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RLUM

Keyword Abbreviation Token (hex) Version(s) Classification
RLUM R{Shift+L} CE 3.5 Function
DCLOSE DC{Shift+L} CE 4.x Command and Statement
varies varies CE ?? 7.0 2-byte Functions

  Syntax  
RLUM colorSource )
 
Parameters Type Legal Value(s) Note(s)
colorSource Integer
  1. background (bitmap & text)
  2. bitmap foreground
  3. bitmap multi-color 1
  4. bitmap multi-color 2
  5. border
 
Returns Type Value(s) Note(s)
chipLuminance Integer
0 ~ 7
 
  Purpose  
Returns the chipLuminance assigned to a graphic screen element.
 
  Remarks  
The RLUM function is a partial complement to COLOR.  It returns the chipLuminance that has been assigned to a colorSource.  The other (primary) partial complement to COLOR is RCLR.  See COLOR for a list of colors and their assigned luminance values.
 
The text color can be changed by printing special "control codes" with CHR$ (or some reverse-font characters in text strings).  However, RLUM does not work with text colors, only bitmap colors and the screen background/border.  You have to PEEK a secret variable, "Text Attribute" (?) to get the luminance of the text. 
 
In an act of incompatibility, BASIC v7.0 does not support RLUM of v3.5... the token $CE (or 206 decimal) is used as the first byte for new 2-byte extended functions.  Any version 3.5 program using RLUM will cause SYNTAX ERROR in v7.0.  Although I won't argue with the decision to use $CE for an extension (all tokens except $FE were in use, but $FE is used for extended commands/statements, not functions), it is not necessary to generate SYNTAX ERROR.  A code of $CE $28 could have been used in version 7.0 to maintain compatibility... well limited compatibility!  The reason they choose to sacrifice RLUM (I'm betting) is because neither video chip in the target machine (C128) has 8 levels of luminance per color.
 
If colorSource is omitted or is not a valid expression, SYNTAX ERROR occurs.  If colorSource is a string, TYPE MISMATCH ERROR occurs; you can use VAL to make a string numeric.  Otherwise the colorSource is converted with INT.  If the result is not a LegalValue (see table above) then ILLEGAL QUANTITY ERROR occurs.
 
Examples (from Plus/4, default values):
PRINT RLUM("0")
 
?TYPE MISMATCH ERROR 
READY.
PRINT RLUM(0) : REM bitmap/text background
 7                   maximum (for white)

READY.
PRINT RLUM(1) : REM bitmap background
 1                   very low (black)

READY.
PRINT RLUM(2) : REM bitmap multi-color 1
 3                   medium (blue)

READY.
PRINT RLUM(3) : REM bitmap multi-color 2
 5                   a bit high (for pink)

READY.
PRINT RLUM(4) : REM 40-column border
 6                   very high (light grape)

READY.
PRINT RLUM(5)
 
?ILLEGAL QUANTITY ERROR 
READY.
 
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© H2Obsession, 2014
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