Yep that coverd more then enough.

I said that the width and height are not know because it will be a rectangle like the ones to select the icons etc. And at first i said them because i wanted to be more concrete.

But this helps alot.

Thank you very much

Bogdan

Thank you wcy but i don't think that will help you.

Anyway thank for trying to help

Bogdan

I don't think so. If you didn't need b to calculate a then b could take any value and a would still work out to be the same thing, so obviously the proportions would be lost.

bogdan wrote:

Click and drag the mouse. You see a rectangle chanching dimension according the mouse position. Well i need it to constrain proportionsand it's final dimenions from top left of the screen to bottom right is 800x600

To contrain proportions, can't you just say that b = (800/600) * a ?

My first attempt at programming this would be to track the mouse, and as the user moves his mouse up, down, left or right, you will need to calculate two different sizes and pick the smallest (I think that would look most natural, but you may find otherwise).

This is just straight from my fingers, and may be wrong, but is the start of an idea:

Δx = x1-x0
Δy = y1-y0

if  ( Δx > (800/600) * Δy ) { // Δx too big
    Δx = (800/600) * Δy
} else {
    Δy = (600/800) * Δx
}

Example: Rectange starts at (100,100) then user drags his mouse to (210,200):

Δx = (210-100) = 110
Δy = (200-100) = 100

Δx = 110    is less than    (800/600) * Δy  = 133.3,  so:
Δy = (600/800) * Δx = 82.5

Result:
Δx = 110
Δy = 82.5

If the user moves again to (250,270) and is still dragging, then always refer to the original point (100,100), so you would have:

Δx = (250-100) = 150
Δy = (270-100) = 170

Δx = 150    is less than    (800/600) * Δy  = 226.7,  so:
Δy = (600/800) * Δx = 112.5

Just keep recalculating with respect to the original (x,y) [when the user first left-clicked] until the user lets go of the mouse button.

All I can say is: that would be the first bit of code I would try, then based on how it behaved I would know what to do next!

Maths - I may be reading your code wrong, but I don't think that's quite the effect he was looking for.

?x = 150    is less than    (800/600) * ?y  = 226.7,  so:
?y = (600/800) * ?x = 112.5

In order to constrain the proportion to 4:3, you need to change one side in proportion to the side whose delta is greater. That way, the user's mouse stays connected with at least one side of the rectangle. So:

if (deltaX > deltaY)  {
       deltaX = (3/4) * deltaX
}
else  {
       deltaX = (4/3) * deltaY
}

However, that algorithm falls down when the user drags the mose above or to the left of the top left corner of the rectangle, since the deltas become negative. So, we need to work in terms of the coordinates of the corners, instead of the width/height of the rectangle.

Declare variables  (xTL, yTL);  (xBR, yBR);  to hold the coordinates of the top left and bottom right corners of the rectangle. xM and yM refer to the position of the mouse. The deltas will also make an appearance.

if (xM > xBR)  {
     deltaX = xM - xBR;
     deltaY = yM - yBR;
     
     if (deltaX > deltaY)  { xBR = xM;  yBR = (3/4) * xM; }
     else  { xBR = (4/3) * yM;  yBR = yM; }

     if (yM < yTL)  {  //The user is above the top left; invert!
          yBR -= 2 * (yBR - yTL);
     }
}

if ((xM < xBR) && (yM > yBR))  {
     //The user is below and to the left of the bottom right
     //Adjust yBR and xBR accordingly, and
     //make sure to check for the user being to the left of xTL
}

And keep going until you've covered every possible situation. Sorry I ran out of time and couldn't finish it.