C# 调整索引颜色位图的大小并维护颜色
我有一个 256 色索引格式的位图，我需要调整它的大小，因此我创建了一个 24 位 RGB 格式的新位图，并使用 < code>Graphics 对象，因为我无法从索引颜色位图创建图形对象。然后，我需要将调整大小的图像另存为索引颜色格式，因此我使用 FormatConvertedBitmap 转换为索引颜色，如下所示：
BitmapSource bitmapSource = Imaging.CreateBitmapSourceFromHBitmap(bmp.GetHbitmap(), IntPtr.Zero, Int32Rect.Empty, BitmapSizeOptions.FromEmptyOptions());
FormatConvertedBitmap converted = new FormatConvertedBitmap();
converted.BeginInit();
converted.Source = bitmapSource;
converted.DestinationFormat = System.Windows.Media.PixelFormats.Indexed8;
converted.DestinationPalette = new BitmapPalette(bitmapSource, 256);
converted.EndInit();
这可行，但纯色现在呈颗粒状并包含其他颜色像素。是否有更好的方法来调整索引彩色图像的大小或保持纯色？
I have a Bitmap which is in 256-colour indexed format, I need to resize it so I create a new Bitmap in 24-bit RGB format and draw it using a Graphics object as I cannot create a graphics object from an indexed colour bitmap. I then need to save the resized image back as an indexed colour format so I use FormatConvertedBitmap to convert to indexed colour like this:
BitmapSource bitmapSource = Imaging.CreateBitmapSourceFromHBitmap(bmp.GetHbitmap(), IntPtr.Zero, Int32Rect.Empty, BitmapSizeOptions.FromEmptyOptions());
FormatConvertedBitmap converted = new FormatConvertedBitmap();
converted.BeginInit();
converted.Source = bitmapSource;
converted.DestinationFormat = System.Windows.Media.PixelFormats.Indexed8;
converted.DestinationPalette = new BitmapPalette(bitmapSource, 256);
converted.EndInit();
This works but the solid colours are now grainy and contain other colour pixels. Is there a better method to either resize an indexed colour image or to maintain the solid colours?
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是的，这是不可避免的。您想要控制以影响此效果的核心属性是 Graphics.InterpolationMode。像双三次这样的高质量选择将为调整大小的图像添加很多颜色。尝试将其恢复为 256 色将会产生颗粒状图像。您可以使用 NearestNeighbor 来减少添加颜色的数量，但最终会得到块状图像。
您可以尝试 GIF 编码器，它也会强制将图像重新采样回 256 色。它使用抖动算法。但往往会产生斑点伪影，当图像具有大面积的相似颜色（例如蓝天）时，这些伪影就会变得明显。除了不重新采样之外，没有什么神奇的治疗方法。对于我们如今拥有的出色硬件来说，这样做是没有意义的。
Yes, this is pretty inevitable. The core property you want to control to influence this is Graphics.InterpolationMode. A high quality selection like Bicubic will add a lot of colors to the resized image. Trying to whack it back to 256 colors is going to produce a grainy image. You could use NearestNeighbor instead to reduce the number of added colors but you'll end up with a blocky image.
You could try the GIF encoder, it is forced to resample the image back to 256 colors as well. It uses a dithering algorithm. But tends to produce flecky artifacts that become noticeable when the image has large areas of similar colors, like a blue sky. There is no magical cure beyond simply not resampling back. There is just no point in doing that with the wonderful hardware we have these days.