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https://github.com/QB64-Phoenix-Edition/QB64pe.git
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69 lines
3.5 KiB
Plaintext
69 lines
3.5 KiB
Plaintext
{{DISPLAYTITLE:_SETALPHA}}{{DISPLAYTITLE:}}
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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]] {{Parameter|alpha&}}[, {{Parameter|color1&}}][ [[TO]] {{Parameter|colour2&}}] [, {{Parameter|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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* {{Parameter|color1&}} designates the 32-bit [[LONG]] color value or range of color values {{Parameter|color1&}} TO {{Parameter|colour2&}} 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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{{PageDescription}}
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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|color1&}} 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|color1&}} TO {{Parameter|color2&}}] 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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{{PageExamples}}
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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}}("qb64_trans.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.
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{{PageSeeAlso}}
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* [[_ALPHA]], [[_ALPHA32]]
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* [[_RGBA]], [[_RGBA32]]
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* [[_CLEARCOLOR]]
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* [[_CLEARCOLOR (function)]]
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* [[_BLEND]], [[_DONTBLEND]]
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* [[COLOR]], [[Images]]
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{{PageNavigation}}
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