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reading_20the_20palette_20contents

Reading the palette contents

by Richard Russell, October 2011

In BBC BASIC you can change the contents of the colour palette using either VDU 19 or a variant of the COLOUR statement. However, no standard method of reading back the contents of the palette is provided, other than by plotting the colour and sampling it using TINT.

Reading the contents of the palette can be avoided by keeping a local copy, and updating it every time you change the palette. For example the copy could be in the form of a structure array declared as follows:

        DIM palette{(15)r&,g&,b&}

Then each time you modify the palette you update the local copy as well:

        COLOUR i, r, g, b
        palette{(i)}.r& = r
        palette{(i)}.g& = g
        palette{(i)}.b& = b

If the palette is modified from many places in your program you would probably want to put this code in a procedure.

However you may consider the overhead of keeping a local copy unattractive. In that case you can alternatively read the palette contents using the Windows API. To read a single palette entry use code similar to the following:

        SYS "GetPaletteEntries", @hpal%, index%, 1, ^rgb%

Here index% is the palette index (0-15) and the current value is returned in the variable rgb%.

Alternatively you could read the entire 16 entries at once, for example into an array:

        DIM rgb%(15)
        SYS "GetPaletteEntries", @hpal%, 0, 16, ^rgb%(0)

In both cases the returned value contains the red component in bits 0 to 7, the green in bits 8 to 15 and the blue in bits 16 to 23. You could separate them out as follows:

        rgb% = rgb%(index%) : REM Needed only in the second case
        r = rgb% AND &FF
        g = (rgb% >> 8) AND &FF
        b = (rgb% >> 16) AND &FF
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reading_20the_20palette_20contents.txt · Last modified: 2018/04/13 15:56 by richardrussell