Why isn't QPainterPath::intersects giving me the expected result?

[blooglet]

I made the following test code for the QPainterPath::intersects test function. I wanted a way to detect if a point is in the vicinity (let's say 2 pixels) of a line stored in a QPainterPath.

QPainterPath path;
path.moveTo(0, 0);
path.lineTo(0, -20);
path.lineTo(40, -20);
path.lineTo(40, -95);
int testX = 4;
int testY = -17;
QRectF rect(testX - 2, testY - 2, 4, 4);
bool intersects = path.intersects(rect);
qDebug() << "Intersects?" << ((intersects) ? "true" : "false");
qDebug() << "Rect: x =" << rect.x() << "| y =" << rect.y() << "| w =" << rect.width() << "| h =" << rect.height();

QPainterPath path;
path.moveTo(0, 0);
path.lineTo(0, -20);
path.lineTo(40, -20);
path.lineTo(40, -95);
int testX = 4;
int testY = -17;
QRectF rect(testX - 2, testY - 2, 4, 4);
bool intersects = path.intersects(rect);
qDebug() << "Intersects?" << ((intersects) ? "true" : "false");
qDebug() << "Rect: x =" << rect.x() << "| y =" << rect.y() << "| w =" << rect.width() << "| h =" << rect.height();

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This produces the following output:

Intersects? true
Rect: x = 2 | y = -19 | w = 4 | h = 4

I'd expect the intersects function to return false in this case because no point in the rectangle overlaps with the drawn line. Am I using the function incorrectly?

[koan]

I think that your rectangle is top left -> bottom right = (2,-19) -> (6, -23) and therefore intersects at (2,-20) and (6,-20). I think you are assuming that from the start point of the rectangle it is drawn in a positive direction but Qt is drawing from a top down point of view.

[MarekR22]

But you point is inside! Take look on a shape! last sector intersects with second sector of path it looks like you mistyped a shape data.

[blooglet]

I think you are assuming that from the start point of the rectangle it is drawn in a positive direction but Qt is drawing from a top down point of view.

Wait... so a QRectF with x = 2, y = -19, w = 4 and h = 4 doesn't stretch to (6, -15) but rather (6, -23)? So the height is counted towards lower Y values? I'm confused.

[wysota]

Why guess? Draw both the path and the rect using QPainter and you'll see what gets drawn where.

[blooglet]

Here's the reference drawing I obtained.

QPainterPath path;
path.moveTo(0, 0);
path.lineTo(0, -20);
path.lineTo(40, -20);
path.lineTo(40, -95);
int testX = 4;
int testY = -17;
QRectF rect(testX - 2, testY - 2, 4, 4);
bool intersects = path.intersects(rect);
qDebug() << "Intersects?" << ((intersects) ? "true" : "false");
qDebug() << "Rect: x =" << rect.x() << "| y =" << rect.y() << "| w =" << rect.width() << "| h =" << rect.height();
 
scene->addPath(path);
scene->addRect(rect);

QPainterPath path;
path.moveTo(0, 0);
path.lineTo(0, -20);
path.lineTo(40, -20);
path.lineTo(40, -95);
int testX = 4;
int testY = -17;
QRectF rect(testX - 2, testY - 2, 4, 4);
bool intersects = path.intersects(rect);
qDebug() << "Intersects?" << ((intersects) ? "true" : "false");
qDebug() << "Rect: x =" << rect.x() << "| y =" << rect.y() << "| w =" << rect.width() << "| h =" << rect.height();

scene->addPath(path);
scene->addRect(rect);

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Intersects? true
Rect: x = 2 | y = -19 | w = 4 | h = 4

They don't intersect in the drawing, do they?

[wysota]

"It depends". If the path is treated as a closed path then they intersect. Try moving the rectangle to Y>0 (e.g. x=2, y=1, w=4, h=4) and see if it is still considered intersecting with the path.

[blooglet]

It appears that the rectangle is considered "not intersecting" when it is outside the bounding rectangle of the painter path. This is not the behavior I want. I want to check if the rectangle overlaps with the line. Am I using the wrong method?

[wysota]

The docs say:

Set operations on paths will treat the paths as areas. Non-closed paths will be treated as implicitly closed.

I don't think there is a method in QPainterPath that does directly what you want.

[blooglet]

Does Qt provide a way to, given a path consisting of lines that isn't necessarily closed and a mouse coordinate, calculate if I clicked on the path? I then want to extend this concept to "in the vicinity of the path" by using a QRectF instead of a QPointF.

[wysota]

This is a mathematical problem that can easily be solved without using any dedicated frameworks. Each line conforms to the ax+by+c = 0 equation. Using coordinates of ends of the lines you can calculate a, b and c and then it is easy to compute the solution by substituting x and y with coordinates of the point you wish to check for intersection. It is similar when talking about intersection of lines. And of course there is QLineF::intersect() to help you.