Question

I do have rectangle, which had information about topx, topy, width and height.

I want to scale this rectangle based on an origin other than top-left. Is there already existing algo to do that ?

Currently I work on Eclipse GEF & SWT. In GEF, the all rectangle operations are assumed that top-left is where the drawing starts and they scale/resize from top-left. But I want to do scale/resize from center.

eg : my rectangle have info like {100,100,50,50}. If I do scaling of 1.5 in both x&y from top-left I’ll get the resultant rectangle as {100,100,100,100} ( First two are x,y and rest are width,height).

Thanks
J

Answer 1

My definition of scale is different from yours because if I scale by 1.5 from the top left my resulting rectangle would be {100, 100, 75, 75} -> the origin stays the same and the size of each side is multiplied by the scale.

Using these definitions, if (x, y) are the top left co-ordinates of the rectangle, scaling from the center and keeping the origin constant:
{x, y, width, height} -> { x + width * (1 – scale)/2, y + height * (1 – scale)/2, width * scale, height * scale}

I suggest scale > 0 although the result is defined for zero and negative values.

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Worked example: Scale {100, 100, 50, 50} by 1.5 from the center.

x: 100 -> 100 + 50 * (1 - 1.5)/2 = 100 + 50 * (-0.5)/2 = 100 - 50/4 = 87.5
y: 100 -> 100 + 50 * (1 - 1.5)/2 = 100 + 50 * (-0.5)/2 = 100 - 50/4 = 87.5
width:  50 -> 50 * 1.5 = 75
height: 50 -> 50 * 1.5 = 75

Result: {100, 100, 50, 50} -> {87.5, 87.5, 75, 75}