_ALPHA

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The _ALPHA function returns the alpha channel transparency level of a color value used on a screen page or image.


Syntax

result& = _ALPHA(color~& [, imageHandle&])


Description

  • If imageHandle& is omitted, it is assumed to be the current write page. Invalid handles will create Illegal function call errors.
  • _NEWIMAGE 32 bit SCREEN modes will always use an _UNSIGNED LONG color~& value.
    • Color values that are set as a _CLEARCOLOR always have an alpha level of 0 (transparent).
    • _SETALPHA can set any alpha level from 0 (or fully transparent) to 255 (or opaque).
    • Normal color values that are set by _RGB or _RGB32 always have an alpha level of 255(opaque).
  • In 4 (16 color) or 8 (256 color) bit palette screens the function will always return 255.
  • _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 them (opposed to dividing then ANDing 32-bit color values manually) makes code easy to read.
  • NOTE: 32 bit _NEWIMAGE screen page backgrounds are transparent black or _ALPHA 0. Use _DONTBLEND or CLS for opaque.


Examples

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

SCREEN 13 clr~& = _RGBA(255, 0, 255, 192) 'returns closest palette color attribute 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



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