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75 lines
2.6 KiB
Markdown
75 lines
2.6 KiB
Markdown
The [_ALPHA](_ALPHA) function returns the alpha channel transparency level of a color value used on a screen page or image.
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## Syntax
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> result& = [_ALPHA](_ALPHA)(color~& [, imageHandle&])
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## Description
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* If imageHandle& is omitted, it is assumed to be the current write page. Invalid handles will create [ERROR Codes](ERROR-Codes) errors.
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* [_NEWIMAGE](_NEWIMAGE) 32 bit [SCREEN](SCREEN) modes will always use an [_UNSIGNED](_UNSIGNED) [LONG](LONG) *color~&* value.
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* Color values that are set as a [_CLEARCOLOR](_CLEARCOLOR) always have an alpha level of 0 (transparent).
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* [_SETALPHA](_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](_RGB) or [_RGB32](_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](_RED32), [_GREEN32](_GREEN32), [_BLUE32](_BLUE32) and [_ALPHA32](_ALPHA32) are all equivalent to [_RED](_RED), [_GREEN](_GREEN), [_BLUE](_BLUE) and [_ALPHA](_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](_NEWIMAGE) screen page backgrounds are transparent black or [_ALPHA](_ALPHA) 0. Use [_DONTBLEND](_DONTBLEND) or [CLS](CLS) for opaque.**
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## Example(s)
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Alpha transparency levels are always 255 in 4 or 8 bit screen modes.
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```vb
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SCREEN 13
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clr~& = _RGBA(255, 0, 255, 192) 'returns closest palette color attribute
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PRINT "Color:"; clr~&
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COLOR clr~&
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PRINT "Alpha:"; _ALPHA(clr~&)
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END
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```
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```text
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Color 36
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Alpha: 255
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```
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> *Explanation:* [_RGBA](_RGBA) cannot change the [_ALPHA](_ALPHA) level. [_ALPHA32](_ALPHA32) would return 0 on any non-32 bit image or page.
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Finding the transparency of a 32 bit screen mode's background before and after [CLS](CLS).
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```vb
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SCREEN _NEWIMAGE(640, 480, 32)
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BG& = POINT(1, 1)
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PRINT "Alpha ="; _ALPHA(BG&); "Press a key to use CLS!"
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K$ = INPUT$(1)
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CLS
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BG& = POINT(1, 1)
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PRINT "CLS Alpha ="; _ALPHA(BG&)
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```
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```text
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CLS Alpha = 255
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```
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> *Explanation:* Set the ALPHA value to 255 using [CLS](CLS) to make the background opaque when overlaying pages.
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## See Also
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* [_ALPHA32](_ALPHA32), [_SETALPHA](_SETALPHA)
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* [_RGBA](_RGBA), [_RGBA32](_RGBA32) (set color with alpha)
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* [_CLEARCOLOR](_CLEARCOLOR), [_CLEARCOLOR (function)](_CLEARCOLOR-(function))
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* [_RED](_RED), [_GREEN](_GREEN), [_BLUE](_BLUE)
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* [_RED32](_RED32), [_GREEN32](_GREEN32). [_BLUE32](_BLUE32)
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* [CLS](CLS), [COLOR](COLOR), [Images](Images)
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