Easiest way to combine several PNG8 images in .NET

问题

I'm trying to combine a bunch of 8-bit PNG images into a larger PNG image in C#. Oddly enough, this seems to be particularly difficult.

Since Graphics does not support indexed color, you can't use that, so I tried building a non-indexed Bitmap (using Graphics) and converting that to indexed color bitmap. The conversion is fine, but I can't figure out how to set the palette of the output image. It defaults to some pre-defined palette which has little to do with what I'm looking for.

So:

Is there a way to control the bitmap palette? Or is there another method (System.Windows.Media.Imaging.WriteableBitmap, for example) which can support this?

Re: WriteableBitmap: I can't seem to find any examples online of how one would go about combining PNGs in this context, or even if it makes any sense to attempt it.

回答1:

Disclaimer, I work for Atalasoft.

Our product, DotImage Photo, is free and can do this.

To read a PNG

 AtalaImage img = new AtalaImage("image.png");

To convert to 24 bpp

 img = img.GetChangedPixelFormat(newPixelFormat);

Create an image of the size you want

 AtalaImage img2 = new AtalaImage(width, height, color);

use OverlayCommand to overlay img onto img2

 OverlayCommand cmd = new OverlayCommand(img);
 cmd.Apply(img2, point);

To save

 img2.Save("new.png", new PngEncoder(), null);

If you need help either comment on this answer or make a forum entry.

回答2:

It turns out that I was able to build out a non-indexed bitmap and convert using PngBitmapEncoder like so:

    byte[] ConvertTo8bpp(Bitmap sourceBitmap)
    {
        // generate a custom palette for the bitmap (I already had a list of colors
        // from a previous operation
        Dictionary<System.Drawing.Color, byte> colorDict = new Dictionary<System.Drawing.Color, byte>(); // lookup table for conversion to indexed color
        List<System.Windows.Media.Color> colorList = new List<System.Windows.Media.Color>(); // list for palette creation
        byte index = 0;
        unchecked
        {
            foreach (var cc in ColorsFromPreviousOperation)
            {
                colorDict[cc] = index++;
                colorList.Add(cc.ToMediaColor());
            }
        }
        System.Windows.Media.Imaging.BitmapPalette bmpPal = new System.Windows.Media.Imaging.BitmapPalette(colorList);

        // create the byte array of raw image data
        int width = sourceBitmap.Width;
        int height = sourceBitmap.Height;
        int stride = sourceBitmap.Width;
        byte[] imageData = new byte[width * height];

        for (int x = 0; x < width; ++x)
            for (int y = 0; y < height; ++y)
            {
                var pixelColor = sourceBitmap.GetPixel(x, y);
                imageData[x + (stride * y)] = colorDict[pixelColor];
            }

        // generate the image source
        var bsource = BitmapSource.Create(width, height, 96, 96, PixelFormats.Indexed8, bmpPal, imageData, stride);

        // encode the image
        PngBitmapEncoder encoder = new PngBitmapEncoder();
        encoder.Interlace = PngInterlaceOption.Off;
        encoder.Frames.Add(BitmapFrame.Create(bsource));

        MemoryStream outputStream = new MemoryStream();
        encoder.Save(outputStream);

        return outputStream.ToArray();
    }

Plus the helper extension method:

    public static System.Windows.Media.Color ToMediaColor(this System.Drawing.Color color)
    {
        return new System.Windows.Media.Color()
        {
            A = color.A,
            R = color.R,
            G = color.G,
            B = color.B
        };
    }

Note for the wary: the PngBitmapEncoder actually seems to reduce the bpp count from 8 to 4 when it can. When I test with 6 colors, for example, the output PNG is only 4-bit. When I use a more color-rich image, it's 8-bit. Looks like a feature so far... though it would be nice if I had explicit control over it.

来源：https://stackoverflow.com/questions/4356973/easiest-way-to-combine-several-png8-images-in-net

标签

.net

graphics

gdi+