Grey Scale Bitmaps

Grey scale is necessary for compatibility with QBasic's legacy SCREEN modes when viewing 24/32 bit bitmaps! You may also desire grey scale images for aesthetic or artistic reasons.

Set each color attribute's RGB intensities to the same value while incrementing the attribute values and intensity levels.

Don't exceed 63 when using OUT in legacy screen modes 12 or 13. Screens 7 and 9 use DAC color settings.

If using a QB64 32 bit _NEWIMAGE Screen, use _RGB or RGB32 values no higher than 255.

Then set the color attributes to reflect the number of colors available in that legacy screen mode.

24/32 bit do not use color attributes, so the intensity levels must be set to an average for all three.

SCREEN 12 (4 BPP - 16 colors)

SUB TrueColor12  'Screen 12 greyscale of 24 bit 'code by Bob Seguin
IF BMP.PWidth * 3) MOD 4) THEN
ZeroPAD$ = SPACE$((4 - ((BMP.PWidth * 3) MOD 4)))
END IF
n = 3
FOR Colr = 0 TO 15  '16 colors
        OUT &H3C8, Colr
        OUT &H3C9, n        'color intensities are each equal in grey scale
        OUT &H3C9, n
        OUT &H3C9, n
        n = n + 4                     'increment intensities. maximum intensity = 63
NEXT Colr
y = BMP.PDepth - 1: o$ = " "
GET #1, BMP.Offset, o$
R$ = " ": G$ = " ": B$ = " "
DO
x = 0                               'set image to left of screen
  DO
    GET #1, , B$
    GET #1, , G$
    GET #1, , R$
    red = ASC(R$)            'intensities from 0 to 255
    grn = ASC(G$)
    blu = ASC(B$)
    average = (red + grn + blu) \ 48    'divide by 16 colors and divide by 3 for average
    PSET (x, y), average                '255 * 3 divided by 16 = maximum color of 15
    x = x + 1
    LOOP WHILE x < BMP.Pwidth
    GET #1, , ZeroPAD$           'move up one row from bottom
    y = y - 1
LOOP UNTIL y = -1
END SUB

SCREEN 13 (4 or 8 BPP - 256 colors)

SUB TrueColor13    'Screen 13 greyscale of 24 bit 'code by Bob Seguin
IF ((BMP.PWidth * 3) MOD 4) <> 0 THEN
ZeroPAD$ = SPACE$((4 - ((BMP.PWidth * 3) MOD 4)))
END IF
FOR Colr = 0 TO 255 STEP 4         '63 different intensities
    FOR Reps = 0 TO 3              'set 3 colors at a time
        OUT &H3C8, Colr + Reps
        OUT &H3C9, n     'color intensities are each equal in grey scale
        OUT &H3C9, n
        OUT &H3C9, n
    NEXT Reps
    n = n + 1                          'increment intensities. maximum = 63
NEXT Colr
y = BMP.PDepth - 1: o$ = " "
GET #1, BMP.Offset, o$          'offset + 1 immediately after header
R$ = " ": G$ = " ": B$ = " "
DO
x = 0                                 'set image to left side of screen
  DO
    GET #1, , B$
    GET #1, , G$
    GET #1, , R$
    red = ASC(R$)
    grn = ASC(G$)
    blu = ASC(B$)
    average = (red + grn + blu) \ 3 'divide by 3 for average as colors match intensities
    PSET (x, y), average            '3 * 255 divided by 3 = maximum color attribute of 255
    x = x + 1
  LOOP WHILE x < BMP.PWidth
  GET #1, , ZeroPAD$                  'skip padder if used
  y = y - 1                             'move up one row from bottom
LOOP UNTIL y = -1
END SUB

SCREEN _NEWIMAGE(BMP.PWidth, BMP.PDepth, 32) (24 BPP - 16 million colors)


SUB TrueGreyScale   'convert a 24 bit bitmap image to grey scale
IF ((BMP.PWidth * 3) MOD 4) <> 0 THEN
ZeroPAD$ = SPACE$((4 - ((BMP.PWidth * 3) MOD 4)))
END IF
y = BMP.PDepth - 1: o$ = " "
GET #1, BMP.Offset, o$   'offset + 1 immediately after header
R$ = " ": G$ = " ": B$ = " "
DO
x = 0                                 'set image to left side of screen
    DO
    GET #1, , B$
    GET #1, , G$
    GET #1, , R$
    red = ASC(R$)
    grn = ASC(G$)
    blu = ASC(B$)
    average = (red + grn + blu) \ 3   'divide by 3 for average as colors match intensities
    PSET (x, y), _RGB32(average, average, average)  'maximum color intensity = 255
    x = x + 1
    LOOP WHILE x < BMP.PWidth
    GET #1, , ZeroPAD$                  'skip padder if used
    y = y - 1                           'move up one row from bottom
LOOP UNTIL y = -1
END SUB

Tinting Grey Scale Images

To tint a SCREEN 12 or 13 greyscale image, keep the palette intensity settings of 1 or 2 colors less than the others.

FOR Colr = 0 TO 255 STEP 4         '63 different intensities
    FOR Reps = 0 TO 3              'set 3 colors at a time
        OUT &H3C8, Colr + Reps
        OUT &H3C9, n - tint    'decrease color intensities of red and green
        OUT &H3C9, n - tint
        OUT &H3C9, n           'higher blue intensity will blue the image
    NEXT Reps
    n = n + 1                          'increment intensities. maximum = 63
    IF n > 5 THEN tint = 5          'keep red and green intensity values above 0!
NEXT Colr

In 24/32 bit SCREENs, subtract some intensity from one or two of the 3 image pixel settings.

DO
  GET #1, , B$
  GET #1, , G$
  GET #1, , R$
  red = ASC(R$)
  grn = ASC(G$)
  blu = ASC(B$)
  average = (red + grn + blu) \ 3   'divide by 3 for average as colors match intensities
  IF average > 5 THEN tint = 5 ELSE tint = 0
  PSET (x, y), _RGB32(average - tint, average - tint, average) 'maximum color intensity = 255
   x = x + 1
LOOP WHILE x < BMP.PWidth

You could keep the red and green intensity settings higher than blue to yellow, brown, or antique the image too.

Grey Scale Bitmaps

See also

Navigation menu

Grey Scale Bitmaps

See also

Navigation menu

Search