Rendering a large QGraphicsScene on a QImage clips it off - qt

I am creating some images rendering the contents of a QGraphicsScene.
My project requirement is that it should handle a canvas size of 10 ft by 8 inches. On screen, and scene size, that is 8640 x 576 pixels.
I can render it fine.
The thing is, the output images need to have 300 resolution.
That means, the rendered image will have a width of 36000, which is over 2^15 - 1 = 32767 pixels.....
The output is clipped - in the code below, I would get a QImage of correct expected size (36000) but the QGraphicsScene only renders to 32767 pixels.
That is confusing... I cannot explain the outcome - if the QImage limitations were 32767 pixels, then I should not be able to create one in the first place. But I checked and the QImage "sanity check" is much higher.
Once the image is created, I do not see anything in the code for rendering QGraphicsScene that would clip at any value....
This is a simple code that is trying to expose my problem.
It creates a QImage of required size, and fills with yellow (for control).
Then it renders a QGraphicsScene with blue background brush and a red rectangle close to the right margin.
If it works correctly, the result should be: an image of width 36000, blue with a tiny red rectangle at the far right.
But... as it is, the result is an image of width 36000, blue for the first 32766 pixels then yellow for the rest, no red rectangle.
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsRectItem>
#include <QPainter>
void printScene(QGraphicsScene* s, qreal ratio) {
qreal w = s->width() * ratio;
qreal h = s->height() * ratio;
QRectF target(0, 0, w, h);
QImage image = QImage(w, h, QImage::Format_ARGB32_Premultiplied);
image.fill(QColor(Qt::yellow).rgb());
QPainter painter;
painter.begin(&image);
s->render(&painter, target);
painter.end();
image.save("image.png");
}
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
QGraphicsScene s;
s.setSceneRect(0, 0, 8640, 576);
s.setBackgroundBrush(Qt::blue);
QGraphicsView view(&s);
view.show();
QGraphicsRectItem* r = s.addRect(8530, 250, 100, 100);
r->setBrush(Qt::red);
qreal ratio = 300/72.;
printScene(&s, ratio);
return app.exec();
}
As seen in sample images, the QImage is created successfully, QGraphicsScene though only renders to 2^15 - 1... But I stepped through their code and I didn't see it stop....
(I also tried creating the original scene 36000 x something (and setting the ratio to 1), and it displays fine... it just won't render to QImage anything beyond 32767 pixels)
Am I missing some setting ? What could be the cause of the QGraphicsScene::render() to not render more ?
I would love to find out how I can render the size I want - width of 36000 pixels - or a reason why this is not possible.
I am running this in Windows 7, 32 bit Qt 5.5.1 or 4.7.4

I have found the reason for the clipping - and imagined 2 workarounds.
Why:
Stepping through the rendering code, the clip rect gets limited to 32767:
bool QRasterPaintEngine::setClipRectInDeviceCoords(const QRect &r, Qt::ClipOperation op)
{
Q_D(QRasterPaintEngine);
QRect clipRect = r & d->deviceRect;
...
}
Where deviceRect is set by
void QRasterPaintEnginePrivate::systemStateChanged()
{
deviceRectUnclipped = QRect(0, 0,
qMin(QT_RASTER_COORD_LIMIT, device->width()),
qMin(QT_RASTER_COORD_LIMIT, device->height()));
QRegion clippedDeviceRgn = systemClip & deviceRectUnclipped;
deviceRect = clippedDeviceRgn.boundingRect();
baseClip->setClipRegion(clippedDeviceRgn);
...
}
and
// This limitations comes from qgrayraster.c. Any higher and
// rasterization of shapes will produce incorrect results.
const int QT_RASTER_COORD_LIMIT = 32767;
Options:
1) Render to a max of 32767 and, if the target must be bigger, scale result. (should give slightly lower quality)
2) Create 2 images and combine them (I still need to figure that out but I think it is the better fix)

Related

Setting custom paper size with QPrinter doesn't print correctly

I need to be able to print from qt (the rendered contents of a QGraphicsScene, or a QImage), to scale, on normal printer, pdf, but also on custom printers, including roll fed.
It seems that anything that works for standard printers fails on custom printers, and the reverse.
I have made the printing work as expected on custom printers now (going back and forth between what works in the different printers).
I set the custom size required, and the preferred orientation, based on length/width ratio.
I open a print dialog (and even check the supply- the paper is set to desired size, and the orientation is set as expected)
print:
On custom printer, I get the correct size, and if the supply is smaller, the print clips as needed. margins are set correctly too.
On Pdf, I get a document of custom size as requested, printed correctly - but the orientation is not respected !!! (even though print dialog showed it correctly) - see image
On HP printer, I get a white page - nothing gets printed.
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsRectItem>
#include <QPrinter>
#include <QPrintDialog>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
// some scene to print - show rectangle for easy measure
QGraphicsScene* s = new QGraphicsScene();
s->setSceneRect(-500, -500, 1500, 1500);
s->setBackgroundBrush(Qt::red);
QGraphicsView* view = new QGraphicsView();
view->setScene(s);
view->show();
qreal in = 72;
QRectF canvasRect(-0.1*in, -0.1*in, 6*in, 4*in);
qreal margins = 0.1*in;
QRectF actualCanvasRect = canvasRect.adjusted(margins,margins,-margins,-margins);
// this is to show actual scene
QGraphicsRectItem* contourItem = new QGraphicsRectItem(actualCanvasRect);
contourItem->setBrush(Qt::blue);
s->addItem(contourItem);
// an item partially on canvas (so I can check margins)
QGraphicsRectItem* item = new QGraphicsRectItem(-.5*in, -in, 2*in, 3*in);
item->setBrush(Qt::yellow);
s->addItem(item);
// actual printing:
// print the scene, to scale, using user margins, on given printer
QPrinter printer;
QPrinter::Orientation orient = (actualCanvasRect.width() >
actualCanvasRect.height() ?
QPrinter::Landscape : QPrinter::Portrait);
printer.setOrientation(orient);
printer.setPaperSize(canvasRect.size(), QPrinter::Point);
printer.setPageMargins(margins, margins, margins, margins, QPrinter::Point);
QPrintDialog printDialog(&printer);
if (printDialog.exec() != QDialog::Accepted)
{
qDebug("dialog canceled");
return 1;
}
QPainter painter;
if (! painter.begin(&printer))
{
qDebug("failed to open printer");
return 1;
}
// render the contents, clipped to printer page size, and scaled from point to device pixel
QRectF source = actualCanvasRect;
// convert target rect to DevicePixel and clip to page
QRectF page = printer.pageRect(QPrinter::DevicePixel);
qreal scale = printer.resolution()/in;
QRectF target = QRectF(page.topLeft(), source.size() * scale);
target &= page; // clip target rect to page
// clip source rect to page - without this, if printer paper is smaller I get unwanted scaling
source &= printer.pageRect(QPrinter::Point);
s->render(&painter, target, source);
painter.end();
return app.exec();
}
I don't understand why pdf creates a portrait page even though I clearly requested landscape (without changing the print dialog: see image). (the width and height are reversed, yet correct - Document Properties shows 4x6, and the page attempts to print correctly and to scale)
More important, I don't understand why a typical laser jet printer prints nothing - blank page - or sometimes for a simple canvas it scales to fit.
BUT if I change any properties in the print dialog from HP, anything irrelevant (like paper source, or paper type.... ), it prints correctly.
What am I doing wrong ?
(using Qt 4.7 and 5.5, must work on 4.7 - Windows, have yet to try Linux)

Saving a QGraphicsScene to Svg changes scaling

I need to save the items from my QGraphicsScene to an svg, and be able to load that svg back on the scene.
I can do it...
But each time the canvas is saved to svg, upon load the items are somewhat bigger (and repeatedly saving and loading the same svg causes it to grow).
I can't find the cause.
I am attaching a sample code - and the result.
test1.pro
QT += gui svg
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets core
TARGET = test1
TEMPLATE = app
SOURCES += \
svggenerator.cpp
svggenerator.cpp
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsEllipseItem>
#include <QGraphicsSvgItem>
#include <QSvgGenerator>
#include <QSvgRenderer>
#include <QFile>
#include <QByteArray>
#include <QMessageBox>
void saveSceneToSvg(QGraphicsScene* s, const QString &filename) {
QRectF newSceneRect;
QGraphicsScene *tempScene = new QGraphicsScene(s->sceneRect());
tempScene->setBackgroundBrush(QBrush(Qt::transparent));
tempScene->setItemIndexMethod(QGraphicsScene::BspTreeIndex);
foreach(QGraphicsItem* item, s->items()) {
newSceneRect |= item->mapToScene(item->boundingRect()).boundingRect();
tempScene->addItem(item);
}
tempScene->setSceneRect(newSceneRect);
tempScene->clearSelection();
QSize sceneSize = newSceneRect.size().toSize();
QSvgGenerator generator;
generator.setFileName(filename);
generator.setSize(sceneSize);
generator.setViewBox(QRect(0, 0, sceneSize.width(), sceneSize.height()));
generator.setDescription(QObject::tr("My canvas exported to Svg"));
generator.setTitle(filename);
QPainter painter;
painter.begin(&generator);
tempScene->render(&painter);
painter.end();
tempScene->clear();
delete tempScene;
}
void loadSvg(QGraphicsScene* s, const QString &filename, const QPointF& p) {
QGraphicsSvgItem* item = new QGraphicsSvgItem();
QFile file(filename);
file.open(QFile::ReadOnly);
QByteArray contents = file.readAll();
item->setSharedRenderer(new QSvgRenderer(contents));
file.close();
item->setPos(p);
s->addItem(item);
}
void processScene(QGraphicsScene* s) {
QGraphicsEllipseItem* eli = new QGraphicsEllipseItem();
eli->setRect(QRectF(0, 0, 100, 100));
eli->setPen(Qt::NoPen);
eli->setBrush(Qt::red);
eli->setPos(100, 300);
s->addItem(eli);
QGraphicsEllipseItem* eli1 = new QGraphicsEllipseItem();
eli1->setRect(QRectF(0, 0, 100, 100));
eli1->setPen(Qt::NoPen);
eli1->setBrush(Qt::yellow);
eli1->setPos(150, 300);
s->addItem(eli1);
QMessageBox::information(NULL, "hi", "click");
saveSceneToSvg(s, "abcd.svg");
loadSvg(s, "abcd.svg", QPointF(100,300));
QMessageBox::information(NULL, "hi", "click");
saveSceneToSvg(s, "abcd1.svg"); // saved with a dif name so I can see
loadSvg(s, "abcd1.svg", QPointF(100,300));
QMessageBox::information(NULL, "hi", "click");
saveSceneToSvg(s, "abcd2.svg");
loadSvg(s, "abcd2.svg", QPointF(100,300));
// .... each time i call them they grow larger
}
int main(int argc, char *argv[]) {
QApplication app(argc, argv);
QGraphicsScene s;
s.setSceneRect(50, 0, 1000, 800);
QGraphicsView view(&s);
view.show();
processScene(&s);
return app.exec();
}
Result:
Looking at the svgs themselves, I can see that the svgs increase in size by approx 1.25... I can't explain, and can't be sure this will be true for other examples. (it seems to)
What is causing this growth ? How can I stop it ?
(Also I notice the ordering is different ... I just noticed and that is a different problem... But since in my "real" code I also have z order I don't care.)
Saving I think is fine - the resulting svg has size and view box of expected size.
Loading other svgs is fine - saving an svg from outside source creates an svg similar in size.
It seems the problem is when I am loading an svg created by the svg generator that it increases in size.
(If I could be sure it is always the case I could try scaling it down on load, but the ratio is not exactly 1.25 each time, close though... and I don't know how to tell the difference between an outside svg and a generated one).
It looks like a bug of SvgGenerator. The size which you provide to SVG generator is used only to form the header of svg. The actual size of .svg file differs from one written in the header. The only workaround I found is simple 25 percent decrease of size on save similar to:
int width = qCeil(qreal(sceneSize.width()/1.25));
int height = qCeil(qreal(sceneSize.height()/1.25));
generator.setSize(QSize(width, height));
generator.setViewBox(QRect(0, 0, width, height));

QGraphicsItemGroup moves during transformations

I have QGraphicsItems on a QGraphicsScene. I need to group them. The main reason for grouping: I need to provide a way to apply transformations on a group.
It is all very simple if I need to apply a single transformation, and the items bounding rectangle top-left is at 0,0.
I have seen how grouping and ungrouping works, preserving items' transformations relative to the scene, and that is great. But on grouping, no matter what positions my items have, the group has coordinates (0,0).
When I try any transformations, no matter what I tried with the transformation origin point, the item moves.
Simple program to show:
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsRectItem>
#include <QDebug>
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QGraphicsScene s;
QGraphicsView view(&s);
s.setSceneRect(90, -10, 420, 140);
view.show();
QGraphicsRectItem* sceneR = new QGraphicsRectItem(s.sceneRect());
s.addItem(sceneR);
QTransform t; // simple scaling transform
t.scale(2, 1); // adding rotation scares me
QGraphicsRectItem* r0 = new QGraphicsRectItem(QRectF(0, 0, 150, 100)); // for compare, not scaled
r0->setBrush(QBrush(Qt::red));
r0->setPos(100, 0);
s.addItem(r0);
QGraphicsRectItem* r1 = new QGraphicsRectItem(QRectF(0, 0, 150, 100)); // for compare, not scaled
r1->setBrush(QBrush(Qt::blue));
r1->setPos(100, 10);
s.addItem(r1);
QGraphicsRectItem* r2 = new QGraphicsRectItem(QRectF(0, 0, 150, 100)); // will make a group of a single item
r2->setFlags(QGraphicsItem::ItemIsSelectable);
r2->setBrush(QBrush(Qt::green));
r2->setPos(100, 20);
r2->setSelected(true);
s.addItem(r2);
QGraphicsItemGroup* g = s.createItemGroup(s.selectedItems());
// QPointF topLeft = g->mapToScene(g->boundingRect().topLeft());
// g->setTransformOriginPoint(topLeft);
r1->setTransform(t);
g->setTransform(t);
qDebug() << r1->pos() << r1->transform();
qDebug() << g->pos() << g->transform();
return app.exec();
}
If I apply transformations, the grouped item moves. I have not been able to figure out a logic or how to compensate.
(red = reference, not scaled; blue, reference, scaled; green, grouped and scaled)
In image above I expect the green rectangle to have the same x coordinate as the blue (I shifted vertically to see what is going on)
How can I make my scaling (and rotation) in place ?
(I am considering the alternative of just setting a common parent, and performing all the work to retrieve items transformations, but not sure how to combine them properly, especially scaling a group how to find the new position and new scale of inside items)
Found a solution - even though I am counting on the group to perform item transformations and calculate its own edges, I must do it manually as well.
Steps:
I find the top-left most point (minX, minY) of selected items' bounding rectangles (mapped to scene),
then move all items by -minX, -minY,
then after grouping, move group by minX, minY.
This sets a correct center of transformations, so the group item behaves like regular items. It also reports a pos() in its top left corner, similar to the default pos() of other QGraphicsItems.

Resizing and rotating a QGraphicsItem results in odd shape

I can't understand how scaling and rotation are applied to QGraphicsItem.
I need to be able to apply rotation and scaling (not necessarily keeping aspect ratio) and I get fully unexpected results.
Rotation must be around the item center. I seem to have no problem doing that - yet if I try to debug the bounding rectangle, I get seemingly wrong values.
If I don't keep aspect ratio, instead of rotation I get a very weird skew, and I have been struggling for quite a while to find the cause and correct it. I hope anybody can find a solution.
For many items - like rectangles - my solution was to give up on resize - and just replace the item with a new one of given size. I even did that for pixmap (though it probably will affect performance a lot).
But I don't know how to do that for text, or a few other types (svg...).
I am trying to understand how scaling is applied, on rotated items, and how to get it applied correctly.
The code below is an experiment where I scale and rotate a text item, and the result... see attached image
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsTextItem>
void experimentScaling(QGraphicsScene* s)
{
QGraphicsTextItem* ref = new QGraphicsTextItem(); // a reference, not resized
ref->setPlainText("hello world");
s->addItem(ref);
ref->setDefaultTextColor(Qt::red);
ref->setRotation(45);
QGraphicsTextItem* t = new QGraphicsTextItem(); // text item to be experimented on
t->setPlainText("hello world");
s->addItem(t);
QTransform transform; // scale
transform.scale(10, 1);
t->setTransform(transform);
t->update();
QPointF _center = t->boundingRect().center();
qDebug("%f %f %f %f", t->boundingRect().left(), t->boundingRect().top(), t->boundingRect().right(), t->boundingRect().bottom()); // seems to be unscaled...
t->setTransformOriginPoint(_center); // rotation must be around item center - and seems to work even though the bounding rect gives wrong values above
t->setRotation(45); // skewed
t->update();
}
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QGraphicsScene s;
QGraphicsView view(&s);
s.setSceneRect(-20, -20, 800, 600);
view.show();
experimentScaling(&s);
return app.exec();
}
Reference (red) text rotated 45 degrees, text rotated 45 degrees and resized 10,1:
The resized (black) text should have the same height as the reference (red) - yet is much taller;
The bounding rectangle is no longer a rectangle - it is skewed;
The angle looks much smaller than 45;
Added a resized but not rotated reference as well:
Please help me understand why this behavior is happening and what can I do about it.
I have tried looking into QGraphicsRotation but I can't figure out how to apply it... All I get is a move instead of rotation.
As documented, the item's transformations are mathematically applied in a certain order - this is the order you'd be multiplying the transform matrices in and is, conceptually, the reverse of the order you'd normally think of.
The transform is applied. The origin point must be included in the transform itself, by applying translations during the transform.
The transformations are applied - each of them can specify its own center.
rotation then scale are applied, both relative to transformOriginPoint.
When you set transform to scaling, and set rotation, the rotation is performed before scaling. The scaling applies to the rotated result - it simply stretches the rotated version horizontally in your case.
You need to somehow enforce the reverse order of operations. The only two ways to do that are:
Stack the transforms in correct order and pass them to transform, or.
Pass a list of correct transformations to transformations.
I'll demonstrate how to do it either way, in an interactive fashion where you can adjust the transform parameters using sliders.
To obtain the correct result using transform:
QGraphicsItem * item = ....;
QTransform t;
QPointF xlate = item->boundingRect().center();
t.translate(xlate.x(), xlate.y());
t.rotate(angle);
t.scale(xScale, yScale);
t.translate(-xlate.x(), -xlate.y());
item->setTransform(t);
To obtain the correct result using transformations:
QGraphicsItem * item = ....;
QGraphicsRotation rot;
QGraphicsScale scale;
auto center = item->boundingRect().center();
rot.setOrigin(QVector3D(center));
scale.setOrigin(QVector3D(center()));
item->setTransformations(QList<QGraphicsTransform*>() << &rot << &scale);
Finally, the example:
// https://github.com/KubaO/stackoverflown/tree/master/questions/graphics-transform-32186798
#include <QtWidgets>
struct Controller {
public:
QSlider angle, xScale, yScale;
Controller(QGridLayout & grid, int col) {
angle.setRange(-180, 180);
xScale.setRange(1, 10);
yScale.setRange(1, 10);
grid.addWidget(&angle, 0, col + 0);
grid.addWidget(&xScale, 0, col + 1);
grid.addWidget(&yScale, 0, col + 2);
}
template <typename F> void connect(F && f) { connect(f, f, std::forward<F>(f)); }
template <typename Fa, typename Fx, typename Fy> void connect(Fa && a, Fx && x, Fy && y) {
QObject::connect(&angle, &QSlider::valueChanged, std::forward<Fa>(a));
QObject::connect(&xScale, &QSlider::valueChanged, std::forward<Fx>(x));
QObject::connect(&yScale, &QSlider::valueChanged, std::forward<Fy>(y));
}
QTransform xform(QPointF xlate) {
QTransform t;
t.translate(xlate.x(), xlate.y());
t.rotate(angle.value());
t.scale(xScale.value(), yScale.value());
t.translate(-xlate.x(), -xlate.y());
return t;
}
};
int main(int argc, char **argv)
{
auto text = QStringLiteral("Hello, World!");
QApplication app(argc, argv);
QGraphicsScene scene;
QWidget w;
QGridLayout layout(&w);
QGraphicsView view(&scene);
Controller left(layout, 0), right(layout, 4);
layout.addWidget(&view, 0, 3);
auto ref = new QGraphicsTextItem(text); // a reference, not resized
ref->setDefaultTextColor(Qt::red);
ref->setTransformOriginPoint(ref->boundingRect().center());
ref->setRotation(45);
scene.addItem(ref);
auto leftItem = new QGraphicsTextItem(text); // controlled from the left
leftItem->setDefaultTextColor(Qt::green);
scene.addItem(leftItem);
auto rightItem = new QGraphicsTextItem(text); // controlled from the right
rightItem->setDefaultTextColor(Qt::blue);
scene.addItem(rightItem);
QGraphicsRotation rot;
QGraphicsScale scale;
rightItem->setTransformations(QList<QGraphicsTransform*>() << &rot << &scale);
rot.setOrigin(QVector3D(rightItem->boundingRect().center()));
scale.setOrigin(QVector3D(rightItem->boundingRect().center()));
left.connect([leftItem, &left]{ leftItem->setTransform(left.xform(leftItem->boundingRect().center()));});
right.connect([&rot](int a){ rot.setAngle(a); },
[&scale](int s){ scale.setXScale(s); }, [&scale](int s){ scale.setYScale(s); });
right.angle.setValue(45);
right.xScale.setValue(3);
right.yScale.setValue(1);
view.ensureVisible(scene.sceneRect());
w.show();
return app.exec();
}
I was able to make that work by using two separate QTransforms and multiplying them together. Note that the order of transformations matter:
QTransform transform1;
transform1.scale(10, 1);
QTransform transform2;
transform2.rotate(45);
t->setTransform(transform1 * transform2);

Adjusting QPen thickness when scaling QGraphicsView?

One amazing feature of QGraphicsView is its ability to scale itself with its scene's content (every QGraphicsItem inserted in the scene actually). The QPixmap that I have inserted scales correctly, meaning that if I provide a scale factor of 4x4 with this:
view->scale(4,4);
Pixmap are zoomed as I want to do.
But this is not the case of the rects that I am used to drawing; they aims to surrounds the pixmaps that I draw on my scene and regardless of the scale factor, they keep a thickness of 1 instead of - I guess - 4.
I have been searching documentation about all of that stuff, trying to figure out the exact purpose of "cosmetics pen", but I still can't manage to make my rectangle go thicker.
Last notice: I have a custom QGraphicsItem and the QPen which is used to draw the rectangled is instanciated on-the-fly in the
virtual void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
method.
Does it matter?
Thanks in advance and apologies for my lack of experience / knowledge in both the Qt framework and the drawing algorithms fields...
It doesn't really matter where you instantiate the QPen.
QPen has a default width of 0. This is a special value that means cosmetic is true and the width is actually 1. So if you don't want the pen to be cosmetic you have to set it to the desired width. You might also need to set cosmetic to false.
Here is a simple example:
#include <QtGui>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QGraphicsView view;
QGraphicsScene scene;
QGraphicsRectItem *item1 = scene.addRect(20, 20, 20, 20);
QGraphicsRectItem *item2 = scene.addRect(50, 20, 20, 20);
QPen pen1, pen2;
pen1.setWidth(5);
pen2.setWidth(5);
pen1.setCosmetic(true);
pen2.setCosmetic(false);
item1->setPen(pen1);
item2->setPen(pen2);
view.setScene(&scene);
view.scale(4, 4); // both rects are the same size, but one a has wider pen
view.show();
return a.exec();
}

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