImage.size.width, originalImage.size.height, 8, 4*originalImage.size.width, colorSpace,  kCGImageAlphaPremultipliedLast | kCGBitmapByteOrderDefault);
  CFRelease(colorSpace);
  free(pixelBuffer);
  CGImageRef cgImage=CGBitmapContextCreateImage(bitmapContext);
  CGContextRelease(bitmapContext);

  UIImage *bwImage = [UIImage imageWithCGImage:cgImage];

我通过写入屏幕外的上下文获取像素数据（获取原始数据的新方法是Apple建议对我不起作用）

I get the pixel data by writing to an offscreen context (the new way of getting the raw data which Apple suggests does not work for me)

例如

  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

  unsigned char *rawData = malloc(imageHeight * imageWidth * 4);
  CGContextRef offscreenContext = CGBitmapContextCreate(rawData, imageWidth, imageHeight,
                                                        bitsPerComponent, bytesPerRow, colorSpace,
                                                        kCGImageAlphaPremultipliedLast | kCGBitmapByteOrderDefault);

  CGColorSpaceRelease(colorSpace);

  CGContextDrawImage(offscreenContext, CGRectMake(0, 0, imageWidth, imageHeight), cgCropped);
  CGContextRelease(offscreenContext);

以下是获取Pixel Data的代码

Here's the code to get Pixel Data

+ (unsigned char *) getPixelData: (CGImageRef) cgCropped {

  size_t imageWidth = CGImageGetWidth(cgCropped);

  size_t imageHeight = CGImageGetHeight(cgCropped);
  size_t bitsPerComponent = 8;
  size_t bytesPerPixel = 4;
  size_t bytesPerRow = bytesPerPixel * imageWidth;
  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

  unsigned char *rawData = malloc(imageHeight * imageWidth * 4);
  CGContextRef offscreenContext = CGBitmapContextCreate(rawData, imageWidth, imageHeight,
                                                        bitsPerComponent, bytesPerRow, colorSpace,
                                                        kCGImageAlphaPremultipliedLast | kCGBitmapByteOrderDefault);

  CGColorSpaceRelease(colorSpace);

  CGContextDrawImage(offscreenContext, CGRectMake(0, 0, imageWidth, imageHeight), cgCropped);
  CGContextRelease(offscreenContext);

  return rawData;
}

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