Scaling problem : newbie ahead :)

[MiguelSanches]

So let’s go over the steps:

1. You draw a rectangle at corners (c1, c2), with dimensions (w1,h1) (width and height respectively)
2. The center of that same rectangle is the point (c1 + w1/2 , c2 + h1/2)
3. Let’s say now that you want the inner rectangle to have dimensions w2, h2
4. To find the corner and maintain the same center you need to go -w2/2, -h2/2

Thus the corner of the rectangle would need to be
(c1 + w1/2 -w2/2 , c2 + h1/2-h2/2)

So the code became:

void setup() {
  size(600, 600);
}
void draw() {
  background(240);
  noStroke();
  // draw a (blue) rectangle
  int w = 200;
  int h = 200;
  int c1 = 0;
  int c2 = 0;

  fill(0, 0, 255);
  rect(c1, c2, w, h, 10, 10, 10, 10);

  // from now on - no Fill
  noFill();

  // draw a second smaller rectangle
  // so let's assume s in ]0, 1[
  float s = 0.75;
  //SO find the dimensions of width and height
  float w2 = s * w;
  float h2 = s * h;

  stroke(255);
  strokeWeight(3);
  // w - s*s is the x-difference length between both rect
  float tx = c1 + w/2 - w2/2;
  float ty = c2 + h/2 - h2/2;
  //scale(s);
  translate(tx, ty);
  rect(0, 0, w2, h2, 10, 10, 10, 10);
}

Result:

Best regards!