256color bitmap to 16 color bitmap

how do I convert a 256 color bitmap to 16 color bitmap using Delphi
bjpAsked:
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gysbert1Commented:
This is not as simple as you might think. What has to be done is that the 16 most representitive colors out of the 256 be chosen, and the rest be dithered to look good.

Have a look at the API calls assosiated with DrawDib. These calls provide some color conversion for you if necesarry. I cannot have a look at this now since I am on a Linux machine till monday but I am sure it will do the trick.
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mheacockCommented:
If you want to convert to the 16 system colours (or if you
already know the 16 colours you want to convert to) then the
answer is trivial.

You can view the Windows colour compoents as three-dimensional
space.  Perhaps Red=X, Green=Y, Blue=Z.  Each of your 256
colours has an RGB value.  Each of your 16 colours has an
RGB value.

Viewing colour as 3D space, you simply imagine lines drawn
from each single 256 colour to every 16 colour.

Find the distance from a 256 colour RGB pixel to every 16 colour
RGB.  The shortest distance is the 16 colour RGB to convert to.

The distance formula can be found in any Algebra text book.
Distance = sqrt((R256 - R16)^2 + (G256 - G16)^2 + (B256 - B16)^2)

There are built-in function in Delphi for retrieving the specific
R and G and B values from a pixel...I think they are GetRedValue, GetGreenValue, etc...or something like that.  They are in the
help files.
 
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bjpAuthor Commented:
Thanks for your answers. I,m not sure though whether I would have to use the Winapi functions GetDIB etc
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mheacockCommented:
Why not just the load the bitmap you want to convert into
a TBitmap...you can alter the pixels via its canvas...
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bjpAuthor Commented:
Thanks again mheacock, I will try to convert the bitmap to a TBitmap
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