Amount to adjust a rectangle

Perdrix

Assume that I have a selected Rectangle in image coordinates.

To draw this I do:

void SelectRect::paintEvent(QPaintEvent*)
{
    QPainter painter(this);
    QPen pen(Qt::red, 1.0);
    QRectF rect(imageView->imageToScreen(selectRect.normalized()));
    painter.setPen(pen);
    //
    // If there's actually a selected area
    //
    if (!rect.isEmpty())
    {
        painter.drawRect(rect
            .adjusted(0, 0, -1, -1));

Which is correct for a pen width of 1.0. If however the pen width is 3.0, what should the adjustment be (I don't think (0, 0, -3, -3) is correct)?

Thanks
D.

Chris Kawa

It depends on what result you're looking for. The line grows equally to both sides i.e. for the same rectangle and width 3.0 it's gonna be 1 px inward and 1 px outward from the line of width 1.0. if you want an outline you'll have to further adjust by (width-1)/2 outward , so adjusted(-1,-1,0,0) and to grow inward adjusted(1,1,-2,-2).

It's a little wobbly for even and fractional widths so it's best to use odd values.

Perdrix

If the selection is e.g. (0,0,10,10), using the code above the drawn rectangle fits exactly inside the selection rectangle and encloses a rectangle (1, 1, 9, 9).

For larger pen sizes I think I would still want the drawn rectangle to fit exactly inside the selection rectangle.

If the pen width were 2.0 I would want the drawn rectangle to enclose a rectangle (2, 2, 8, 8).

If the pen size were 3.0 the enclosed rectangle should be (3, 3, 7, 7).

I'm sure there's a good algorithm to express that but I can't figure it out right now.

What's the normal convention for drawing a selection rect ? Inside the selection, or outside, or directly on top of?

Thanks
David

Chris Kawa

Ok, so basically you want the rectangle to grow inward. Like I said the width grows both ways so you just have to shift it inward i.e.

float halfWidth = (pen.widthF() - 1.f) / 2.f;

and then instead of

painter.drawRect(rect.adjusted(0, 0, -1, -1));

you do

painter.drawRect(rect.adjusted(halfWidth , halfWidth , -halfWidth -1 , -halfWidth -1 ));

It's floating point, so like I said - you might see some wobble for some widths due to imperfect rounding.

Perdrix

Many thanks