Showing video in GUI does not work (black screen)

SGaist

Did you consider implementing my suggestion rather than moving theses pointers around ?

I don't think there's a guarantee that the buffer they are pointing to will be valid when the callback method is done.

makopo

@SGaist

Did you mean building a QImage in an own method instead of using the DrawFrame method from the callback interface?

SGaist

Build the QImage in the callback and send it further using invokeMethod.

makopo

@SGaist

I found by accident that my program renders a single frame of the camera input, after I start the loop and minimize the GUI of the program. When I reopen the GUI a new frame is rendered and also, as I can see in the console, the paintGL() method of the QOpenGLWidget interface is called. I think the paintGL()method is not implemented well. I will take look to the OpenGL part.

Do you agree that there is a problem with OpenGL? Or do you think the cause of this is the event loop?
Sorry for all this silly questions. I'am not very experienced in programming and this is a studyproject from university. Some thinks sounds new to me.

SGaist

That QOpenGLWidget is new to me, there's no reference to it in your code.

I would recommend minimizing your application so that you can concentrate on the Decklink integration part.

makopo

I checked the DeckLink intergration and I think there is no problem. I come bck to the OpenGl part and tried to draw a triangle with OpenGL in QOpenGLWidget::paintGL(). The function is called, but the graphic isn't rendered.

void VideoScreen::paintGL() {
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	glBegin(GL_TRIANGLES);
		glColor3f(1, 0, 0);
		glVertex3f(-0.5, -0.5, 0);
		glColor3f(0, 1, 0);
		glVertex3f(0.5, -0.5, 0);
		glColor3f(1, 0, 0);
		glVertex3f(0.0, -0.5, 0);
	glEnd();
	qDebug() << "VideoScreen: paintGL works";
}

I also checked QOpenGLWidget::initializeGL() and the call of glClearColor works.

void VideoScreen::initializeGL() {
	initializeOpenGLFunctions();
	glClearColor(1, 1, 0, 1); //after calling this the widget is yellow
	m_previewHelper->InitializeGL();
	qDebug() << "VideoScreen: initalizeGL works.";
}

The OpenGL context is created in the main-method.

QSurfaceFormat format;
format.setDepthBufferSize(24);
format.setStencilBufferSize(8);
format.setVersion(3, 2);
format.setProfile(QSurfaceFormat::CoreProfile);
QSurfaceFormat::setDefaultFormat(format);

The QOpenGLWidget part is implemented in a own class.

class OpenGLScreen : public QOpenGLWidget, protected QOpenGLFunctions {};

I create a instance of the class which inherits from QMainWindow. Then the OpenGLScreen Widget is added to a layout:

QScreen::QScreen(QWidget* parent)
    : QMainWindow(parent), 
    m_ui(new Ui::MainWindow)
{
    m_screen = new OpenGLScreen(parent);
    m_horizontalScreen->addWidget(m_screen);

QWidget::show();

Thanks for a little hint whats going wrong here.

SGaist

I don't see any projection matrix initialization.

In your code are you sure you are properly setting up the texture to be painted if using a texture for the video image ?

makopo

@SGaist

Thanks for advice. I tried to initialize the projection matrix in QOpenGLWidget::resizeGL(int w, int h), but I also get an empty widget.

void VideoScreen::resizeGL(int w, int h) {
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
    glMatrixMode(GL_MODELVIEW);
    qDebug() << "VideoScreen: resizeGl works";
}

According to the decklink documention the texture should be generated with a special interface, called IDeckLinkScreenPreviewHelper.
So in VideoScreen::paintGL() a call of m_screenPreviewHelper->PaintGL() should render Texture and Shader. But it is not deeply descripted in the documentation. Also in concerning sample code Shaders and Textures are not used.
When I include m_screenPreviewHelper->PaintGL() I get a black widget. After minimizing the main window paintGL is called and after reopening the window too. With this I get a single (freezed) frame rendered.

SGaist

Can you provide a link to the actual Decklink example you are using as reference ?

makopo

@SGaist

You can finde the sample code here: QuadPreview. The SDK is not shared with github e.g. , so I uploaded it to private cloud.

In this Qt example the matrix identity and the perspective are set with the QMatrix4x4 class. Is this modern way for initializing a matrix? Or is there no difference between

glViewport(0, 0, w, h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
glMatrixMode(GL_MODELVIEW);

and

m_proj.setToIdentity();
m_proj.perspective(45.0f, GLfloat(w) / h, 0.01f, 100.0f);

makopo

I fixed the issue with calling QWidget::update() in QOpenGLWidget::paintGL(). Now the videostream works.

SGaist

Glad you found a solution and thanks for sharing !

Note that this is a rather unexpected solution has you should not need to do that.

makopo

@SGaist said in Showing video in GUI does not work (black screen):

Note that this is a rather unexpected solution has you should not need to do that.

That sounds unsatisfactory. In fact signal an slot runs in the same thread.
In another source I found that signal and connect should run in the same thread. In my code these are different.

SGaist

Signals and slots are not required to be handled in objects belonging to the same thread.