<![CDATA[Amount to adjust a rectangle]]>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.

]]>https://forum.qt.io/topic/138535/amount-to-adjust-a-rectangleRSS for NodeMon, 14 Sep 2026 05:41:56 GMTThu, 11 Aug 2022 16:23:38 GMT60<![CDATA[Reply to Amount to adjust a rectangle on Fri, 12 Aug 2022 09:19:24 GMT]]>Many thanks

]]>https://forum.qt.io/post/724631https://forum.qt.io/post/724631Fri, 12 Aug 2022 09:19:24 GMT<![CDATA[Reply to Amount to adjust a rectangle on Fri, 12 Aug 2022 09:10:48 GMT]]>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.

]]>https://forum.qt.io/post/724630https://forum.qt.io/post/724630Fri, 12 Aug 2022 09:10:48 GMT<![CDATA[Reply to Amount to adjust a rectangle on Fri, 12 Aug 2022 09:01:06 GMT]]>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

]]>https://forum.qt.io/post/724629https://forum.qt.io/post/724629Fri, 12 Aug 2022 09:01:06 GMT<![CDATA[Reply to Amount to adjust a rectangle on Thu, 11 Aug 2022 19:33:10 GMT]]>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.

]]>https://forum.qt.io/post/724586https://forum.qt.io/post/724586Thu, 11 Aug 2022 19:33:10 GMT