c# - Bitmap Palette throws index out of bounds exception when the index is not out of bounds

Question

I know you must here the IndexOutOfBoundsException loads, and I wouldn't normally post stuff about it, but I have just come across it whilst trying to parse an array as a palette using the following code. It throws the exception when i = 0 and palette.Length = 768, I can't see why and I'm sure this code worked before:

        ColorPalette palette1 = bmp.Palette;

        for (int i = 0; i < palette.Length; i += 3)
        {
            if (i != 0)
            {
                Color b = Color.FromArgb(255, palette[i], palette[i + 1], palette[i + 2]);
                palette1.Entries[i/3] = b;
            }

            else
            {
                Color b = Color.FromArgb(255, palette[i], palette[i + 1], palette[i + 2]);
                palette1.Entries[i] = b;
            }
        }

        bmp.Palette = palette1;

The following code DOES work, but uses a smaller palette in a separate function:

        ColorPalette palette1 = bmp.Palette;

        for (int i = 0; i < 48; i += 3)
        {
            if (i != 0)
            {
                Color b = Color.FromArgb(255, palette[i], palette[i + 1], palette[i + 2]);
                palette1.Entries[i / 3] = b;
            }

            else
            {
                Color b = Color.FromArgb(255, palette[i], palette[i + 1], palette[i + 2]);
                palette1.Entries[i] = b;
            }
        }

        bmp.Palette = palette1;

The `if/else` in your code is unnecessary, by the way, since `0/3 == 0`. — JSBձոգչ, Apr 14 '11 at 15:51
Which line is throwing the exception? What's the relationship between `palette` and `palette1`? — Jon Skeet, Apr 14 '11 at 15:54

JSBձոգչ · Answer 1 · 2011-04-14T15:59:19.107

1

Your loop variable i is bounded by palette.Length, but you're trying to assign to palette1. There's no guarantee that palette1 has the same length as palette, and my guess is that it does not -- hence your problem.

edited Apr 14 '11 at 15:59

answered Apr 14 '11 at 15:54

JSBձոգչ

40,684
18
101
169

I've tried this, but palette.Entries.Length is zero and now can't be added to the bitmap again. Anyway, that doesn't explain why it worked for the last loop. – user646265 Apr 14 '11 at 15:58
@user: try adding a line to explicitly check that `(palette.Length / 3) == palette1.Entries.Length`, and see if it fails. Your second entry would succeed as long as there are at least 48 entries in `palette1.Entries`, which is evidently true. – JSBձոգչ Apr 14 '11 at 16:01
It's not even entering the loop now, instead it's throwing an error (parameter is not valid) when trying to re-assign the palette with 'bmp.Palette = palette1;'. It seems that palette1's length is zero, so it won't loop if 256 doesn't equal 0. – user646265 Apr 14 '11 at 16:08
If the existing bitmap palette has length 0, then you can't modify it in place. You'll have to create a new bitmap with a new palette and copy the data from the old bitmap over, then reassign the palette entries. – JSBձոգչ Apr 14 '11 at 16:13
I have copied 'bmp' to a new bitmap 'bmp2' and then used 'bmp2.Palette', but I still get the same issue. Why would this be different from the other loop, it's the same code, but with a different amount of data? – user646265 Apr 14 '11 at 16:29
I'm guessing, because you're not really giving me the information I need, but I'm guessing that you just used `bmp2 = new Bitmap(bmp)`. That won't work, since it creates a bitmap that's identical to the old one and has the same problem. You need to use the constructor `System.Drawing.Bitmap(Int32, Int32, PixelFormat)` and specify `PixelFormat.Format8bppIndexed` as the last argument, then copy the actual pixel array, *then* modify the color table. – JSBձոգչ Apr 14 '11 at 16:38
Alright, found the issue: I was calling this function from a seperate class and when i called it to create an 8 bit per pixel file but I called px (the pixel format) as a 24bppRGB. 24-bit files don't have palettes, so the palette index was empty, which explains why it worked on the 4bpp example. Oops, still thanks for the help! – user646265 Apr 15 '11 at 16:18

c# - Bitmap Palette throws index out of bounds exception when the index is not out of bounds

1 Answers1