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为了减小应用程序的大小，我在 png 图像上使用 jpg 和 alpha。我可以合并 jpg 和 alpha 图像以获取 png，但问题是它会留下边缘不清晰的 alpha 出血（哑光）。请帮我解决一下这个。

下面我编写的代码可以帮助我从 jpg 和 alpha 图像中获取 png 图像。请帮助我摆脱 alpha bleed（matte）。谢谢。 

+ (UIImage*)pngImageWithJPEG:(UIImage*)jpegImage compressedAlphaFile:(UIImage*)alphaImage
{
CGRect imageRect = CGRectMake(0, 0, jpegImage.size.width, jpegImage.size.height);

//Pixel Buffer
uint32_t* piPixels = (uint32_t*)malloc(imageRect.size.width * imageRect.size.height * sizeof(uint32_t));

memset(piPixels, 0, imageRect.size.width * imageRect.size.height * sizeof(uint32_t));

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(piPixels, imageRect.size.width, imageRect.size.height, 8, sizeof(uint32_t) * imageRect.size.width, colorSpace, kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedLast);

//Drawing the alphaImage to the pixel buffer to seperate the alpha values
CGContextDrawImage(context, imageRect, alphaImage.CGImage);

//Buffer to store the alpha values from the alphaImage
uint8_t* piAlpha = (uint8_t*)malloc(sizeof(uint8_t) * imageRect.size.width * imageRect.size.height);

//Copying the alpha values from the alphaImage to the alpha buffer
for (uint32_t y = 0; y < imageRect.size.height; y++) 
{
    for (uint32_t x = 0; x < imageRect.size.width; x++)
    {
        uint8_t* piRGBAPixel = (uint8_t*)&piPixels[y * (uint32_t)imageRect.size.width + x];

        //alpha = 0, red = 1, green = 2, blue = 3.
        piAlpha[y * (uint32_t)imageRect.size.width + x] = piRGBAPixel[1];
    }
}

//Drawing the jpegImage in the pixel buffer.
CGContextDrawImage(context, imageRect, jpegImage.CGImage);

//Setting alpha to the jpegImage
for (uint32_t y = 0; y < imageRect.size.height; y++) 
{
    for (uint32_t x = 0; x < imageRect.size.width; x++)
    {
        uint8_t* piRGBAPixel = (uint8_t*)&piPixels[y * (uint32_t)imageRect.size.width + x];

        float fAlpha0To1 = piAlpha[y * (uint32_t)imageRect.size.width + x] / 255.0f;

        //alpha = 0, red = 1, green = 2, blue = 3.
        piRGBAPixel[0] = piAlpha[y * (uint32_t)imageRect.size.width + x];
        piRGBAPixel[1] *= fAlpha0To1;
        piRGBAPixel[2] *= fAlpha0To1;
        piRGBAPixel[3] *= fAlpha0To1;
    }
}

//Creating image from the pixel buffer
CGImageRef cgImage = CGBitmapContextCreateImage(context);

//Releasing resources
CGContextRelease(context);
CGColorSpaceRelease(colorSpace);
free(piPixels);
free(piAlpha);

//Creating the pngImage to return from the cgImage
UIImage* pngImage = [UIImage imageWithCGImage:cgImage];

//Releasing the cgImage.
CGImageRelease(cgImage);

return pngImage;
}
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1 回答 1

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预乘 alpha 颜色空间可以更好地处理颜色溢出。尝试像这样预处理源像素：

r = r*a;
g = g*a;
b = b*a;

要获得原始值，您通常会在读取图像后将其反转（将 RGB 值除以aif a> 0），但由于 iOS 本身以预乘 alpha 颜色工作，您甚至不必这样做。

也可以试试PNG8+Alpha格式，有时可能比单独的 JPG+mask 小。

于 2013-03-07T13:56:49.270 回答