The _SETALPHA statement sets the alpha channel transparency level of some or all of the pixels of an image.
Syntax
Parameter(s)
- 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 _DEST 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 ERROR Codes error will occur.
- If the image specified by imageHandle& uses a palette, an ERROR Codes error will occur.
- If imageHandle& is an invalid handle, an ERROR Codes error will occur.
- NOTE: 32-bit _NEWIMAGE screen page backgrounds are transparent black or _ALPHA 0. Use _DONTBLEND or CLS for opaque.
Example(s)
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.