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68 lines
2.9 KiB
Markdown
68 lines
2.9 KiB
Markdown
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The [_SETALPHA](_SETALPHA) statement sets the alpha channel transparency level of some or all of the pixels of an image.
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## Syntax
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> [_SETALPHA](_SETALPHA) alpha&[, color1&][ [TO](TO) colour2&] [, imageHandle&]
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## Parameter(s)
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* alpha& is the new alpha level to set, ranging from 0 (transparent) to 255 (opaque).
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* color1& designates the 32-bit [LONG](LONG) color value or range of color values color1& TO 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](_CLEARCOLOR) settings.
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* If imageHandle& is omitted, it is assumed to be the current write page or [_DEST](_DEST) image.
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## Description
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* In the first syntax, the alpha level of all pixels is set to alpha&.
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* In the second syntax, the alpha level of all pixels matching the color color1& is set to 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 [color1& TO color2&] are set.
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* The [_ALPHA](_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 alpha& is outside that range, an [ERROR Codes](ERROR-Codes) error will occur.
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* If the image specified by imageHandle& uses a palette, an [ERROR Codes](ERROR-Codes) error will occur.
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* If imageHandle& is an invalid handle, an [ERROR Codes](ERROR-Codes) error will occur.
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* **NOTE: 32-bit [_NEWIMAGE](_NEWIMAGE) screen page backgrounds are transparent black or [_ALPHA](_ALPHA) 0. Use [_DONTBLEND](_DONTBLEND) or [CLS](CLS) for opaque.**
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## Example(s)
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Using a _SETALPHA color range to fade an image in and out while not affecting the transparent white background.
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```vb
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main = _NEWIMAGE(640, 480, 32)
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SCREEN main
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_SCREENMOVE _MIDDLE
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Image1& = _LOADIMAGE("qb64_trans.png") '<<< PNG file with white background to hide
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_SOURCE Image1&
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clr~& = POINT(0, 0) 'find background color of image
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_CLEARCOLOR clr~&, Image1& 'set background color as transparent
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topclr~& = clr~& - _RGBA(1, 1, 1, 0) 'get topmost color range just below full white
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_DEST main
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a& = 0
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d = 1
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DO
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_LIMIT 10 'regulate speed of fade in and out
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CLS ', _RGB(255, 0, 0)
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a& = a& + d
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IF a& = 255 THEN d = -d
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_SETALPHA a&, 0 TO topclr~&, Image1& 'affects all colors below bright white
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_PUTIMAGE (0, 342), Image1&
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LOCATE 1, 1: PRINT "Alpha: "; a&
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_DISPLAY
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LOOP UNTIL a& = 0
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```
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> *Explanation:* The [POINT](POINT) value minus [_RGBA](_RGBA)(1, 1, 1, 0) subtracts a small amount from the bright white color value so that the top [_SETALPHA](_SETALPHA) color range will not affect the [_CLEARCOLOR](_CLEARCOLOR) transparency of the bright white PNG background.
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## See Also
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* [_ALPHA](_ALPHA), [_ALPHA32](_ALPHA32)
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* [_RGBA](_RGBA), [_RGBA32](_RGBA32)
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* [_CLEARCOLOR](_CLEARCOLOR)
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* [_CLEARCOLOR (function)](_CLEARCOLOR-(function))
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* [_BLEND](_BLEND), [_DONTBLEND](_DONTBLEND)
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* [COLOR](COLOR), [Images](Images)
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