Why does the Processing `fill()` with alpha never completely fill?

Question

Let's say we have the following code:

void setup() {
    background(0);
    size(200, 200);
    fill(255);        
    rect(75, 75, 50, 50);
}  

void draw() {
    fill(0, 2);
    rect(0, 0, width, height);
}

Even after waiting 'forever,' the white 50x50 rectangle is still visible, albeit faded. Why doesn't the fill(0, 2) eventually cover this up?

I suppose this question is twofold:

Why doesn't it eventually fade to black, as in why does drawing another dark rectangle on top of the white one not erase it eventually (I'm thinking along the lines of putting tinted windows over each other; eventually even the brightest light won't shine through), and
Why doesn't it eventually fade to black, as in why is this the behavior intended by the Processing community?

score 3 · Accepted Answer · answered Mar 11 '14 at 14:28

3

Here's a post explaining what's going on: http://processing.org/discourse/beta/num_1138703939.html

Basically, the problem is that Processing stores colors as ints, but takes float arguments. When combining colors, Processing rounds the floats to ints. In your case, your color is getting stuck at a value of 63, 63, 63 because at that point the blending is too slight to make a difference that is detectable after rounding.

The solution is to do the fading from the source, not by overlaying an alpha color over top.

answered Mar 11 '14 at 14:28

Kevin Workman

41,537
9
68
107

Could you expand a bit on what you mean by fading from the source? I'm assuming you mean touch the `pixels` array by hand? – j6m8 Mar 11 '14 at 18:25
Basically, most sketches draw everything from scratch each time. That means you should repaint your background, then calculate yourself how dim the rectangle should be (or if it should be painted at all) based on how long the sketch has been running, and draw the rectangle according to that calculation. – Kevin Workman Mar 11 '14 at 18:31

score 0 · Answer 2 · answered Dec 30 '21 at 14:56

0

I had the same issue with fill(0, 0, 0, 5);.
Interestingly, changing the alpha value to 20 helped (and I'm sure many other values work too).

void draw() {
    fill(0, 0, 0, 20); // Note the value 20 for the alpha channel.
    rect(0, 0, width, height);
}

answered Dec 30 '21 at 14:56

Evgenia Karunus

10,715
5
56
70

score -2 · Answer 3 · answered Mar 11 '14 at 08:30

The default background color is darker than the color you assigned to the first rectangle, thus it gets black sooner.

Why doesn't it eventually fade to black, as in why does drawing another dark rectangle on top of the white one not erase it eventually (I'm thinking along the lines of putting tinted windows over each other; eventually even the brightest light won't shine through), and

Why doesn't it eventually fade to black, as in why is this the behavior intended by the Processing community?

Also, in your original code (not the above sample) you're probably drawing the white rectangle continuously, so it will never fade.

Note that the `rect` in the `setup()` is white; the continuously drawn one is black with an alpha value. Additionally, I set the background to black in my `setup()`, so it's not a concern of default values. — j6m8, Mar 11 '14 at 13:09

Why does the Processing `fill()` with alpha never completely fill?

3 Answers3

Linked