qt - 如何在 Qt 5 中使用带有实例化的 VAO

Question

我试图围绕如何正确使用 VAO 进行实例渲染(特别是在 Qt 5.2 中，使用 OpenGL 3.3)。我的理解是，VAO 保存了 VBO 的状态和相关属性，这样您就不必担心在绘图时绑定(bind)和启用所有内容，您只需绑定(bind) VAO。但是通过实例化，您通常会有多个 VBO。你如何绕过需要绑定(bind)它们？还是我只需要为每个顶点数据和每个实例数据都使用一个 VBO？

我一直在看一些教程，例如:http://ogldev.atspace.co.uk/www/tutorial33/tutorial33.html

在我看来，他所做的是对每个顶点数据使用 VAO，而不是对每个实例数据使用 VAO。我试过用我的基于 Qt 的代码做同样的事情，但它对我不起作用(可能是因为我不完全理解它是如何工作的......当绘制发生时他的实例数据是否仍然需要绑定(bind)？ ?)

一些虚拟代码......这有点傻，我只是在绘制两个三角形的单个实例，并将透视矩阵作为每个实例的属性。

glwindow.cpp:

#include "glwindow.h"

#include <QColor>
#include <QMatrix4x4>
#include <QVector>
#include <QVector3D>
#include <QVector4D>

#include <QDebug>


GLWindow::GLWindow(QWindow *parent) 
  : QWindow(parent)
  , _vbo(QOpenGLBuffer::VertexBuffer)
  , _matbo(QOpenGLBuffer::VertexBuffer)
  , _context(0)
{
  setSurfaceType(QWindow::OpenGLSurface);
}

GLWindow::~GLWindow()
{}

void GLWindow::initGL()
{  
  setupShaders();
  _program->bind();
  _positionAttr = _program->attributeLocation("position");
  _colourAttr = _program->attributeLocation("colour");
  _matrixAttr = _program->attributeLocation("matrix");

  QVector<QVector3D> triangles;
  triangles << QVector3D(-0.5, 0.5, 1) << QVector3D(-0.5, -0.5, 1) << QVector3D(0.5, -0.5, 1);
  triangles << QVector3D(0.5, 0.5, 0.5) << QVector3D(-0.5, -0.5, 0.5) << QVector3D(0.5, -0.5, 0.5);

  QVector<QVector3D> colours;
  colours << QVector3D(1, 0, 0) << QVector3D(0, 1, 0) << QVector3D(0, 0, 1);
  colours << QVector3D(1, 1, 1) << QVector3D(1, 1, 1) << QVector3D(1, 1, 1);

  _vao.create();
  _vao.bind();

  _vbo.create();
  _vbo.setUsagePattern(QOpenGLBuffer::StaticDraw);
  _vbo.bind();

  size_t positionSize = triangles.size() * sizeof(QVector3D);
  size_t colourSize = colours.size() * sizeof(QVector3D);
  _vbo.allocate(positionSize + colourSize);
  _vbo.bind();
  _vbo.write(0, triangles.constData(), positionSize);
  _vbo.write(positionSize, colours.constData(), colourSize);
  _colourOffset = positionSize;

  _program->setAttributeBuffer(_positionAttr, GL_FLOAT, 0, 3, 0);
  _program->setAttributeBuffer(_colourAttr, GL_FLOAT, _colourOffset, 3, 0);

  _program->enableAttributeArray(_positionAttr);  
  _program->enableAttributeArray(_colourAttr);

  _vao.release();

  _matbo.create();
  _matbo.setUsagePattern(QOpenGLBuffer::StaticDraw);
  _matbo.bind();

  _matbo.allocate(4 * sizeof(QVector4D));
  _program->setAttributeBuffer(_matrixAttr, GL_FLOAT, 0, 4, 4 * sizeof(QVector4D));
  _program->enableAttributeArray(_matrixAttr);

  _func330->glVertexAttribDivisor(_matrixAttr, 1);
  _matbo.release();

  _program->release();
  resizeGL(width(), height());
}

void GLWindow::resizeGL(int w, int h)
{
  glViewport(0, 0, w, h);
}

void GLWindow::paintGL()
{
  if (! _context) // not yet initialized
    return;

  _context->makeCurrent(this);
  QColor background(Qt::black);

  glClearColor(background.redF(), background.greenF(), background.blueF(), 1.0f);
  glClearDepth(1.0f);
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  QMatrix4x4 matrix;
  matrix.perspective(60, 4.0/3.0, 0.1, 100.0);
  matrix.translate(0, 0, -2);

  _program->bind();

  _matbo.bind();
  _matbo.write(0, matrix.constData(), 4 * sizeof(QVector4D));

  _vao.bind();

  glEnable(GL_DEPTH_TEST);
  _func330->glDrawArraysInstanced(GL_TRIANGLES, 0, 6, 1);

  _vao.release();

  _program->release();

  _context->swapBuffers(this);
  _context->doneCurrent();

}

void GLWindow::setupShaders()
{

  QString vShaderSrc("#version 330\n"
    "layout(location = 0) in vec4 position;\n"
    "layout(location = 1) in vec4 colour;\n"
    "layout(location = 2) in mat4 matrix;\n"
    "smooth out vec4 col;\n"
    "void main() {\n"
    "   col = colour;\n"
    "   gl_Position = matrix * position;\n"
    "}\n");

  QString fShaderSrc("#version 330\n"
    "smooth in vec4 col;\n"
    "void main() {\n"
    "   gl_FragColor = col;\n"
    "}\n");


  _program = new QOpenGLShaderProgram(this);
  _program->addShaderFromSourceCode(QOpenGLShader::Vertex, vShaderSrc);
  _program->addShaderFromSourceCode(QOpenGLShader::Fragment, fShaderSrc);
  _program->link();


}

void GLWindow::exposeEvent(QExposeEvent *event)
{
  Q_UNUSED(event);

  if (isExposed())
  {
    if (! _context)
    {
      _context = new QOpenGLContext(this);
      QSurfaceFormat format(requestedFormat());
      format.setVersion(3,3);
      format.setDepthBufferSize(24);

      _context->setFormat(format);
      _context->create();

      _context->makeCurrent(this);
      initializeOpenGLFunctions();

      _func330 = _context->versionFunctions<QOpenGLFunctions_3_3_Core>();
      if (_func330)
        _func330->initializeOpenGLFunctions();
      else
      {
        qWarning() << "Could not obtain required OpenGL context version";
        exit(1);
      }

      initGL();
    }

    paintGL();
  }
}

glwindow.h:

#ifndef GL_WINDOW_H
#define GL_WINDOW_H

#include <QExposeEvent>
#include <QSurfaceFormat>
#include <QWindow>

#include <QOpenGLBuffer>
#include <QOpenGLContext>
#include <QOpenGLFunctions>
#include <QOpenGLFunctions_3_3_Core>
#include <QOpenGLShaderProgram>
#include <QOpenGLVertexArrayObject>


class GLWindow : public QWindow, protected QOpenGLFunctions
{
  Q_OBJECT
public:
  GLWindow(QWindow * = 0);
  virtual ~GLWindow();

  void initGL();
  void paintGL();
  void resizeGL(int, int);

protected:
  virtual void exposeEvent(QExposeEvent *);


private:

  void setupShaders();

  QOpenGLBuffer _vbo;
  QOpenGLBuffer _matbo;
  QOpenGLContext *_context;
  QOpenGLShaderProgram *_program;
  QOpenGLVertexArrayObject _vao;
  QOpenGLFunctions_3_3_Core *_func330;

  GLuint _positionAttr;
  GLuint _colourAttr;
  GLuint _matrixAttr;

  size_t _colourOffset;

} ; 

#endif

glbuffertest.cpp:

#include <QGuiApplication>
#include <QSurfaceFormat>

#include "glwindow.h"



int main(int argc, char **argv)
{

  QGuiApplication app(argc, argv);

  GLWindow window;  
  window.resize(400, 400);
  window.show();

  return app.exec();

}

glbuffertest.pro:

######################################################################
# Automatically generated by qmake (3.0) Fri May 16 09:49:41 2014
######################################################################

TEMPLATE = app
TARGET = glbuffertest
INCLUDEPATH += .

CONFIG += qt debug

# Input
SOURCES += glbuffertest.cpp glwindow.cpp
HEADERS += glwindow.h

更新:
我试过摆脱我的 _matbo缓冲区，而是将矩阵数据放入与位置和颜色属性相同的 VBO 中，但这对我不起作用。我的 initGL函数现在看起来像:

void GLWindow::initGL()
{  
  setupShaders();
  _program->bind();
  _positionAttr = _program->attributeLocation("position");
  _colourAttr = _program->attributeLocation("colour");
  _matrixAttr = _program->attributeLocation("matrix");

  QVector<QVector3D> triangles;
  triangles << QVector3D(-0.5, 0.5, 1) << QVector3D(-0.5, -0.5, 1) << QVector3D(0.5, -0.5, 1);
  triangles << QVector3D(0.5, 0.5, 0.5) << QVector3D(-0.5, -0.5, 0.5) << QVector3D(0.5, -0.5, 0.5);

  QVector<QVector3D> colours;
  colours << QVector3D(1, 0, 0) << QVector3D(0, 1, 0) << QVector3D(0, 0, 1);
  colours << QVector3D(1, 1, 1) << QVector3D(1, 1, 1) << QVector3D(1, 1, 1);

  _vao.create();
  _vao.bind();

  _vbo.create();
  _vbo.setUsagePattern(QOpenGLBuffer::StaticDraw);
  _vbo.bind();

  size_t positionSize = triangles.size() * sizeof(QVector3D);
  size_t colourSize = colours.size() * sizeof(QVector3D);
  size_t matrixSize = 4 * sizeof(QVector4D);
  _vbo.allocate(positionSize + colourSize + matrixSize);
  _vbo.bind();
  _vbo.write(0, triangles.constData(), positionSize);
  _vbo.write(positionSize, colours.constData(), colourSize);


  _colourOffset = positionSize;
  _matrixOffset = positionSize + colourSize;

  _program->setAttributeBuffer(_positionAttr, GL_FLOAT, 0, 3, 0);
  _program->setAttributeBuffer(_colourAttr, GL_FLOAT, _colourOffset, 3, 0);
  _program->setAttributeBuffer(_matrixAttr, GL_FLOAT, _matrixOffset, 4, 4 * sizeof(QVector4D));

  _program->enableAttributeArray(_positionAttr);  
  _program->enableAttributeArray(_colourAttr);
  _program->enableAttributeArray(_matrixAttr);
  _func330->glVertexAttribDivisor(_matrixAttr, 1);

  _vao.release();

  _program->release();
  resizeGL(width(), height());
}

和 paintGL :

void GLWindow::paintGL()
{
  if (! _context) // not yet initialized
    return;

  _context->makeCurrent(this);
  QColor background(Qt::black);

  glClearColor(background.redF(), background.greenF(), background.blueF(), 1.0f);
  glClearDepth(1.0f);
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  QMatrix4x4 matrix;
  matrix.perspective(60, 4.0/3.0, 0.1, 100.0);
  matrix.translate(0, 0, -2);


  _program->bind();

  _vao.bind();
  _vbo.write(_matrixOffset, matrix.constData(), 4 * sizeof(QVector4D));

  /* I tried replacing the three preceding lines with the following, without success: */

  /*
  _vao.bind();
  _vbo.bind();
  _vbo.write(_matrixOffset, matrix.constData(), 4 * sizeof(QVector4D));


  _program->bind();
  _program->enableAttributeArray(_matrixAttr);
  _func330->glVertexAttribDivisor(_matrixAttr, 1); */

  glEnable(GL_DEPTH_TEST);
  _func330->glDrawArraysInstanced(GL_TRIANGLES, 0, 6, 1);

  _vao.release();

  _program->release();

  _context->swapBuffers(this);
  _context->doneCurrent();

}

因此，似乎我的实例化问题不仅仅是在错误的时间绑定(bind)了错误的缓冲区。我还有什么做错的？

Answer 1

我认为您必须为位置创建一个 VBO，为颜色创建一个 VBO(或使用跨步的交错数据)。 VAO 允许您使用多个 VBO，每个属性一个。

vao.create();
vao.bind();
// prepare your shader program
// ...
// prepare your VBOs : one VBO for pos, one VBO for colors, one for normals,...
// example for position
vertexPositionBuffer.create();
vertexPositionBuffer.setUsagePattern(QOpenGLBuffer::StaticDraw); 
vertexPositionBuffer.bind();
// if your store the points using QVector<QVector3D>
vertexPositionBuffer.allocate(vertices.constData(), vertices.size() * sizeof(QVector3D));
vertexPositionBuffer.release();
// do the same for colors or other attributes
// ...
// after all buffers are created
shaderProgram.bind();
// Bind the position buffer
vertexPositionBuffer.bind();
shaderProgram.enableAttributeArray("vertexPosition");
shaderProgram.setAttributeBuffer("vertexPosition", GL_FLOAT, 0, 3);
vertexPositionBuffer.release();
// do the same for all other buffers
// ...
shaderProgram.release();
// release vao 
vao.release();

and in your paintGL function:

// update your matrices
// bind your shader program
// set you uniform variables
// then
vao.bind();
glDrawArrays(GL_TRIANGLES, 0, vertices.size());
vao.release();
// release your shader program