Difference between revisions of "ALPHA"

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{{DISPLAYTITLE:_ALPHA}}
 
{{DISPLAYTITLE:_ALPHA}}
The {{KW|_ALPHA}} function returns the alpha channel transparency level of a color value used on a screen page or image.
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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==
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{{PageSyntax}}
:{{Parameter|result&}} = {{KW|_ALPHA}}({{Parameter|color~&}} [, {{Parameter|imageHandle&}}])
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:{{Parameter|result&}} = [[_ALPHA]]({{Parameter|color~&}} [, {{Parameter|imageHandle&}}])
  
  
==Description==
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{{PageDescription}}
 
* If {{Parameter|imageHandle&}} is omitted, it is assumed to be the current write page. Invalid handles will create [[ERROR Codes|Illegal function call]] errors.
 
* If {{Parameter|imageHandle&}} is omitted, it is assumed to be the current write page. Invalid handles will create [[ERROR Codes|Illegal function call]] errors.
 
* [[_NEWIMAGE]] 32 bit [[SCREEN]] modes will always use an [[_UNSIGNED]] [[LONG]] ''color~&'' value.
 
* [[_NEWIMAGE]] 32 bit [[SCREEN]] modes will always use an [[_UNSIGNED]] [[LONG]] ''color~&'' value.
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==Examples==
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{{PageExamples}}
 
''Example 1:'' Alpha transparency levels are always 255 in 4 or 8 bit screen modes.
 
''Example 1:'' Alpha transparency levels are always 255 in 4 or 8 bit screen modes.
 
{{CodeStart}} '' ''
 
{{CodeStart}} '' ''
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==See also==
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{{PageSeeAlso}}
 
* [[_ALPHA32]], [[_SETALPHA]]
 
* [[_ALPHA32]], [[_SETALPHA]]
 
* [[_RGBA]], [[_RGBA32]] {{text|(set color with alpha)}}
 
* [[_RGBA]], [[_RGBA32]] {{text|(set color with alpha)}}

Latest revision as of 03:16, 1 November 2017

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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