Qt3D: Draw Grid/Axis Lines?

[oblivioncth]

Does anyone know a relatively easy way to just render axis/grid lines for X,Y,Z in a Qt3D OpenGL scene? I made a simple model viewer that opens wavefront .obj files and it would be nice to have the axis lines.

[A Former User]

@oblivioncth There is no easy way.

[zespy]

Hi! I know this is old, but in case somebody falls on this one day, here is a solution (made from example here: https://www.youtube.com/watch?v=dWqXLDiL-qk)

First, a method to draw a line:

#include <Qt3DCore/QEntity>
#include <Qt3DCore/QTransform>
#include <Qt3DExtras/QPhongMaterial>
#include <Qt3DRender/QAttribute>
#include <Qt3DRender/QBuffer>
#include <Qt3DRender/QGeometry>

void drawLine(const QVector3D& start, const QVector3D& end, const QColor& color, Q3DCore::QEntity *_rootEntity)
{
	auto *geometry = new Qt3DRender::QGeometry(_rootEntity);

	// position vertices (start and end)
	QByteArray bufferBytes;
	bufferBytes.resize(3 * 2 * sizeof(float)); // start.x, start.y, start.end + end.x, end.y, end.z
	float *positions = reinterpret_cast<float*>(bufferBytes.data());
	*positions++ = start.x();
	*positions++ = start.y();
	*positions++ = start.z();
	*positions++ = end.x();
	*positions++ = end.y();
	*positions++ = end.z();

	auto *buf = new Qt3DRender::QBuffer(geometry);
	buf->setData(bufferBytes);

	auto *positionAttribute = new Qt3DRender::QAttribute(geometry);
	positionAttribute->setName(Qt3DRender::QAttribute::defaultPositionAttributeName());
	positionAttribute->setVertexBaseType(Qt3DRender::QAttribute::Float);
	positionAttribute->setVertexSize(3);
	positionAttribute->setAttributeType(Qt3DRender::QAttribute::VertexAttribute);
	positionAttribute->setBuffer(buf);
	positionAttribute->setByteStride(3 * sizeof(float));
	positionAttribute->setCount(2);
	geometry->addAttribute(positionAttribute); // We add the vertices in the geometry

	// connectivity between vertices
	QByteArray indexBytes;
	indexBytes.resize(2 * sizeof(unsigned int)); // start to end
	unsigned int *indices = reinterpret_cast<unsigned int*>(indexBytes.data());
	*indices++ = 0;
	*indices++ = 1;

	auto *indexBuffer = new Qt3DRender::QBuffer(geometry);
	indexBuffer->setData(indexBytes);

	auto *indexAttribute = new Qt3DRender::QAttribute(geometry);
	indexAttribute->setVertexBaseType(Qt3DRender::QAttribute::UnsignedInt);
	indexAttribute->setAttributeType(Qt3DRender::QAttribute::IndexAttribute);
	indexAttribute->setBuffer(indexBuffer);
	indexAttribute->setCount(2);
	geometry->addAttribute(indexAttribute); // We add the indices linking the points in the geometry

	// mesh
	auto *line = new Qt3DRender::QGeometryRenderer(_rootEntity);
	line->setGeometry(geometry);
	line->setPrimitiveType(Qt3DRender::QGeometryRenderer::Lines);
	auto *material = new Qt3DExtras::QPhongMaterial(_rootEntity);
	material->setAmbient(color);

	// entity
	auto *lineEntity = new Qt3DCore::QEntity(_rootEntity);
	lineEntity->addComponent(line);
	lineEntity->addComponent(material);
}

Now we can draw the origin:

auto *root = new Qt3DCore::QEntity();
// draw origin
drawLine({ 0, 0, 0 }, { 1, 0, 0 }, Qt::red, root); // X
drawLine({ 0, 0, 0 }, { 0, 1, 0 }, Qt::green, root); // Y
drawLine({ 0, 0, 0 }, { 0, 0, 1 }, Qt::blue, root); // Z

[zespy]

Now, I'd like to print the label of the axis next to them ("X" next to the X axis) with a QText2DEntity but I can't find a way to do it... See https://forum.qt.io/topic/92944/qt3d-how-to-print-text-qtext2dentity

[Rahul Das]

@zespy
This is absolutely brilliant explanation and example.
I know the topic is old now, but still a valid reference. Hence i thought to share the correction i had to do on buffer type [I assume its deprecated now], respectively :

auto *buf = new Qt3DRender::QBuffer(Qt3DRender::QBuffer::VertexBuffer, geometry);
auto *indexBuffer = new Qt3DRender::QBuffer(Qt3DRender::QBuffer::IndexBuffer, geometry);

Mine is Qt 5.9 on Win 10.

[sonichy]

void drawLine(const QVector3D& start, const QVector3D& end, const QColor& color, Q3DCore::QEntity *_rootEntity)

Lost a letter:

void drawLine(const QVector3D& start, const QVector3D& end, const QColor& color, Qt3DCore::QEntity *_rootEntity)

[giusirux]

@zespy Hi
I'm trying to use your code for my application. My goal is to insert grid behind an STL image, but it doesn't work and I don't know why.
I copied and pasted drawLine in the main. I also watched the example on youtube and I used his Qt example (basicshapes). So, when I run the project, it compiles without problems, but there is only my STL image without the grid.

[maydin]

@giusirux When I call setInstanceCount(1) I saw the rendered lines. Not sure what other function do in this code, didn't test them.

Qt3DRender::QGeometryRenderer *customMeshRenderer = new Qt3DRender::QGeometryRenderer;
customMeshRenderer->setInstanceCount(1);
customMeshRenderer->setIndexOffset(0);
customMeshRenderer->setFirstInstance(0);
customMeshRenderer->setPrimitiveType(Qt3DRender::QGeometryRenderer::Triangles);
customMeshRenderer->setGeometry(customGeometry);
customMeshRenderer->setVertexCount(12);