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Revision as of 04:22, 16 August 2012 by imported>Clippy
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The _ALPHA function returns the alpha channel transparency level of a color value used on a screen page or image.


result& = _ALPHA(colour& [, imageHandle&])


  • If imageHandle& is omitted, it is assumed to be the current write page.
  • If the image specified by imageHandle& uses a palette, colour& is the palette index of the color value used.
    • Color values that match the clear color set by _CLEARCOLOR have an alpha level of 0 (zero).
    • Color values that don't match the clear color have an alpha level of 255.
    • If colour& value is outside of the range 0 to 255, an Illegal function call error will occur.
  • If the image specified by imageHandle& does not use a palette, colour& is the color value used to retrieve the alpha level.
  • If imageHandle& is an invalid handle, an Illegal function call error will occur.
  • _RED32, _GREEN32, _BLUE32 and _ALPHA32 are all equivalent to _RED, _GREEN, _BLUE and _ALPHA but they are highly optimized and only accept a 32-bit color (B8:G8:R8:A8). Using these in your code (opposed to dividing then ANDing 32-bit color values manually) makes your code easy to read also.
  • NOTE: Default 32 bit backgrounds are clear black or _RGBA(0, 0, 0, 0)! Use CLS to make the black opaque!

Example 1: Alpha transparency levels are always 255 in 4 or 8 bit screen modes.

SCREEN 13 clr~& = _RGBA(255, 0, 255, 192) PRINT "Color:"; clr~& COLOR clr~& PRINT "Alpha:"; _ALPHA(clr~&) END

Color 36 Alpha: 255

Explanation: _RGBA cannot change the _ALPHA level. _ALPHA32 would return 0 on any non-32 bit image or page.

Example 2: Finding the transparency of a 32 bit screen mode's background before and after CLS.

SCREEN _NEWIMAGE(640, 480, 32) BG& = POINT(1, 1) PRINT "Alpha ="; _ALPHA(BG&); "Press a key to use CLS!" K$ = INPUT$(1) CLS BG& = POINT(1, 1) PRINT "CLS Alpha ="; _ALPHA(BG&)

CLS Alpha = 255

Explanation: Set the ALPHA value to 255 using CLS to make the background opaque when overlaying pages.

See also

Keyword Reference - Alphabetical
Keyword Reference - By Usage
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