将8bpp索引位图转换为24bpp，然后使用相同的自定义调色板再次返回

Question

I'm making a tool that does some graphical editing to some bitmaps. I have some 8bppIndexed bitmaps with an existing color palette that I have to convert to 24bppRgb to allow me to draw on it and edit colors of pixels based on some options the user selected. Bitmap graphic is the original 8bpp .bmp file:

//Get Bitmap from file
Bitmap graphic = new Bitmap(source);

//Get palette and pixel format
ColorPalette pal = graphic.Palette;
PixelFormat pix = graphic.PixelFormat; //Format8bppIndexed

//Create datagraphic in 24bppRgb to allow for editing
PixelFormat datapix = PixelFormat.Format24bppRgb;
Bitmap datagraphic = graphic.Clone(new Rectangle(0, 0, graphic.Width, graphic.Height), datapix);

After drawing and editing Bitmap datagraphic, I'm trying to convert it back to 8bppIndexed with the same color palette as Bitmap graphic. I can do that like this:

//Convert back to graphic pixel format
datagraphic = datagraphic.Clone(new Rectangle(0, 0, datagraphic.Width, datagraphic.Height), pix); //Format8bppIndexed

//Apply color palette
datagraphic.Palette = pal;

However, after cloning datagraphic with the same pixel format as graphic, it creates an entirely new color palette. The colors in datagraphic are then all incorrect after applying ColorPalette pal. They only match by their index number between the two palettes. Is there another way of doing this that preserves the colors?

And yes, the bitmaps need to be 8bppIndexed with the custom palette. I'm trying to avoid the need to go through Photoshop to change the color index of all the end result data graphics with the correct palette.

I should say that I'm still fairly new to C#. I hope I was clear as to what I'm trying to do. I appreciate any help here. Thanks in advance.

Answer 1

Converting an 8-bit image to 24bpp is trivial: you just paint it on a new image of the same dimensions and the preferred pixel format.

public static Bitmap PaintOn24bpp(Image image)
{
    Bitmap bp = new Bitmap(image.Width, image.Height, PixelFormat.Format24bppRgb);
    using (Graphics gr = Graphics.FromImage(bp))
        gr.DrawImage(image, new Rectangle(0, 0, bp.Width, bp.Height));
    return bp;
}

This works with any source image no matter its pixel format, though for images containing transparency you may want to preprocess them to fill any transparent areas, because I don't know if painting with transparency taken into account will work if the image you're drawing on doesn't support alpha.

The reverse, to convert a high-colour image to indexed, is a lot trickier, but if you have the palette, it is certainly possible. The main problem in this case is that using Graphics doesn't work, since indexed graphics don't have coloured pixels; they have a coloured palette, and pixels that just reference those colours.

Editing 8-bit data generally boils down to building a byte array containing the image data, and converting that to an image with the palette slapped onto it. The actual conversion is done by making a new 8bpp Bitmap, opening its backing byte array with the LockBits function, and copying your new data into it with Marshal.Copy .

After that, the final missing link is a method to match your image's coloured pixels to their closest palette match, so you can then take the array of resulting matches and bake it into an 8bpp image. This is generally done by calculating the Pythagorean distance (in 3D RGB colour space) of your colour to each of the palette colours, and then taking the index of the colour for which that distance is the smallest.

The whole process, from start to end, is explained in detail in this answer:

A: How to convert a colored image to a image that has only two predefined colors?

This deals with 2 colours, but the method for dealing with 2 or with 256 colours is completely identical, since the end result in both cases was an 8bpp image.

It could be optimised for dealing with 24bpp images directly instead of converting the input to 32bpp, but that's probably not worth the effort, especially since the method posted there works with absolutely any input.