Reallocating QSGGeometryNode Vertex Data - qt

Question

I am using Qt 5.8. I am attempting to draw circular points using QSGGeometry::DrawTriangles (using QSGGeometry::DrawPoints draws points as rectangles/squares--not desired). In order to do this I am drawing 8 triangles that gives the illusion of a circle. Each data point will have 8 triangles associated with it. The number of data points can vary at any given time. After a (user) specified amount of time as a data point is added, one data point is removed. There seems to be an error in the allocation of data when it's drawn. I used setVertexDataPattern(QSGGeometry::StreamPattern); in the construction of the QSGGeometryNode; in hopes of getting the desired output.
On each draw call, I call m_geometry.allocate(m_pts.size() * MAX_VERTICES), where MAX_VERTICES = 24 in case the number of points since the last draw call has changed. I have attempted to use GL_POLYGON (since it would require fewer vertices), but the same problem happens. There seems to be an attempt to draw a shape from one slice of the the first data point to another slice of the last data point. Is there something wrong with reallocating for every draw call? What is the proper way to handle drawing data with varying sizes?
Update I think it may deal with a size issue. I have sample code that only draws 1 triangle (instead of 8) and once you get to about 25000 (times 3 for each triangle) the odd line appears and seems to stop drawing additional triangles. In the following sample code (when using a smaller number of points) the last triangle is white.
main.qml
import QtQuick 2.8
import QtQuick.Window 2.2
import TestModule 1.0
Window {
visible: true
width: 640
height: 480
title: qsTr("Hello World")
Rectangle {
color: "black"
width: parent.width
height: parent.height * .90
anchors.top: parent.top
MouseArea {
anchors.fill: parent
onClicked: {
Qt.quit();
}
}
TestItem {
id: testItem
anchors.fill: parent
ptCount: 25000
color: "green"
}
}
Rectangle {
anchors.horizontalCenter: parent.horizontalCenter
anchors.bottom: parent.bottom
height: parent.height * .10
width: parent.width
border.color: "pink"
color: "lightgray"
TextInput {
anchors.fill: parent
anchors.centerIn: parent
id: textInput
text: "enter max number of points here"
horizontalAlignment: TextInput.AlignHCenter
verticalAlignment: TextInput.AlignVCenter
color: "steelblue"
onEditingFinished: testItem.ptCount = parseInt(textInput.text)
validator: IntValidator{bottom: 1}
}
}
}
TestItem.h
#include <QQuickItem>
class QSGGeometryNode;
class TestItem : public QQuickItem
{
Q_OBJECT
Q_PROPERTY(QColor color READ color WRITE setColor NOTIFY colorChanged)
Q_PROPERTY(qint32 ptCount READ ptCount WRITE setPtCount NOTIFY ptCountChanged)
public:
explicit TestItem(QQuickItem *parent = 0);
QColor color();
void setColor(const QColor &color);
void setPtCount(const qint32& newVal);
qint32 ptCount();
signals:
void colorChanged();
void ptCountChanged();
protected:
QSGNode *updatePaintNode(QSGNode *, UpdatePaintNodeData *);
void geometryChanged(const QRectF &newGeometry, const QRectF &oldGeometry);
QColor m_color;
qint32 m_ptCount;
};
TestItem.cpp
#include "TestItem.h"
#include <QSGNode>
#include <QSGVertexColorMaterial>
TestItem::TestItem(QQuickItem *parent) : QQuickItem(parent), m_color(Qt::green), m_ptCount(25000)
{
setFlag(ItemHasContents, true);
}
QColor TestItem::color()
{
return m_color;
}
void TestItem::setColor(const QColor &color)
{
m_color = color;
update();
emit colorChanged();
}
void TestItem::setPtCount(const qint32 &newVal)
{
if (newVal < 0)
m_ptCount = 25000;
else
m_ptCount = newVal;
update();
emit ptCountChanged();
}
qint32 TestItem::ptCount()
{
return m_ptCount;
}
QSGNode *TestItem::updatePaintNode(QSGNode *oldNode, QQuickItem::UpdatePaintNodeData *)
{
QSGGeometryNode *node = nullptr;
QSGGeometry *geometry = nullptr;
if (!oldNode)
{
node = new QSGGeometryNode;
geometry = new QSGGeometry(QSGGeometry::defaultAttributes_ColoredPoint2D(), m_ptCount * 3);
geometry->setDrawingMode(GL_TRIANGLES);
node->setGeometry(geometry);
node->setFlag(QSGNode::OwnsGeometry);
QSGVertexColorMaterial *material = new QSGVertexColorMaterial;
node->setMaterial(material);
node->setFlag(QSGNode::OwnsMaterial);
}
else
{
node = static_cast<QSGGeometryNode *>(oldNode);
geometry = node->geometry();
geometry->allocate(m_ptCount * 3);
}
QSGGeometry::ColoredPoint2D *vertices = geometry->vertexDataAsColoredPoint2D();
qreal triWidth = 250/boundingRect().width() + 10;
for (int i = 0; i < m_ptCount; ++i)
{
QColor color;
if (i == m_ptCount - 1)
color = Qt::white;
else
color = m_color;
qreal x0 = (boundingRect().width() * .90/m_ptCount) * i ;
qreal y0 = 60 * sinf(x0* 3.14/180); // 60 just varies the height of the wave
qreal x1 = x0 + 0.05 * boundingRect().width(); // 0.05 so that we have 5% space on each side
qreal y1 = y0 + boundingRect().height()/2;
vertices[i * 3].set(x1, y1, color.red(), color.green(), color.blue(), color.alpha());
vertices[i * 3 + 1].set(x1 + triWidth, y1, color.red(), color.green(), color.blue(), color.alpha());
vertices[i * 3 + 2].set(x1 + triWidth, y1 + triWidth, color.red(), color.green(), color.blue(), color.alpha());
}
node->markDirty(QSGNode::DirtyGeometry);
return node;
}
void TestItem::geometryChanged(const QRectF &newGeometry, const QRectF &oldGeometry)
{
update();
QQuickItem::geometryChanged(newGeometry, oldGeometry);
}
Any help with determining if this is a Qt bug or my error somewhere?

Answer 1

It turns out I needed to change the geometry's constructor to use UnsignedIntType; it defaults to UnsignedShortType.
geometry = new QSGGeometry(QSGGeometry::defaultAttributes_ColoredPoint2D(), m_ptCount * 3, 0, QSGGeometry::UnsignedIntType);