Difference between revisions of "ALPHA"

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{{CodeStart}} '' ''
 
{{CodeStart}} '' ''
 
{{Cl|SCREEN}} {{Cl|_NEWIMAGE}}(640, 480, 32)
 
{{Cl|SCREEN}} {{Cl|_NEWIMAGE}}(640, 480, 32)
x = {{Cl|POINT}}(1, 1)
+
BG& = {{Cl|POINT}}(1, 1)
{{Cl|PRINT}} "Alpha ="; {{Cl|_ALPHA}}(x); "Press a key to use CLS!"
+
{{Cl|PRINT}} "Alpha ="; {{Cl|_ALPHA}}(BG&); "Press a key to use CLS!"
 
K$ = INPUT$(1)                   
 
K$ = INPUT$(1)                   
 
{{Cl|CLS}}
 
{{Cl|CLS}}
x = {{Cl|POINT}}(1, 1)
+
BG& = {{Cl|POINT}}(1, 1)
{{Cl|PRINT}} "CLS Alpha ="; {{Cl|_ALPHA}}(x) '' ''
+
{{Cl|PRINT}} "CLS Alpha ="; {{Cl|_ALPHA}}(BG&) '' ''
 
{{CodeEnd}}
 
{{CodeEnd}}
 
: ''Explanation:'' You need to set the ALPHA value to 255 using [[CLS]] to make the black background opaque!
 
: ''Explanation:'' You need to set the ALPHA value to 255 using [[CLS]] to make the black background opaque!

Revision as of 02:08, 4 July 2011

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


Syntax

result& = _ALPHA(colour& [, imageHandle&])


Description

  • If imageHandle& is omitted, it is assumed to be the current write page.
  • If the image specified by imageHandle& uses a palette, colour& is the palette index of the color value used.
    • Color values that match the clear color set by _CLEARCOLOR have an alpha level of 0 (zero).
    • Color values that don't match the clear color have an alpha level of 255.
    • If colour& value is outside of the range 0 to 255, an Illegal function call error will occur.
  • If the image specified by imageHandle& does not use a palette, colour& is the color value used to retrieve the alpha level.
  • If imageHandle& is an invalid handle, an Illegal function call error will occur.
  • _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 these in your code (opposed to dividing then ANDing 32-bit color values manually) makes your code easy to read also.
  • NOTE: Default 32 bit backgrounds are clear black or _RGBA(0, 0, 0, 0)! Use CLS to make the black opaque!


Example: Finding the transparency of a 32 bit screen mode's background.

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&)

Explanation: You need to set the ALPHA value to 255 using CLS to make the black background opaque!


See also



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