How to Convert Object Color Values to RGB Equivalents

MORE INFORMATION

When colors are assigned through the Property Sheet, they are displayed in the sheet as individual red, green, and blue (RGB) values -- separated by commas. However, the actual value assigned to the object internally is the decimal equivalent of a hexadecimal number that has the individual red, green, and blue values encoded positionally.

Evaluating a color property does not display the RGB values; instead it displays the decimal number. An example of this number can be seen by typing the following statement in the Command window:

   ? _SCREEN.BACKCOLOR
				

Each color can be set within a range of 0 to 255, so it is difficult to determine from a single decimal value the color the user chose. With the exception of black (0), it is hard to distinguish all the potential colors ranging up to white (16777215). The individual colors can be parsed to compare with other color settings to avoid potential problems due to extremes in contrast, and reset other colors in the interface to accommodate the custom settings.

The function in this article converts the number to hexadecimal, extracts the individual RGB values, then converts the results for each value to decimal and returns the result as a string in the expected RGB format.

The value that FoxPro assigns to an object's color property consists of a double word in hexadecimal notation. The first byte of the first word is ignored. The remaining three bytes indicate the individual red, green, and blue values in the following format:

   [XX][BB][GG][RR]
				

Note that the blue and red values are reversed when encoded, and must be manipulated to be presented in the expected RGB format.

The first section of the function converts the decimal value to hexadecimal by factoring the next highest exponent of 2 contained within the decimal value, and so on until the remaining factor is 1.

Once converted, the resulting string is parsed and the first and third bytes are swapped so that they are in Red-Green-Blue order.

The final section reverts the hexadecimal value of each pair back to a decimal value between 0 and 255, representing the intensity of the color that it indicates. The colors values are then concatenated and returned to the calling program in the RGB format.

The code may be used as a method within a class or form, or as a function within a .prg or procedure file.

Dec2RGB Function Code

***********************************************************************
*
* Dec2RGB function: returns separate RGB values from object color value
*
* Usage:
*         RGB = Dec2RGB(<Decimal Color>) && such as _SCREEN.BACKCOLOR
*          ? RGB = "192, 192, 192"       && color is light gray if true
*
* FUNCTION dec2rgb
LPARAMETERS tnDec
IF tnDec < 0
     WAIT WINDOW 'Must be a positive value'
     RETURN ""
ENDIF
lTalk=IIF(SET('TALK')='ON',.T.,.F.)
SET TALK OFF

* Determine the hexadecimal equivalent of the decimal parameter passed
lcHex = ""
lnFactor = 24          && set up factor value one exponent greater than
used

FOR lnPos = 6 TO 1 STEP -1
     lnFactor = lnFactor - 4     && decrement factorial
     lnExp = 2 ^ lnFactor        && extrapolate next least power of two
     FOR lnOrd = 15 TO 1 STEP -1
          IF tnDec < lnExp            && no value greater than current one,
               lcHex = lcHex + "0"    && so store a zero in this position
               EXIT                   && go back for the next value
          ENDIF
          IF tnDec >= lnExp * lnOrd  && is value greater than or equal to?
               * find the matching hex value from its ordinal position
               lcHex = lcHex + SUBSTR('123456789ABCDEF', lnOrd, 1)
               EXIT
          ENDIF
     ENDFOR
     tnDec = tnDec % lnExp     && leave remainder of exponential division
ENDFOR

* reverse the order of the individual color indicators
lcHex = RIGHT(lcHex, 2) + SUBSTR(lcHex, 3, 2) + LEFT(lcHex, 2)

* convert the pairs into decimal values
lnPick = 2          && offset to determine which pair to convert
lcRGB = ["]     && start of string delineator

* parse each color indicator and convert to decimal
FOR lnColor = 1 TO 3
     lcHue = SUBSTR(lcHex, (lnPick * lnColor) - 1, 2) && pull out color
     lnMSB = ASC(LEFT(lcHue, 1))     && "Most Significant Bit"
     lnLSB = ASC(RIGHT(lcHue, 1))     && "Least Significant Bit"

     * subtract appropriate value from each to get decimal equivalent
     lnMSB = lnMSB - IIF(lnMSB > 57, 55, 48)
     lnLSB = lnLSB - IIF(lnLSB > 57, 55, 48)

     * then add decimals together
     lcRGB = lcRGB + TRANSFORM( lnMSB * 16 + lnLSB, '999') + ", "
ENDFOR
lcRGB = LEFT(lcRGB, LEN(lcRGB) - 2) + ["]  && replace last comma with quote
RETURN lcRGB