Draw a cv::Mat inside a QGLWidget? - c++

Question

I am wondering how to draw a cv::Mat into a QGLWidget. I´ve found here a method and tried to implement it. But My widget is black.
This is the code of my widget:
h file
#ifndef GLVIEWER_H
#define GLVIEWER_H
#include <QtOpenGL/QGLWidget>
#include <QtOpenGL/QtOpenGL>
#include <opencv2/core/core.hpp>
class GLViewer : public QGLWidget {
Q_OBJECT
public:
GLViewer(QWidget *parent = NULL);
void setNewFrame(cv::Mat newframe);
protected:
void initializeGL();
void resizeGL(int w, int h);
void paintGL();
void mousePressEvent(QMouseEvent *event);
void mouseMoveEvent(QMouseEvent *event);
void keyPressEvent(QKeyEvent *event);
cv::Mat frame;
};
#endif // GLVIEWER_H
cpp file
#include "glviewer.h"
#include <iostream>
#include <QtGui/QMouseEvent>
GLViewer::GLViewer(QWidget *parent) : QGLWidget(parent) {
setMouseTracking(true);
frame = cv::Mat::ones(25, 50, CV_8UC3) * 255;
}
void GLViewer::setNewFrame(cv::Mat newframe) {
frame = newframe;
paintGL();
}
void GLViewer::initializeGL() {
std::cout << "initializeGL" << std::endl;
glViewport(0,0,this->width(), this->height());
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-this->width()/2, this->width()/2, this->height()/2, -this->height()/2, -1, 1);
}
void GLViewer::resizeGL(int w, int h) {
glViewport(0, 0, w, h);
// glMatrixMode(GL_PROJECTION);
// glLoadIdentity();
// gluOrtho2D(0, w, 0, h); // set origin to bottom left corner
// glMatrixMode(GL_MODELVIEW);
// glLoadIdentity();
}
void GLViewer::paintGL() {
//std::cout << "paint" << std::endl;
glClear(GL_COLOR_BUFFER_BIT);
glClearColor(1.0f, 0, 0, 1.0f);
glDrawPixels(frame.rows, frame.cols,
GL_RGB,
GL_UNSIGNED_BYTE,
frame.data);
}
As I have initialized the cv::Mat with ´cv::Mat::ones(25, 50, CV_8UC3) * 255;´ I expect to see a little blue rect but.. nothing is happening, full black widget.
Anyone can point me out to a solution?
EDIT
I have tried to change the background to red and now things are red.. this is the output:
the background is correct but the blue rect.. is not blue even if it seems to a rectangle.. what is happening? i have tried to invert frame.cols and frame.cols, and GL_BGR instead of GL_RGB and some random trials with data types (GL_UNSIGNED_SHORT etc) but nothing is getting a better result..

Answer 1

glDrawPixels is not a wise choice. 2D pixel drawing is amazingly slow in OpenGL. What you really want to do is load the matrix into a texture, then draw a primitive with the texture on it.
However for your simple use case:
I would recommend you use QGLWidget, but still QPainter to draw stuff. It will be fully accelerated, but you don't have to deal with all this transformation stuff etc.!
The code is rather simple. Have a look at:
https://sourceforge.net/p/gerbil/svn/HEAD/tree/trunk/gerbil-gui/scaledview.cpp
(an example of a widget that automatically scales the content)
To convert from Mat to QImage, see:
https://sourceforge.net/p/gerbil/svn/HEAD/tree/trunk/common/qtopencv.cpp
The conversion QImage <-> QPixmap can be done implicitely by constructing one from the other. Note that it essentially will only store the image data on the graphics card as a texture.
The conversion cv::Mat to QImage is a "redundant" data copy. However it is really negligible, try it!

Answer 2

Your resizeGL() is wiping out the matrix setup you're doing in initializeGL(), the glOrtho() call in particular.
Try moving the matrix setup to paintGL():
...
void GLViewer::initializeGL()
{
std::cout << "initializeGL" << std::endl;
glViewport(0,0,width(), height());
}
void GLViewer::resizeGL(int w, int h)
{
glViewport(0, 0, w, h);
}
void GLViewer::paintGL() {
std::cout << "paint" << std::endl;
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-width()/2.0, width()/2.0, height()/2.0, -height()/2.0, -1, 1);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glRasterPos2i(width()/2.0,-height()/2.0);
// also try glRasterPos2i(0,0)
glPixelZoom(-1.0f,-1.0f);
glDrawPixels
(
frame.rows, frame.cols,
GL_RGB,
GL_UNSIGNED_BYTE,
frame.data
);
}
...
EDIT: Try issuing a glPixelStore( GL_UNPACK_ALIGNMENT, 1 ) before the glDrawPixels() call. Or switch to GL_RGBA and a four-component cv::Mat format.