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Topic: applying alpha to color vars (Read 400 times) |
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sspboyd
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applying alpha to color vars
« on: Apr 20th, 2004, 7:24pm » |
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Is it possible to give a color variable an alpha value after it's been declared? Eg. fill(colorVar, alpha);
I want to use a seven colour palate with random alpha values. I created a color array and defined my colours without alpha values. I have attempted to use the colours with an added the alpha in the fill() statement. What I get is a gray colour with the alpah value. Is there another way of doing this?
The following example shows what I am getting:
Code:
color[] carray = new color[7]; // create color array
carray[0]=color(204,204,204); // add colours w/o alpha
carray[1]=color(255,255,0);
carray[2]=color(204,255,255);
carray[3]=color(0,255,0);
carray[4]=color(255,0,255);
carray[5]=color(255,93,15);
carray[6]=color(0,33,255);
fill(carray[1]); // yellow fill
rect(10,10,40,40); // yellow rect
fill(carray[1], 126); // yellow fill, 50% alpha?
rect(30,30,40,40); // not yellow rect
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steve
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gmail.com w/ sspboyd username
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sspboyd
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Re: applying alpha to color vars
« Reply #1 on: Apr 20th, 2004, 9:02pm » |
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this is compromise i've come up with. It works but it isn't very elegant.
Code:
int[][] carray = new int[2][3]; // store RGB colour vals
int rc; // to store the colour selection
carray[0][0]=204;
carray[0][1]=20;
carray[0][2]=47;
carray[1][0]=255;
carray[1][1]=255;
carray[1][2]=0;
rc=0;
fill(carray[rc][0],carray[rc][1],carray[rc][2],random(55,155)); // R,G,B,A
rect(10,10,30,30);
rc=1;
fill(carray[rc][0],carray[rc][1],carray[rc][2],random(25,55)); // R,G,B,A
rect(30,30,30,30);
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gmail.com w/ sspboyd username
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TomC
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Re: applying alpha to color vars
« Reply #2 on: Apr 20th, 2004, 10:39pm » |
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You could define a function which takes a color and an alpha value and returns a new color.
I'm fairly confident this should work:
Code:
color colorWithAlpha(color col, int alpha) {
return color(red(col),green(col),blue(col),alpha);
}
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So now you can say:
Code:
fill(colorWithAlpha(carray[1], 127)); // yellow fill, 50% alpha
rect(30,30,40,40); // 50% yellow rect
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An alternative way to do the same thing a bit faster (probably) using bit shifts, would be...
Code:
color colorWithAlpha(color col, int alpha) {
return (alpha << 24) | ((col << 8) >>> 8);
}
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update I've just noticed that fill(color, alpha) is in the reference section, so it should work. I'll wait a while and see if fry picks it up as a bug, otherwise I'll repost on the bug forum.
I guess it's a problem with the fact that colors are represented internally as ints, so there's no real difference between fill(int grey, int alpha) and fill(color col, int alpha). Here's a fix for fill, so that your original code would work...
Code:
void fill(int col, int alpha) {
if ((col & 0xff000000) != 0) {
// if col has an alpha component (could just check for col > 255?)
super.fill((alpha << 24) | ((col << 8) >>> 8));
}
else {
// otherwise you must have meant grey
super.fill(col,alpha);
}
}
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| « Last Edit: Apr 20th, 2004, 10:50pm by TomC » |
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fry
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Re: applying alpha to color vars
« Reply #3 on: Apr 21st, 2004, 12:13am » |
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right, the distinction is that it's fill(gray, alpha) to go with fill(gray).
however, since we cheat and allow fill(color c), i suppose we could use the same logic to figure out whether it's fill(color c, float alpha) or fill(float gray, float alpha).. though i wonder if we aren't already setting ourselves up for trouble with that..
for the technically minded.. since a 'color' is just an int, internally the fill and stroke functions check to see if higher bits of the value are set, and if so, determines whether you meant:
fill(int gray) or fill(int some_color)
[ the latter is the same as fill(color some_color) ]
this mostly works, but there's a problem where if a value is zero, that comes out as simply opaque black (no higher bits set), instead of, say alpha 0 on a black color. the latter you'd run into a problem with if you tried to, say, fade a black rectangle by setting its alpha.. if that makes sense.
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