Using 8 bpp Screens

When using 8 bpp screen, also known as palletized screens, there are some special functions for setting and using colors. Unlike 16,24 and 32 bpp true color screen the 8 bpp screen uses a palette to store colors and a palette index to specify the color in drawing functions.

The palette

The palette consists of 256 entries (0 - 255) that can store an RGB color. The palette may be set manually or loaded from a bitmap image.

To load a palette from a bitmap image use the LOADPALETTE command. LOADPALETTE only loads the palette data from the image, not the image itself.
 

LOADPALETTE GETSTARTPATH+"pal.bmp"

In order for sprites to display in the correct colors on an 8 bpp screen the screens palette must be identical to the sprites. Only 256 color bitmaps can be used properly as sprite images.

Setting a palette color can be done using the SETPALETTECOLOR command.
 

SETPALETTECOLOR 2, RGB(255,255,0)

Retrieving the color stored in a palette is done with GETPALETTECOLOR
 

col = GETPALETTECOLOR(2)

The entire 255 entry palette can be filled with a single color using FILLPALETTE
 

FILLPALETTE RGB(255,255,255)

Fading the screen

One of the neat tricks to do with a palletized screen is create a smooth fade to color effect by changing palette values on the fly over time. Use the FADEPALETTE command to achieve this.
 

CREATESCREEN 640,480,8
SETPALETTECOLOR 2,RGB(255,0,0)
DO
    FILLSCREEN 2
    WRITETEXT 0,0,"Press ESC to fade and exit"
    FLIP
UNTIL KEYDOWN(1)
'4 second FADE to black. 0 = RGB(0,0,0).
FADEPALETTE 0,50
CLOSESCREEN
END

The time value is specified in 2/25th of a second (0.08).  So to specify 8 seconds the value would be 8/.08 = 100. Some common values are:

13 = about 1 second

25 = 2 seconds

50 = 4 seconds

100 = 8 seconds

Use with drawing commands

The 2D drawing commands such as DRAWLINE and DRAWRECT accept a palette index directly when using 8 bpp screens.
 

SETPALETTECOLOR 1, RGB(255,0,0)
DRAWLINE 0, 0, 100, 100, 1

The Emergence BASIC drawing commands require the conversion function PALETTEINDEX to properly work with 8 bpp screens
 

CIRCLE BackBuffer,110,125,50,PALETTEINDEX(3),PALETTEINDEX(4)

Notes

Alpha blending is not possible with 8 bpp screens in as much drawing functions that use an alpha value will fail. These include DrawAlphaLine, WriteAlphaPixel and the drawing modes @ALPHA, @TRANSALPHA, and @TRANSSHADOW for sprites.

When using direct buffer writing remember that an 8bpp screen uses a single byte for each pixel and each byte is a palette index number, not an RGB color.