Color Extraction Developer Insights

[TerryRitchie]

Thanks for explaining this. I understand little vs big endian from my assembler dabbling days back in the Z80, 6809e, 6502, and x86 days with each processor handling bit presentation differently. I assumed RGB was presented in bit form the same way as the QB64PE commands are set up. I don't know why this didn't cross my mind though when I was getting some strange results in final images. Age I guess.

Your classroom exercise would have been great to use. I left teaching back in 2018 unfortunately.

I incorporated your suggestion into the following subroutine that converts an image to gray scale. Works well.

Code: (Select All)

' _______________________________________________________________________________________________________
'/                                                                                                       \
SUB __GrayScale (i AS LONG) '                                                                __GrayScale |
    ' ___________________________________________________________________________________________________|___
    '/                                                                                                       \
    '| Converts an image to gray scale without affecting original alpha values.                              |
    '| Uses ITU-R 601-2 Luma Transformation (L = R * 299/1000 + G * 587/1000 + B * 114/1000)                 |
    '|                                                                                                       |
    '| i - the image to work with                                                                            |
    '\_______________________________________________________________________________________________________/

    DIM m AS _MEM '            memory block holding image data
    DIM o AS _OFFSET '         4 byte pixel location within memory block
    DIM t AS _OFFSET '         total number of pixels in image
    DIM a AS _UNSIGNED _BYTE ' alpha level of each pixel
    DIM r AS _UNSIGNED _BYTE ' red value of each pixel
    DIM g AS _UNSIGNED _BYTE ' green value of each pixel
    DIM b AS _UNSIGNED _BYTE ' blue value of each pixel
    DIM p AS _UNSIGNED LONG '  new grayscale pixel value

    $CHECKING:OFF
    m = _MEMIMAGE(i) '                            set memory block to image data
    t = m.SIZE \ 4 '                              calculate number of pixels in image
    o = m.OFFSET '                                start address of memory block
    DO '                                          begin pixel count
        _MEMGET m, o + 3, a '                     get alpha (0 to 255)
        _MEMGET m, o + 2, r '                     get red   (0 to 255)
        _MEMGET m, o + 1, g '                     get green (0 to 255)
        _MEMGET m, o, b '                         get blue  (0 to 255)
        p = INT(r * .299 + g * .587 + b * .114) ' calculate gray level
        p = _RGBA32(p, p, p, a) '                 create new pixel
        _MEMPUT m, o, p '                         update pixel in memory block
        o = o + 4 '                               next 4 byte pixel location in memory block
        t = t - 1 '                               decrement pixels remaining
    LOOP UNTIL t = 0 '                            leave when no pixels remain
    _MEMFREE m '                                  free memory block
    $CHECKING:ON

END SUB