访问 Qt3D 中的帧缓冲

我的任务：计算3D网格的像素坐标(例如制作快照)，以从特定的相机角度找到该网格的2D形状。

我目前正在使用带有QGeometryRenderer的Qt3D来渲染包含网格的场景到QWidget，效果很好。 我尝试使用 QWidget：：render() 将 QWidget 的内容渲染到 Pixmap 中，如这篇文章如何创建 QWidget 的屏幕截图？.将像素图保存到.jpg会产生具有默认背景色的空白图像，这是有意义的，因为 QWidget 不持有网格对象本身。

以下是在我的主窗口中设置场景的方式.cpp

// sets the scene objects, camera, lights,...
void MainWindow::setScene() {
scene = custommesh->createScene(mesh->getVertices(), 
mesh->getVerticesNormals(), 
mesh->getFaceNormals(), 
mesh->getVerticesIndex(), 
mesh->getFacesIndex());              // QEntity*                         
custommesh->setMaterial(scene);              // CustomMeshRenderer object
camera = custommesh->setCamera(view);
custommesh->setLight(scene, camera);
custommesh->setCamController(scene, camera);
view->setRootEntity(scene);                  // Qt3DExtras::Qt3DWindow object
// Setting up a QWiget working as a container for the view
QWidget *container = QWidget::createWindowContainer(view);
container->setMinimumSize(QSize(500, 500));
QSizePolicy policy = QSizePolicy(QSizePolicy::Policy(5), QSizePolicy::Policy(5));
policy.setHorizontalStretch(1);
policy.setVerticalStretch(1);
container->setSizePolicy(policy);
container->setObjectName("meshWidget");
this->ui->meshLayout->insertWidget(0, container);
}

至于渲染，这里是自定义网格渲染器类，其中定义了QGeometryRenderer，并在初始化网格时返回QEntity*。

#include "custommeshrenderer.h"
#include <Qt3DRender/QAttribute>
#include <Qt3DExtras>
#include <Qt3DRender/QGeometryRenderer>

CustommeshRenderer::CustommeshRenderer()
{
rootEntity = new Qt3DCore::QEntity;
customMeshEntity = new Qt3DCore::QEntity(rootEntity);
transform = new Qt3DCore::QTransform;
customMeshRenderer = new Qt3DRender::QGeometryRenderer;
customGeometry = new Qt3DRender::QGeometry(customMeshRenderer);
m_pVertexDataBuffer = new Qt3DRender::QBuffer(Qt3DRender::QBuffer::VertexBuffer, customGeometry);
m_pNormalDataBuffer = new Qt3DRender::QBuffer(Qt3DRender::QBuffer::VertexBuffer, customGeometry);
m_pColorDataBuffer = new Qt3DRender::QBuffer(Qt3DRender::QBuffer::VertexBuffer, customGeometry);
m_pIndexDataBuffer = new Qt3DRender::QBuffer(Qt3DRender::QBuffer::IndexBuffer, customGeometry);
}
/**
Set vertices and their normals for the scene
@param vertices List with all vertices of the mesh
@param vertices_normals List with all vertice normals
@param face_normals List with all face normals
@param vertice_idx List with the indices for the vertices
@param face_idx List with all indices for the faces
@return Entity where some components were added
*/
Qt3DCore::QEntity *CustommeshRenderer::createScene(QList<QVector3D> vertices, QList<QVector3D> vertices_normals, QList<QVector3D> face_normals, QList<int> vertices_idx, QList<QVector3D> faces_idx) {
// Setting scale to 8.0
transform->setScale(8.0f);
// Setting all the colors to (200, 0, 0)
QList<QVector3D> color_list;
for(int i = 0; i < vertices.length(); i++) {
color_list.append(QVector3D(200.0f, 0.0f, 0.0f));
}
// Fill vertexBuffer with data which hold the vertices, normals and colors
// Build structure: Size of Verticles List * 3 (x,y,z) * 4 (since x,y,z are floats, which needs 4 bytes each) 
vertexBufferData.resize(vertices.length() * 3 * (int)sizeof(float));
float *rawVertexArray = reinterpret_cast<float *>(vertexBufferData.data());
normalBufferData.resize(vertices_normals.length() * 3 * (int)sizeof(float));
float *rawNormalArray = reinterpret_cast<float *>(normalBufferData.data());
colorBufferData.resize(color_list.length() * 3 * (int)sizeof(float));
float *rawColorArray = reinterpret_cast<float *>(colorBufferData.data());

setRawVertexArray(rawVertexArray, vertices);
setRawNormalArray(rawNormalArray, vertices_normals);
setRawColorArray(rawColorArray, color_list);
//Fill indexBufferData with data which holds the triangulation information (patches/tris/lines)
indexBufferData.resize(faces_idx.length() * 3 * (int)sizeof(uint));
uint *rawIndexArray = reinterpret_cast<uint *>(indexBufferData.data());
setRawIndexArray(rawIndexArray, faces_idx);
//Set data to buffers
m_pVertexDataBuffer->setData(vertexBufferData);
m_pNormalDataBuffer->setData(normalBufferData);
m_pColorDataBuffer->setData(colorBufferData);
m_pIndexDataBuffer->setData(indexBufferData);
// Attributes
Qt3DRender::QAttribute *positionAttribute = new Qt3DRender::QAttribute();
positionAttribute->setAttributeType(Qt3DRender::QAttribute::VertexAttribute);
positionAttribute->setBuffer(m_pVertexDataBuffer);
// positionAttribute->setBuffer(m_pVertexDataBuffer.data());
positionAttribute->setDataType(Qt3DRender::QAttribute::Float);
positionAttribute->setDataSize(3);
positionAttribute->setByteOffset(0);
positionAttribute->setByteStride(3 * sizeof(float));
positionAttribute->setCount(vertices.length());
positionAttribute->setName(Qt3DRender::QAttribute::defaultPositionAttributeName());
Qt3DRender::QAttribute *normalAttribute = new Qt3DRender::QAttribute();
normalAttribute->setAttributeType(Qt3DRender::QAttribute::VertexAttribute);
normalAttribute->setBuffer(m_pNormalDataBuffer);
//normalAttribute->setBuffer(m_pNormalDataBuffer.data());
normalAttribute->setDataType(Qt3DRender::QAttribute::Float);
normalAttribute->setDataSize(3);
normalAttribute->setByteOffset(0);
normalAttribute->setByteStride(3 * sizeof(float));
normalAttribute->setCount(vertices.length());
normalAttribute->setName(Qt3DRender::QAttribute::defaultNormalAttributeName());
Qt3DRender::QAttribute* colorAttribute = new Qt3DRender::QAttribute();
colorAttribute->setAttributeType(Qt3DRender::QAttribute::VertexAttribute);
colorAttribute->setBuffer(m_pColorDataBuffer);
//colorAttribute->setBuffer(m_pColorDataBuffer.data());
colorAttribute->setDataType(Qt3DRender::QAttribute::Float);
colorAttribute->setDataSize(3);
colorAttribute->setByteOffset(0);
colorAttribute->setByteStride(3 * sizeof(float));
colorAttribute->setCount(vertices.length());
colorAttribute->setName(Qt3DRender::QAttribute::defaultColorAttributeName());
Qt3DRender::QAttribute *indexAttribute = new Qt3DRender::QAttribute();
indexAttribute->setAttributeType(Qt3DRender::QAttribute::IndexAttribute);
indexAttribute->setBuffer(m_pIndexDataBuffer);
//indexAttribute->setBuffer(m_pIndexDataBuffer.data());
indexAttribute->setDataType(Qt3DRender::QAttribute::UnsignedInt);
indexAttribute->setDataSize(3);
indexAttribute->setByteOffset(0);
indexAttribute->setByteStride(3 * sizeof(uint));
indexAttribute->setCount(face_normals.length());
customGeometry->addAttribute(positionAttribute);
customGeometry->addAttribute(normalAttribute);
/*customGeometry->addAttribute(colorAttribute);*/
customGeometry->addAttribute(indexAttribute);
//Set the final geometry and primitive type
customMeshRenderer->setPrimitiveType(Qt3DRender::QGeometryRenderer::Triangles);
customMeshRenderer->setVerticesPerPatch(3);
customMeshRenderer->setGeometry(customGeometry);

customMeshRenderer->setVertexCount(faces_idx.length()*3);
customMeshEntity->addComponent(customMeshRenderer);
customMeshEntity->addComponent(transform);
setMaterial(customMeshEntity);
return rootEntity;
}

访问帧缓冲的最佳方法是什么，或者是否有其他方法可以拍摄网格的快照？

我最后的希望是自己实现渲染管线(至少从投影坐标到像素坐标)，但我更喜欢另一种解决方案。不幸的是，我必须依赖Qt3D，无法切换到QOpenGLWidget等其他类。至少我还没有找到集成它的可能性。

我对Qt3D很陌生，缺乏详细的文档并没有让它变得更容易。