转自：http://blog.sina.com.cn/s/blog_64cfe8f00101pm3x.html
//遍历图片像素，更改图片颜色
void ProviderReleaseData (void *info, const void data, size_t size)
{
free((void)data);
}

• (UIImage) imageBlackToTransparent:(UIImage) image
  {
  // 分配内存
  const int imageWidth = image.size.width;
  const int imageHeight = image.size.height;
  size_t bytesPerRow = imageWidth * 4;
  uint32_t* rgbImageBuf = (uint32_t*)malloc(bytesPerRow * imageHeight);

  // 创建context
  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
  CGContextRef context = CGBitmapContextCreate(rgbImageBuf, imageWidth, imageHeight, 8, bytesPerRow, colorSpace,
  kCGBitmapByteOrder32Little | kCGImageAlphaNoneSkipLast);
  CGContextDrawImage(context, CGRectMake(0, 0, imageWidth, imageHeight), image.CGImage);

  // 遍历像素
  int pixelNum = imageWidth * imageHeight;
  uint32_t* pCurPtr = rgbImageBuf;
  for (int i = 0; i < pixelNum; i++, pCurPtr++)
  {
  if ((pCurPtr & 0xFFFFFF00) == 0xffffff00) // 将白色变成透明
  {
  uint8_t ptr = (uint8_t)pCurPtr;
  ptr[0] = 0;
  }
  else
  {
  // 改成下面的代码，会将图片转成想要的颜色
  uint8_t ptr = (uint8_t*)pCurPtr;
  ptr[3] = 0; //0~255
  ptr[2] = 0;
  ptr[1] = 0;

    }
  

  }

  // 将内存转成image
  CGDataProviderRef dataProvider = CGDataProviderCreateWithData(NULL, rgbImageBuf, bytesPerRow * imageHeight, ProviderReleaseData);
  CGImageRef imageRef = CGImageCreate(imageWidth, imageHeight, 8, 32, bytesPerRow, colorSpace,
  kCGImageAlphaLast | kCGBitmapByteOrder32Little, dataProvider,
  NULL, true, kCGRenderingIntentDefault);
  CGDataProviderRelease(dataProvider);

  UIImage* resultUIImage = [UIImage imageWithCGImage:imageRef];

  // 释放
  CGImageRelease(imageRef);
  CGContextRelease(context);
  CGColorSpaceRelease(colorSpace);
  // free(rgbImageBuf) 创建dataProvider时已提供释放函数，这里不用free

  return resultUIImage;
  }

//设置图片透明度

• (UIImage )imageByApplyingAlpha:(CGFloat)alpha image:(UIImage)image
  {
  UIGraphicsBeginImageContextWithOptions(image.size, NO, 0.0f);

  CGContextRef ctx = UIGraphicsGetCurrentContext();
  CGRect area = CGRectMake(0, 0, image.size.width, image.size.height);

  CGContextScaleCTM(ctx, 1, -1);
  CGContextTranslateCTM(ctx, 0, -area.size.height);

  CGContextSetBlendMode(ctx, kCGBlendModeMultiply);

  CGContextSetAlpha(ctx, alpha);

  CGContextDrawImage(ctx, area, image.CGImage);

  UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();

  UIGraphicsEndImageContext();

  return newImage;
  }

//合并2张图片

• (UIImage *)addImage:(UIImage *)image1 toImage:(UIImage *)image2 {
  UIGraphicsBeginImageContext(image2.size);

  // Draw image1
  [image1 drawInRect:CGRectMake(0, 0, image1.size.width, image1.size.height)];

  // Draw image2
  [image2 drawInRect:CGRectMake(0, 0, image2.size.width, image2.size.height)];

  UIImage *resultingImage = UIGraphicsGetImageFromCurrentImageContext();
  UIGraphicsEndImageContext();
  return resultingImage;
  }

//将UIImage缩放到指定大小尺寸：

• (UIImage *)scaleToSize:(UIImage )img size:(CGSize)size{
  // 创建一个bitmap的context
  // 并把它设置成为当前正在使用的context
  UIGraphicsBeginImageContext(size);
  // 绘制改变大小的图片
  [img drawInRect:CGRectMake(0, 0, size.width, size.height)];
  // 从当前context中创建一个改变大小后的图片
  UIImage scaledImage = UIGraphicsGetImageFromCurrentImageContext();
  // 使当前的context出堆栈
  UIGraphicsEndImageContext();
  // 返回新的改变大小后的图片
  return scaledImage;
  }

//根据图片的大小等比例压缩返回图片
-(UIImage *)fitSmallImage:(UIImage *)image size:(CGSize)size
{
if (nil == image)
{
return nil;
}
if (image.size.width<<span class="s5">200 && image.size.height<<span class="s5">200)
{
return image;
}
UIGraphicsBeginImageContext(size);
CGRect rect = CGRectMake(0, 0, size.width, size.height);
[image drawInRect:rect];
UIImage *newing = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
return newing;
}