Question

I get an image screenshot as a byte array. How do I convert this image to a lower bit image so as to reduce size? There are only 10 colors in the whole image so I think there must be some workaround to do this. Any hints?

Answer 1

You can save it as a palettized image file. Assuming your image is held in a variable bmp of type Bitmap. mStream is a MemoryStream in this case so you can store it in memory:

bmp.Save(mStream, Imaging.ImageFormat.Gif)
bmp.Dispose

The gif format support various palette sizes up-to 256 colors as it uses bitplanes rather than RGB-buffers. In your case it would probably be saved as a 16 color file. The GIF uses LZW compression so the resulting files tend to be small in size.

There is also PNG-8 format which do approximately the same as with the GIF.

To recall initialize a Bitmap with:

mStream.Seek(0, SeekOrigin.Begin)
Dim bmp As Bitmap = Bitmap.FromStream(mStream)
...
bmp.Dispose

Unfortunately you cannot create a Graphics object with indexed bitmaps – otherwise the logic approach would be to copy the data from screen into a bitmap of pixel format PixelFormat.Format4bppIndexed.

However, creating a Graphics on a bitmap in indexed format will raise an exception – basically you will need to convert and copy the data manually. Therefor my suggestion in this answer will probably be more convenient if the image isn’t needed frequently.