[Development] Issues with QPainter::drawPolygon (off by one error?)
Henry Skoglund
henry at tungware.se
Sun Apr 28 11:39:42 CEST 2024
On 2024-04-28 11:00, Christian Ehrlicher via Development wrote:
>
> Am 26.04.2024 um 22:48 schrieb Henry Skoglund:
>> On 2024-04-26 21:52, Christian Ehrlicher via Development wrote:
>>> Hello,
>>>
>>> I'm currently trying to investigate a painting problem within the
>>> windowsvista / common style:
>>>
>>> QImage img(7, 7, QImage::Format_ARGB32_Premultiplied);
>>> QPolygon poly{ QPoint(1, 1), {5, 1}, {3, 5} };
>>> QPainter p(&img);
>>> p.setPen(Qt::NoPen);
>>> p.setBrush(QColor(Qt::black));
>>> p.drawPolygon(poly);
>>> p.setPen(QColor(0, 255, 0, 128));
>>> p.drawPoints(poly.data(), poly.size());
>>> p.end();
>>> img.save("output.png");
>>> ...
>>>
>> Hi, I think the QPainter draws very poorly using direct polygon paths.
>> You could try going via a QPainterPath, say:
>>
>> QImage img(7, 7, QImage::Format_ARGB32_Premultiplied);
>> img.fill(QColor(0, 255, 0, 128)); // to be sure the img
>> contains no junk values
>> QPolygon poly{ QPoint(1, 1), {5, 1}, {3, 5} };
>> QPainter p(&img);
>> p.setPen(Qt::NoPen);
>> p.setBrush(QColor(Qt::black));
>> p.setPen(QColor(0, 255, 0, 128));
>> QPainterPath path;
>> path.addPolygon(poly);
>> p.drawPath(path);
>> p.end();
>> img.save("output.png");
>>
>> Rgrds Henry
>>
>
> This helped a little bit but it then fails again for other sizes. It's a
> mess :(
> I'm shortly before drawing the triangles pixel by pixel by myself...
>
If you close the polygon by adding the first point again, it will help
to draw a better triangle:
QImage img(7, 7, QImage::Format_ARGB32_Premultiplied);
img.fill("white"); // clear the scene
QPolygon poly{ QPoint(1, 1), {5, 1}, {3, 5}, {1, 1} }; // added a
point
QPainter p(&img);
p.setPen(Qt::NoPen);
p.setBrush(QColor(Qt::black));
p.setPen(QColor(0, 255, 0, 128));
QPainterPath path;
path.addPolygon(poly);
p.drawPath(path);
p.end();
img.save("output.png");
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