ios - 从 UIImage 中移除 alpha channel

Question

我使用以下方法从文件系统中获取解压后的 uiimage。然而，当我打开颜色混合层时，UIImageView 被着色为红色，即使 UIImageView 设置为不透明。

文件系统上的图像没有 alpha channel 。我尝试设置 CGContextSetAlpha(bitmapContext, 1)，但仍然有混合层。

有人知道如何在使用 CGContextDrawImage 时删除 alpha channel 吗？

- (UIImage *)decompressedImage
{
    CGImageRef imageRef = self.CGImage;
    CGRect rect = CGRectMake(0.f, 0.f, CGImageGetWidth(imageRef), CGImageGetHeight(imageRef));
    CGContextRef bitmapContext = CGBitmapContextCreate(NULL,
                                                       rect.size.width,
                                                       rect.size.height,
                                                       CGImageGetBitsPerComponent(imageRef),
                                                       CGImageGetBytesPerRow(imageRef),
                                                       CGImageGetColorSpace(imageRef),
                                                       kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little
                                                       );
    // kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrder32Little are the bit flags required
    //  so that the main thread doesn't have any conversions to do.

    CGContextDrawImage(bitmapContext, rect, imageRef);

    CGImageRef decompressedImageRef = CGBitmapContextCreateImage(bitmapContext);
    UIImage* decompressedImage = [UIImage imageWithCGImage:decompressedImageRef
                                                     scale:[[UIScreen mainScreen] scale]
                                               orientation:UIImageOrientationUp];
    CGImageRelease(decompressedImageRef);
    CGContextRelease(bitmapContext);

    return decompressedImage;
}

Answer 1

在上下文的选项中，尝试选项

kCGImageAlphaNoneSkipLast

或

kCGImageAlphaNoneSkipFirst

取决于您使用的是 RGBA 还是 ARGB

即，

CGContextRef bitmapContext = CGBitmapContextCreate(NULL,
                                                       rect.size.width,
                                                       rect.size.height,
                                                       CGImageGetBitsPerComponent(imageRef),
                                                       CGImageGetBytesPerRow(imageRef),
                                                       CGImageGetColorSpace(imageRef),
                                                       kCGImageAlphaNoneSkipLast | kCGBitmapByteOrder32Little
                                                       );