c# - 使用 NQUANT 进行图像量化时出现的错误

Question

我最近获得了 NuGet 包 Nquant .

我打算用它来减少位图的文件大小并将其保存为PNG。但我收到此错误:

The image you are attempting to quantize does not contain a 32 bit ARGB palette. This image has a bit depth of 8 with 256 colors.

这里有人用过Nquant吗？您遇到过这个错误吗？您是如何解决的？

我的代码供您引用:

var bitmap = new Bitmap(width, jbgsize / height, PixelFormat.Format8bppIndexed);
        ColorPalette pal = bitmap.Palette;
        for (int i = 0; i <= 255; i++)
        {
            // create greyscale color table
            pal.Entries[i] = Color.FromArgb(i, i, i);
        }
        bitmap.Palette = pal; // you need to re-set this property to force the new ColorPalette

        var bitmap_data = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), ImageLockMode.WriteOnly, PixelFormat.Format8bppIndexed);
        Marshal.Copy(output, 0, bitmap_data.Scan0, output.Length);
        bitmap.UnlockBits(bitmap_data);
        MemoryStream stream = new MemoryStream();
var quantizer = new WuQuantizer();
        using(var bmp = new Bitmap(bitmap))
        {
            using (var quantized = quantizer.QuantizeImage(bitmap))
            {
                quantized.Save(stream, ImageFormat.Png);
            }
        }

        byteArray = stream.ToArray();
        return byteArray.Concat(output).ToArray();

Answer 1

您可以将图像转换为 Format32bppPArgb，然后对其进行量化。

这是我将图像尺寸减小约 3 倍的工作示例。

public static byte[] CompressImageStream(byte[] imageStream)
{
    using (var ms = new MemoryStream(imageStream))
    using (var original = new Bitmap(ms))
    using (var clonedWith32PixelsFormat = new Bitmap(
        original.Width,
        original.Height,
        PixelFormat.Format32bppPArgb))
    {
        using (Graphics gr = Graphics.FromImage(clonedWith32PixelsFormat))
        {
            gr.DrawImage(
                original,
                new Rectangle(0, 0, clonedWith32PixelsFormat.Width, clonedWith32PixelsFormat.Height));
        }

        using (Image compressedImage = new WuQuantizer().QuantizeImage(clonedWith32PixelsFormat))
        {
            return ImageToByteArray(compressedImage);
        }
    }
}

public static byte[] ImageToByteArray(Image image)
{
    if (image == null)
    {
        throw new ArgumentNullException(nameof(image));
    }

    using (var stream = new MemoryStream())
    {
        image.Save(stream, ImageFormat.Png);
        return stream.ToArray();
    }
}