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https://github.com/QB64-Phoenix-Edition/QB64pe.git
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65 lines
3.5 KiB
Plaintext
65 lines
3.5 KiB
Plaintext
The '''_SETALPHA''' statement sets the alpha channel transparency level of some or all of the pixels of an image.
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{{PageSyntax}}
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::: '''_SETALPHA ''alpha&'''''[, ''colour1&''][ {{KW|TO}} ''colour2&''] [, ''imageHandle&'']
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{{Parameters}}
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* {{Parameter|alpha&}} is the new alpha level to set, ranging from 0 (transparent) to 255 (opaque).
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* ''co1or&'' designates the 32 bit [[LONG]] color value or range of color values ''c1&'' TO ''c2&'' to set the transparency.
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* If no color value or range of colors is given, the entire image's alpha is changed, including any [[_CLEARCOLOR]] settings.
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* If {{Parameter|imageHandle&}} is omitted, it is assumed to be the current write page or [[_DEST|destination]] image.
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''Usage:''
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* In the first syntax, the alpha level of all pixels is set to {{Parameter|alpha&}}.
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* In the second syntax, the alpha level of all pixels matching the color {{Parameter|c&}} is set to {{Parameter|alpha&}}.
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* In the third syntax, the alpha level of all pixels with red, green, blue and alpha channels in the range [{{Parameter|c1&}} TO {{Parameter|c2&}}] are set.
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* 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.
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* If {{Parameter|alpha&}} is outside that range, an [[ERROR Codes|illegal function call]] error will occur.
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* If the image specified by {{Parameter|imageHandle&}} uses a palette, an [[ERROR Codes|invalid handle]] error will occur.
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* If {{Parameter|imageHandle&}} is an invalid handle, an [[ERROR Codes|illegal function call]] error will occur.
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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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''Example:'' Using a _SETALPHA color range to fade an image in and out while not affecting the transparent white background.
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{{CodeStart}} '' ''
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main = {{Cl|_NEWIMAGE}}(640, 480, 32)
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{{Cl|SCREEN}} main
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{{Cl|_SCREENMOVE}} {{Cl|_SCREENMOVE|_MIDDLE}}
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Image1& = {{Cl|_LOADIMAGE}}("qb64bee.png") '<<< PNG file with white background to hide
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{{Cl|_SOURCE}} Image1&
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clr~& = {{Cl|POINT}}(0, 0) 'find background color of image
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{{Cl|_CLEARCOLOR}} clr~&, Image1& 'set background color as transparent
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topclr~& = clr~& - {{Cl|_RGBA}}(1, 1, 1, 0) 'get topmost color range just below full white
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{{Cl|_DEST}} main
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a& = 0
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d = 1
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DO
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{{Cl|_LIMIT}} 10 'regulate speed of fade in and out
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{{Cl|CLS}} ', {{Cl|_RGB}}(255, 0, 0)
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a& = a& + d
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{{Cl|IF...THEN|IF}} a& = 255 {{Cl|THEN}} d = -d
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{{Cl|_SETALPHA}} a&, 0 {{Cl|TO}} topclr~&, Image1& 'affects all colors below bright white
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{{Cl|_PUTIMAGE}} (0, 342), Image1&
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{{Cl|LOCATE}} 1, 1: {{Cl|PRINT}} "Alpha: "; a&
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{{Cl|_DISPLAY}}
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{{Cl|LOOP}} {{Cl|UNTIL}} a& = 0 '' ''
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{{CodeEnd}}
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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. [https://dl.dropbox.com/u/8440706/QB64bee.png QB64bee.png].
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{{PageSeeAlso}}
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* [[_ALPHA]], [[_ALPHA32]] {{text|(read transparency level)}}
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* [[_RGBA]], [[_RGBA32]] {{text|(set color levels and alpha)}}
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* [[_CLEARCOLOR]] {{text|(sets a transparent color)}}
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* [[_CLEARCOLOR (function)]] {{text|(finds transparent color)}}
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* [[_BLEND]], [[_DONTBLEND]]
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* [[COLOR]], [[Images]]
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{{PageNavigation}} |