Processing 1.0 - Processing Discourse

We closed this forum 18 June 2010. It has served us well since 2005 as the ALPHA forum did before it from 2002 to 2005. New discussions are ongoing at the new URL http://forum.processing.org. You'll need to sign up and get a new user account. We're sorry about that inconvenience, but we think it's better in the long run. The content on this forum will remain online.

Index › Programming Questions & Help › Syntax Questions › help with circle

‹ Previous Topic | Next Topic ›

Pages: 1

help with circle (Read 677 times)

elictricocean YaBB Newbies Offline Posts: 2	help with circle Nov 11^th, 2006, 10:07pm Hi, I wondering if there was a way to make a circle with a different color for every pixel within the circle. Would I need trig for this? I am not in that high of a math class yet... elictricocean

rick.giles

Re: help with circle
Reply #1 - Nov 12^th, 2006, 2:14pm

Suppose the circle has center (cx, cy) and radius r.

1. Call function loadPixels() to load the pixel data for the display window into the system variable pixels[] array.
2. For each value of y that intersects the circle, and for each value of x such that (x, y) is in the circle, set the color of the corresponding element of pixels[].
3. Call function updatePixels to update the display window with the current data from pixels[].

Write nested for statements in step 2. y has values between (cy - r) and (cy + r). For each value of y, x has values between (cx - w) and (cx + w), where w = sqrt(r * r + sq(cy - y)). The element of pixels[] that corresponds to (x, y) has index (x + y * width). You do have to check that the pixels are in the window.

/Rick

st33d

Re: help with circle
Reply #2 - Nov 12^th, 2006, 4:42pm

How about cheating?
Code:


void setup(){
  size(200, 200);
  background(200);
  ellipse(100, 100, 200, 200);
  smooth();
  loadPixels();
  for(int i = 0; i < pixels.length; i++){
    if(pixels[i] != color(200)){
	pixels[i] = color(random(255), random(255), random(255));
    }
  }
  updatePixels();
}

elictricocean YaBB Newbies Offline Posts: 2	Re: help with circle Reply #3 - Nov 12^th, 2006, 9:29pm thanks. i think I like the first way better. If you could explain it a little more that would be great thanks. elictricocean

rick.giles

Re: help with circle
Reply #4 - Nov 12^th, 2006, 10:24pm

Drawing a picture for step 2 helps. Draw a circle with center (cx, cy) and radius r. Draw a horizontal line for a value of y that intersects the circle at points (x1, y) and (x2, y), where
x1 <= x2.

x1 is (cx - w) and x2 is (cx + w), where w = sqrt(r * r + sq(cy - y)) (Pythagoras' Theorem)

So, for every value of y such that the horizontal line at that value of y intersects the circle, you need to draw the pixels in the span between x1 and x2.

Code:


// determine y1, and y2, the minimum and maximum values of y
// such that the line at y intersects the circle and is
// in the window
for (int y = y1; y < y2; y++) {
  // compute w
  // determine x1 and x2, the minimum and maximum values
  // of x such that (x, y) is inside the circle and
  // is in the window
  for (int x = x1; x < x2; x++) {
    // assign a color to the pixel at (x, y)
  }
}

/Rick

Pages: 1

‹ Previous Topic | Next Topic ›