Minimal OpenGL offscreen rendering using QT

Question

I am trying to make a simple offscreen renderer to produce a bunch of image files using QT. There are lots of examples around, though I haven't found any dealing with this sort of one-shot rendering, without a loop and without a visible window. Plus QT provides you with convenient wrappers, which is great but on the other side it becomes harder to follow examples written with C++glew+glfw.

I tried using this to set the offscreen context and it works. After creating an FBO and rendering(like here) no triangle is drawn inside the image (fbo->ToImage).

My current code is just a mix of two:

#include <QtGui/QSurfaceFormat>
#include <QtGui/QOffscreenSurface>
#include <QtGui/QOpenGLFunctions>
#include <QtGui/QOpenGLFramebufferObject>
#include <QtGui/QOpenGLShaderProgram>
#include <QApplication>
#include <QDebug>
#include <QImage>
#include <QOpenGLBuffer>
#include <QOpenGLTexture>

int main(int argc, char* argv[])
{
   QApplication a(argc, argv);

   QSurfaceFormat surfaceFormat;
   surfaceFormat.setMajorVersion(4);
   surfaceFormat.setMinorVersion(3);

   QOpenGLContext openGLContext;
   openGLContext.setFormat(surfaceFormat);
   openGLContext.create();
   if(!openGLContext.isValid()) return -1;

   QOffscreenSurface surface;
   surface.setFormat(surfaceFormat);
   surface.create();
   if(!surface.isValid()) return -2;

   openGLContext.makeCurrent(&surface);

   QSize vpSize = QSize(300, 300);

   qDebug("Hi");



   QOpenGLFramebufferObjectFormat fboFormat;
   fboFormat.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
   QOpenGLFramebufferObject fbo(vpSize, fboFormat);

   fbo.bind();

   static const float vertexPositions[] = {
       -0.8f, -0.8f, 0.0f,
        0.8f, -0.8f, 0.0f,
        0.0f,  0.8f, 0.0f
   };

   static const float vertexColors[] = {
       1.0f, 0.0f, 0.0f,
       0.0f, 1.0f, 0.0f,
       0.0f, 0.0f, 1.0f
   };

   QOpenGLBuffer vertexPositionBuffer(QOpenGLBuffer::VertexBuffer);
   vertexPositionBuffer.create();
   vertexPositionBuffer.setUsagePattern(QOpenGLBuffer::StaticDraw);
   vertexPositionBuffer.bind();
   vertexPositionBuffer.allocate(vertexPositions, 9 * sizeof(float));

   QOpenGLBuffer vertexColorBuffer(QOpenGLBuffer::VertexBuffer);
   vertexColorBuffer.create();
   vertexColorBuffer.setUsagePattern(QOpenGLBuffer::StaticDraw);
   vertexColorBuffer.bind();
   vertexColorBuffer.allocate(vertexColors, 9 * sizeof(float));

   QOpenGLShaderProgram program;
   program.addShaderFromSourceCode(QOpenGLShader::Vertex,
                                   "#version 330\r\n"
                                   "in vec3 position;\n"
                                   "in vec3 color;\n"
                                   "out vec3 fragColor;\n"
                                   "void main() {\n"
                                   "    fragColor = color;\n"
                                   "    gl_Position = vec4(position, 1.0);\n"
                                   "}\n"
                                   );
   program.addShaderFromSourceCode(QOpenGLShader::Fragment,
                                   "#version 330\r\n"
                                   "in vec3 fragColor;\n"
                                   "out vec4 color;\n"
                                   "void main() {\n"
                                   "    color = vec4(fragColor, 1.0);\n"
                                   "}\n"
                                   );
   program.link();
   program.bind();

   vertexPositionBuffer.bind();
   program.enableAttributeArray("position");
   program.setAttributeBuffer("position", GL_FLOAT, 0, 3);

   vertexColorBuffer.bind();
   program.enableAttributeArray("color");
   program.setAttributeBuffer("color", GL_FLOAT, 0, 3);

   openGLContext.functions()->glClearColor(0.3f, 0.0f, 0.7f, 1.0f);
   openGLContext.functions()->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

   openGLContext.functions()->glDrawArrays(GL_TRIANGLES, 0, 3);


   program.disableAttributeArray("position");
   program.disableAttributeArray("color");

   program.release();

   fbo.release();

   qDebug("FBO released");

   QImage im = fbo.toImage();

   if (im.save("asd.png")){
       qDebug("Image saved!!");
   }



   a.exec();
}

Any ide23edas on how to put the together the offscreen part with the drawing part?

Update 1:

#include <QGuiApplication>
#include <QOffscreenSurface>
#include <QOpenGLFunctions>
#include <QOpenGLFramebufferObject>
#include <QOpenGLShaderProgram>
#include <QOpenGLBuffer>
#include <QOpenGLVertexArrayObject>
#include <QDebug>
#include <QImage>
#include <QLoggingCategory>

int main(int argc, char* argv[])
{
    QGuiApplication a(argc, argv);
//    QLoggingCategory::setFilterRules("qt.qpa.gl=true");

//   =======CONTEXT SETUP======

//   Set OpenGL version to use
    QSurfaceFormat surfaceFormat;
    surfaceFormat.setMajorVersion(4);
    surfaceFormat.setMinorVersion(3);

    QOpenGLContext openGLContext;
    openGLContext.setFormat(surfaceFormat);
    openGLContext.create();
    if(!openGLContext.isValid()) qDebug("Unable to create context");

    QOffscreenSurface surface;
    surface.setFormat(surfaceFormat);
    surface.create();
    if(!surface.isValid()) qDebug("Unable to create the Offscreen surface");

    openGLContext.makeCurrent(&surface);

//   Viewport size
    QSize vpSize = QSize(300, 300);

    QOpenGLFramebufferObjectFormat fboFormat;
    fboFormat.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
    QOpenGLFramebufferObject fbo(vpSize, fboFormat);
    openGLContext.functions()->glViewport(0,0,vpSize.width(), vpSize.height());
    fbo.bind();

//    ========GEOMEETRY SETUP========

    static const float vertexPositions[] = {
        -1.0f, -1.0f,
        -0.0f,  1.0f,
         1.0f, -1.0f,
    };

    static const float vertexColors[] = {
       1.0f, 0.0f, 0.0f,
       0.0f, 1.0f, 0.0f,
       0.0f, 0.0f, 1.0f
    };

    QOpenGLBuffer vertexPositionBuffer(QOpenGLBuffer::VertexBuffer);
    vertexPositionBuffer.create();
    vertexPositionBuffer.setUsagePattern(QOpenGLBuffer::StaticDraw);
    vertexPositionBuffer.bind();
    vertexPositionBuffer.allocate(vertexPositions, 6 * sizeof(float));

    QOpenGLBuffer vertexColorBuffer(QOpenGLBuffer::VertexBuffer);
    vertexColorBuffer.create();
    vertexColorBuffer.setUsagePattern(QOpenGLBuffer::StaticDraw);
    vertexColorBuffer.bind();
    vertexColorBuffer.allocate(vertexColors, 9 * sizeof(float));

    QOpenGLShaderProgram program;
    program.addShaderFromSourceCode(QOpenGLShader::Vertex,
                                   "#version 330\r\n"
                                   "in vec2 position;\n"
                                   "in vec3 color;\n"
                                   "out vec3 fragColor;\n"
                                   "void main() {\n"
                                   "    fragColor = color;\n"
                                   "    gl_Position = vec4(position, 0.0, 1.0);\n"
                                   "}\n"
                                   );
    program.addShaderFromSourceCode(QOpenGLShader::Fragment,
                                   "#version 330\r\n"
                                   "in vec3 fragColor;\n"
                                   "out vec4 color;\n"
                                   "void main() {\n"
                                   "    color = vec4(fragColor, 1.0);\n"
                                   "}\n"
                                   );
    program.link();
    program.bind();

    QOpenGLVertexArrayObject vao;
    vao.create();
    vao.bind();

    vertexPositionBuffer.bind();
    program.enableAttributeArray("position");
    program.setAttributeBuffer("position", GL_FLOAT, 0, 2);

    vertexColorBuffer.bind();
    program.enableAttributeArray("color");
    program.setAttributeBuffer("color", GL_FLOAT, 0, 3);

//    ==============DRAWING TO THE FBO============

    openGLContext.functions()->glClearColor(0.3f, 0.0f, 0.7f, 1.0f);
    openGLContext.functions()->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    openGLContext.functions()->glDrawArrays(GL_TRIANGLES, 0, 3);

//   =============CLEANUP================== maybe not necessary
    program.disableAttributeArray("position");
    program.disableAttributeArray("color");
    program.release();
    fbo.release();

//    ========SAVE IMAGE===========

    QImage im = fbo.toImage();

    if (im.save("asd.png")){
       qDebug("Image saved!!");
    }

    a.quit();
}

Thanks to G.M's tip. This is a working code that will save an image of a colored triangle, for anyone interested.

Please edit your question to provide a [mcve]. As things stand all we can do is guess. — G.M., Dec 14 '19 at 19:07

score 0 · Accepted Answer · answered Dec 15 '19 at 10:14

0

I suspect the problem is simply that you are getting a 4.3 core profile but aren't creating/using a vertex array object. Creating a VAO immediately after binding the shader program should suffice, so change...

program.link();
program.bind();

to...

program.link();
program.bind();

QOpenGLVertexArrayObject vao;
vao.create();
vao.bind();

(You'll also need to add #include <QOpenGLVertexArrayObject> at the top of your source file.)

Also note that error checking is important. Consider using glGetError or perhaps a debug context.

answered Dec 15 '19 at 10:14

G.M.

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2
15
18

Wow!! It worked. You brightened my day. I would like to clean up the code and publish something easy to follow. Is "openGLContext.functions()->" a proper way to call OpenGL functions? For debugging may "QLoggingCategory::setFilterRules("qt.qpa.gl=true");" be useful? Thank you – canaldin Dec 15 '19 at 17:44
@canaldin Did you ever publish anything on this? A guide on offscreen rendering using opengl and QTOffscreenSurface would fill a missing hole in the internet :) – Geronimo Mar 09 '20 at 21:49

Minimal OpenGL offscreen rendering using QT

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