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64 lines
2.7 KiB
Plaintext
64 lines
2.7 KiB
Plaintext
{{DISPLAYTITLE:_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.
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{{PageSyntax}}
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:{{Parameter|result&}} = [[_ALPHA]]({{Parameter|color~&}} [, {{Parameter|imageHandle&}}])
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{{PageDescription}}
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* If {{Parameter|imageHandle&}} is omitted, it is assumed to be the current write page. Invalid handles will create [[ERROR Codes|Illegal function call]] errors.
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* [[_NEWIMAGE]] 32 bit [[SCREEN]] modes will always use an [[_UNSIGNED]] [[LONG]] ''color~&'' value.
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** Color values that are set as a [[_CLEARCOLOR]] always have an alpha level of 0 (transparent).
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** [[_SETALPHA]] can set any alpha level from 0 (or fully transparent) to 255 (or opaque).
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** Normal color values that are set by [[_RGB]] or [[_RGB32]] always have an alpha level of 255(opaque).
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* In 4 (16 color) or 8 (256 color) bit palette screens the function will always return 255.
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*[[_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.
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* '''NOTE: 32 bit [[_NEWIMAGE]] screen page backgrounds are transparent black or [[_ALPHA]] 0. Use [[_DONTBLEND]] or [[CLS]] for opaque.'''
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{{PageExamples}}
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''Example 1:'' Alpha transparency levels are always 255 in 4 or 8 bit screen modes.
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{{CodeStart}} '' ''
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{{Cl|SCREEN}} 13
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clr~& = {{Cl|_RGBA}}(255, 0, 255, 192) 'returns closest palette color attribute
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{{Cl|PRINT}} "Color:"; clr~&
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{{Cl|COLOR}} clr~&
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{{Cl|PRINT}} "Alpha:"; {{Cl|_ALPHA}}(clr~&)
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{{Cl|END}}
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{{CodeEnd}}
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{{OutputStart}}{{text|Color 36|#FFFFFF}}
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{{text|Alpha: 255|#FF00FF}}
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{{OutputEnd}}
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: ''Explanation:'' [[_RGBA]] cannot change the [[_ALPHA]] level. [[_ALPHA32]] would return 0 on any non-32 bit image or page.
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''Example 2:'' Finding the transparency of a 32 bit screen mode's background before and after [[CLS]].
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{{CodeStart}} '' ''
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{{Cl|SCREEN}} {{Cl|_NEWIMAGE}}(640, 480, 32)
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BG& = {{Cl|POINT}}(1, 1)
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{{Cl|PRINT}} "Alpha ="; {{Cl|_ALPHA}}(BG&); "Press a key to use CLS!"
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K$ = INPUT$(1)
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{{Cl|CLS}}
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BG& = {{Cl|POINT}}(1, 1)
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{{Cl|PRINT}} "CLS Alpha ="; {{Cl|_ALPHA}}(BG&) '' ''
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{{CodeEnd}}
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{{OutputStart}}CLS Alpha = 255 {{OutputEnd}}
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: ''Explanation:'' Set the ALPHA value to 255 using [[CLS]] to make the background opaque when overlaying pages.
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{{PageSeeAlso}}
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* [[_ALPHA32]], [[_SETALPHA]]
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* [[_RGBA]], [[_RGBA32]] {{text|(set color with alpha)}}
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* [[_CLEARCOLOR]], [[_CLEARCOLOR (function)]]
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* [[_RED]], [[_GREEN]], [[_BLUE]]
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* [[_RED32]], [[_GREEN32]]. [[_BLUE32]]
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* [[CLS]], [[COLOR]], [[Images]]
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{{PageNavigation}}
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