about color alpha problem

General Discussion Other

tomtomton..

in General Discussion • Other • 5 months ago

Dear all

I have execute the code below.

void draw()
{
for (int i=0; i<255; i++)
{
fill(255,0,0,1);
ellipse(width/2,height/2,400,400);
}
}

But I could not get the pure red color after repeating more than 255 times.

Is it the bug for Processing?

Tom Tong

Replies(4)

GoToLoop

Re: about color alpha problem

5 months ago

It's something that took me by surprise and some time to figure out by myself:

The main Processing's PGraphics surface doesn't use any alpha property at all!

Rather, it blends the color being drawn + color of background, to result in a 100% opaque color!!!

Check below your example code displaying the current surface's color through get() .

You'll notice that all of them are 100% opaque red-like.

Original 0x1FF0000 becomes an analogous opaque red!

As another 0x1FF0000 circle is drawn above previous ones over and over, resultant blended color changes.

And after a while, it reaches a plateau and stays that way.

Initial background() influences color blending sequence you obtain too!

Tip: Callback function draw() is itself a "big" loop. Your for loop block is quite redundant!

void setup() {
  size(300, 300);
  fill(0x1FF0000);
  background(-1);
  //background(0);
}

void draw() {
  final int w = width >> 1, h = height >> 1;
  ellipse(w, h, w, h);
  print(hex( get(w, h) ) + " - ");
}

PhiLho

Re: about color alpha problem

5 months ago

As said. See also

I display images in sequence but I see only the last one. Why?

tomtomton..

Re: about color alpha problem

5 months ago

Thank you for all your reply.

I try to make gradient with the following code. But I could not get the 100% opaque color in the center.

How could I solve this?

int dim=400;
int i=dim;
void setup()
{
size(640, 480);
background(255);
colorMode(HSB, 360, 100, 100,100);
fill(0,99,99,1);
noStroke();
}
void draw()
{
if (i>0)
{
ellipse(320,240,i,i);
}
i--;
}

P.S. The opaque circle is added from Paint.

GoToLoop

Re: about color alpha problem

5 months ago

All colors drawn on the main surface are 100% opaque.

They are just blended in a way to be analogous to transparent ones!

You can confirm that by the initial FF pair from a color code like -> FF FF 3F 3F

FF means 100% opaque!

To reach a plateau, you have to keep laying transparent colors upon previous ones,

until the result blending is compared to an opaque color.

And like I also said before, the background() color determines how far a color can reach opaqueness!

final static short W = 320, H = 240, DIM = 400;
short dim = DIM;

void setup() {
  size(W << 1, H << 1);
  noStroke();
  background(-1);
  
  colorMode(HSB, 360, 100, 100, 100);
  fill(0, 99, 99, 1);
}

void draw() {
  if (dim-- > 0)  ellipse(W, H, dim, dim);
  print(hex( get(W, H) ) + " - ");
}

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