SETALPHA: Difference between revisions

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(Created page with "{{DISPLAYTITLE:_SETALPHA}}{{DISPLAYTITLE:}} The _SETALPHA statement sets the alpha channel transparency level of some or all of the pixels of an image. {{PageSyntax}} : _SETALPHA {{Parameter|alpha&}}[, {{Parameter|color1&}}][ TO {{Parameter|colour2&}}] [, {{Parameter|imageHandle&}}] {{Parameters}} * {{Parameter|alpha&}} is the new alpha level to set, ranging from 0 (transparent) to 255 (opaque). * {{Parameter|color1&}} designates the 32-bit LONG color...")
 
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{{Cl|LOOP}} {{Cl|UNTIL}} a& = 0 '' ''
{{Cl|LOOP}} {{Cl|UNTIL}} a& = 0 '' ''
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: ''Explanation:'' The [[POINT]] value minus [[_RGBA]](1, 1, 1, 0) subtracts a small amount from the bright white color value so that the top [[_SETALPHA]] color range will not affect the [[_CLEARCOLOR]] transparency of the bright white PNG background. [http://www.qb64.net/qb64_trans.png qb64_trans.png].
: ''Explanation:'' The [[POINT]] value minus [[_RGBA]](1, 1, 1, 0) subtracts a small amount from the bright white color value so that the top [[_SETALPHA]] color range will not affect the [[_CLEARCOLOR]] transparency of the bright white PNG background.





Revision as of 23:23, 21 April 2022

The _SETALPHA statement sets the alpha channel transparency level of some or all of the pixels of an image.


Syntax

_SETALPHA alpha&[, color1&][ TO colour2&] [, imageHandle&]


Template:Parameters

  • alpha& is the new alpha level to set, ranging from 0 (transparent) to 255 (opaque).
  • color1& designates the 32-bit LONG color value or range of color values color1& TO colour2& to set the transparency.
  • If no color value or range of colors is given, the entire image's alpha is changed, including any _CLEARCOLOR settings.
  • If imageHandle& is omitted, it is assumed to be the current write page or destination image.


Description

  • In the first syntax, the alpha level of all pixels is set to alpha&.
  • In the second syntax, the alpha level of all pixels matching the color color1& is set to alpha&.
  • In the third syntax, the alpha level of all pixels with red, green, blue and alpha channels in the range [color1& TO color2&] are set.
  • The _ALPHA setting makes a 32-bit color transparent, opaque or something in between. Zero is clear and 255 totally blocks underlying images. Use it to see through backgrounds or image colors.
  • If alpha& is outside that range, an illegal function call error will occur.
  • If the image specified by imageHandle& uses a palette, an invalid handle error will occur.
  • If imageHandle& is an invalid handle, an illegal function call error will occur.
  • NOTE: 32-bit _NEWIMAGE screen page backgrounds are transparent black or _ALPHA 0. Use _DONTBLEND or CLS for opaque.


Examples

Example: Using a _SETALPHA color range to fade an image in and out while not affecting the transparent white background.

  
main = _NEWIMAGE(640, 480, 32) 
SCREEN main
_SCREENMOVE _MIDDLE

Image1& = _LOADIMAGE("qb64_trans.png") '<<< PNG file with white background to hide
_SOURCE Image1&
clr~& = POINT(0, 0) 'find background color of image
_CLEARCOLOR clr~&, Image1& 'set background color as transparent

topclr~& = clr~& - _RGBA(1, 1, 1, 0)  'get topmost color range just below full white
_DEST main

a& = 0
d = 1
DO
  _LIMIT 10 'regulate speed of fade in and out
  CLS ', _RGB(255, 0, 0)
  a& = a& + d
  IF a& = 255 THEN d = -d
  _SETALPHA a&, 0 TO topclr~&, Image1& 'affects all colors below bright white
  _PUTIMAGE (0, 342), Image1& 
  LOCATE 1, 1: PRINT "Alpha: "; a&
  _DISPLAY
LOOP UNTIL a& = 0  
Explanation: The POINT value minus _RGBA(1, 1, 1, 0) subtracts a small amount from the bright white color value so that the top _SETALPHA color range will not affect the _CLEARCOLOR transparency of the bright white PNG background.


See also



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