Difference between revisions of "ALPHA"

From QB64 Wiki
Jump to navigation Jump to search
imported>Cyperium
m
imported>Odin
(Created page with "The '''_ALPHA''' function returns the alpha channel transparency level of a color value used on a screen page or image. == Syntax == result& = '''_ALPHA'''(color~&[, imageHan...")
Line 1: Line 1:
{{DISPLAYTITLE:_ALPHA}}
+
The '''_ALPHA''' function returns the alpha channel transparency level of a color value used on a screen page or image.
The {{KW|_ALPHA}} function returns the alpha channel transparency level of a color value used on a screen page or image.
 
  
 +
== Syntax ==
 +
result& = '''_ALPHA'''(color~&[, imageHandle&])
  
{{PageSyntax}}
+
== Example ==
:{{Parameter|result&}} = {{KW|_ALPHA}}({{Parameter|colour~&}} [, {{Parameter|imageHandle&}}])
+
<source lang="qbasic">
 +
SCREEN 13
  
 +
clr~& = _RGBA(255, 0, 255, 192) 'returns closest palette color attribute
 +
PRINT "Color:"; clr~&
  
{{PageDescription}}
+
COLOR clr~&
* If {{Parameter|imageHandle&}} is omitted, it is assumed to be the current write page. Invalid handles will create [[ERROR Codes|Illegal function call]] errors.
+
PRINT "Alpha:"; _ALPHA(clr~&)
* [[_NEWIMAGE]] 32 bit [[SCREEN]] modes will always use an [[_UNSIGNED]] [[LONG]] ''colour'' 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!'''
 
  
 +
END
 +
</source>
  
''Example 1:'' Alpha transparency levels are always 255 in 4 or 8 bit screen modes.
+
''Output:''
{{CodeStart}} '' ''
+
<pre>Color 36
{{Cl|SCREEN}} 13
+
Alpha: 255</pre>
 
 
clr~& = {{Cl|_RGBA}}(255, 0, 255, 192) 'returns closest palette color attribute
 
{{Cl|PRINT}} "Color:"; clr~&
 
 
 
{{Cl|COLOR}} clr~&
 
{{Cl|PRINT}} "Alpha:"; {{Cl|_ALPHA}}(clr~&)
 
 
 
{{Cl|END}}
 
{{CodeEnd}}
 
{{OutputStart}}{{text|Color 36|#FFFFFF}}
 
{{text|Alpha: 255|#FF00FF}}
 
{{OutputEnd}}
 
: ''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]].
 
{{CodeStart}} '' ''
 
{{Cl|SCREEN}} {{Cl|_NEWIMAGE}}(640, 480, 32)
 
BG& = {{Cl|POINT}}(1, 1)
 
{{Cl|PRINT}} "Alpha ="; {{Cl|_ALPHA}}(BG&); "Press a key to use CLS!"
 
K$ = INPUT$(1)                 
 
{{Cl|CLS}}
 
BG& = {{Cl|POINT}}(1, 1)
 
{{Cl|PRINT}} "CLS Alpha ="; {{Cl|_ALPHA}}(BG&) '' ''
 
{{CodeEnd}}
 
{{OutputStart}}CLS Alpha = 255  {{OutputEnd}}
 
: ''Explanation:'' Set the ALPHA value to 255 using [[CLS]] to make the background opaque when overlaying pages.
 
 
 
 
 
{{PageSeeAlso}}
 
* [[_ALPHA32]], [[_SETALPHA]]
 
* [[_RGBA]], [[_RGBA32]] {{text|(set color with alpha)}}
 
* [[_CLEARCOLOR]], [[_CLEARCOLOR (function)]]
 
* [[_RED]], [[_GREEN]], [[_BLUE]]
 
* [[_RED32]], [[_GREEN32]]. [[_BLUE32]]
 
* [[CLS]], [[COLOR]], [[Images]]
 
 
 
 
 
{{PageNavigation}}
 

Revision as of 03:16, 17 July 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&])

Example

<source lang="qbasic"> SCREEN 13

clr~& = _RGBA(255, 0, 255, 192) 'returns closest palette color attribute PRINT "Color:"; clr~&

COLOR clr~& PRINT "Alpha:"; _ALPHA(clr~&)

END </source>

Output:

Color 36
Alpha: 255