在使用 CGBitmapContext 操作像素时，我遇到了一些非常奇怪的事情。基本上我正在改变 PNG 的 Alpha 值。我有一个例子，我可以成功地将 PNG 像素的 Alpha 值归零，除非我的 ImageView 的超级视图是带有图像的 ImageView……这是为什么？我不确定这是错误还是上下文中的某些内容设置不正确。
下面是一个例子。它成功地将 alpha 值归零，但是当你给超级视图的 UIImageView 一个图像时，它不能正常工作。应该具有零 Alpha 的像素是稍微可见的。但是，当您从 superview 的 UIImageView 中删除图像时，Alpha 值被正确归零，您可以完美地看到它背后的白色背景。我尝试过使用几种不同的 CGBlendModes，但似乎并没有这样做。
这是演示此奇怪事件的示例项目的保管箱链接： https ://www.dropbox.com/s/e93hzxl5ru5wnss/TestAlpha.zip
这是代码复制/粘贴：
// Create a Background ImageView
UIImageView Background = new UIImageView(this.View.Bounds);
// Comment out the below line to see the Pixels alpha values change correctly, UnComment the below line to see the pixel's alpha values change incorrectly. Why is this?
// When Commented out, The pixels whose alpha value I set to zero become transparent and I can see the white background through the image.
// When Commented in, I SHOULD see the "GrayPattern.png" Image through the transparent pixels. I can kind of see it, but for some reason I still see the dirt pixels who's alpha is set to zero! Is this a bug? Is the CGBitmapContext set up incorrectly?
Background.Image = UIImage.FromFile("GrayPattern.png");
this.View.AddSubview(Background);
UIImageView MyImageView = new UIImageView(UIScreen.MainScreen.Bounds);
UIImage sourceImage = UIImage.FromFile("Dirt.png");
MyImageView.Image = sourceImage;
Background.AddSubview(MyImageView);
CGImage image = MyImageView.Image.CGImage;
Console.WriteLine(image.AlphaInfo);
int width = image.Width;
int height = image.Height;
CGColorSpace colorSpace = CGColorSpace.CreateDeviceRGB();
int bytesPerRow = image.BytesPerRow;
int bytesPerPixel = bytesPerRow / width;
int bitmapByteCount = bytesPerRow * height;
int bitsPerComponent = image.BitsPerComponent;
CGImageAlphaInfo alphaInfo = CGImageAlphaInfo.PremultipliedLast;
// Allocate memory because the BitmapData is unmanaged
IntPtr BitmapData = Marshal.AllocHGlobal(bitmapByteCount);
CGBitmapContext context = new CGBitmapContext(BitmapData, width, height, bitsPerComponent, bytesPerRow, colorSpace, alphaInfo);
context.SetBlendMode(CGBlendMode.Copy);
context.DrawImage(new RectangleF(0, 0, width, height), image);
UIImageView MyImageView_brush = new UIImageView(new RectangleF(0, 0, 100, 100));
UIImage sourceImage_brush = UIImage.FromFile("BB_Brush_Rect_MoreFeather.png");
MyImageView_brush.Image = sourceImage_brush;
MyImageView_brush.Hidden = true;
Background.AddSubview(MyImageView_brush);
CGImage image_brush = sourceImage_brush.CGImage;
Console.WriteLine(image_brush.AlphaInfo);
int width_brush = image_brush.Width;
int height_brush = image_brush.Height;
CGColorSpace colorSpace_brush = CGColorSpace.CreateDeviceRGB();
int bytesPerRow_brush = image_brush.BytesPerRow;
int bytesPerPixel_brush = bytesPerRow_brush / width_brush;
int bitmapByteCount_brush = bytesPerRow_brush * height_brush;
int bitsPerComponent_brush = image_brush.BitsPerComponent;
CGImageAlphaInfo alphaInfo_brush = CGImageAlphaInfo.PremultipliedLast;
// Allocate memory because the BitmapData is unmanaged
IntPtr BitmapData_brush = Marshal.AllocHGlobal(bitmapByteCount_brush);
CGBitmapContext context_brush = new CGBitmapContext(BitmapData_brush, width_brush, height_brush, bitsPerComponent_brush, bytesPerRow_brush, colorSpace_brush, alphaInfo_brush);
context_brush.SetBlendMode(CGBlendMode.Copy);
context_brush.DrawImage(new RectangleF(0, 0, width_brush, height_brush), image_brush);
for ( int x = 0; x < width_brush; x++ )
{
for ( int y = 0; y < height_brush; y++ )
{
int byteIndex_brush = (bytesPerRow_brush * y) + x * bytesPerPixel_brush;
byte alpha_brush = GetByte(byteIndex_brush+3, BitmapData_brush);
// Console.WriteLine("alpha_brush = " + alpha_brush);
byte setValue = (byte)(255 - alpha_brush);
// Console.WriteLine("setValue = " + setValue);
int byteIndex = (bytesPerRow * y) + x * bytesPerPixel;
SetByte(byteIndex+3, BitmapData, setValue);
}
}
// Set the MyImageView Image equal to the context image, but I still see all top image, I dont' see any bottom image showing through.
MyImageView.Image = UIImage.FromImage (context.ToImage());
context.Dispose();
context_brush.Dispose();
// Free memory used by the BitmapData now that we're finished
Marshal.FreeHGlobal(BitmapData);
Marshal.FreeHGlobal(BitmapData_brush);
public unsafe byte GetByte(int offset, IntPtr buffer)
{
byte* bufferAsBytes = (byte*) buffer;
return bufferAsBytes[offset];
}
public unsafe void SetByte(int offset, IntPtr buffer, byte setValue)
{
byte* bufferAsBytes = (byte*) buffer;
bufferAsBytes[offset] = setValue;
}
更新 我找到了一种解决方法（尽管它的性能对于我正在做的事情来说是非常不可接受的）解决方法只是从我上面的代码中获取结果图像，将其转换为 NSData，然后将 NSData 转换回 UIIMage .. . 神奇地，图像看起来正确！这种解决方法；但是，对于我正在做的事情是不可接受的，因为用我的图像执行此操作实际上需要 2 秒，这对我的应用程序来说太长了。几百毫秒会很有趣, 但是 2 秒对我来说太长了。如果这是一个错误，希望它得到解决！
这是解决方法（性能不佳）代码。只需将此代码添加到上面代码的末尾（或下载我的保管箱项目并将此代码添加到它的末尾并运行）：
NSData test = new NSData();
test = MyImageView.Image.AsPNG();
UIImage workAroundImage = UIImage.LoadFromData(test);
MyImageView.Image = workAroundImage;
这是之前和之后的照片。之前没有解决方法，之后包括解决方法。
请注意，灰色并没有完全出现。它被一些浅棕色覆盖，好像 alpha 值没有完全归零。
请注意解决方法，您可以完美清晰地看到灰色，根本没有浅棕色。我唯一做的就是将图像转换为 NSData，然后再次转换回 UIImage！很奇怪...