ARGB model - alpha always 255 when saved

Asked May 06 '13 at 16:47

Active May 06 '13 at 17:12

Viewed 42 times

I have a list of colors and I am making a bitmap from it. Each color has a specific value for A, R, G and B. The problem is that when I save the bitmap and when I load it from the bitmap image file, all values for (A) Alpha are changed automatically to 255.

Here is the code for making the bitmap:

private Bitmap PaintImage(List<Color> colors, int width, int height)
{
    Bitmap bitmap = new Bitmap(width, height);
    int colorIndex = 0;

    for (int y = 0; y < height; y++)
    {
        for (int x = 0; x < width; x++)
        {
            bitmap.SetPixel(x, y, colors[colorIndex])
            colorIndex += 1;
        }
    }

    return bitmap;
}

Here is the code for saving the bitmap:

bitmap.Save(path, ImageFormat.Bmp);

Solution: I fixed the problem by using the PNG format.

edited May 06 '13 at 17:12

asked May 06 '13 at 16:47

Nikolay Dakov

What is the value of the [`PixelFormat` property](http://msdn.microsoft.com/en-us/library/system.drawing.image.pixelformat.aspx) of the bitmap? – O. R. Mapper May 06 '13 at 16:50
4

The Windows API's used for reading and writing BMP files don't support an alpha value. See http://stackoverflow.com/questions/16077947/alpha-value-not-persisting-while-saving-bitmap – CodeCaster May 06 '13 at 16:57
Thanks for this information but is there a solution? Or I have to leave it like that... – Nikolay Dakov May 06 '13 at 16:59
1

Use another format, for example PNG, or use a different API that does support the alpha byte (as the BMP format does support it). – CodeCaster May 06 '13 at 17:00
The problem is that the PNG format is using compression and the values will be changed (not sure, will check). I making some kind of cryptography. – Nikolay Dakov May 06 '13 at 17:02
1

PNG compression is lossless. – CodeCaster May 06 '13 at 17:04
1

Fixed the problem, thanks. – Nikolay Dakov May 06 '13 at 17:13

ARGB model - alpha always 255 when saved

0 Answers0