How to change screen resolution using QT, OpenGL, C++, and Linux?

Question

I just want to make a full screen game. I know how to change resolution on Windows but how do I change the resolution under Linux? Is there a cross platform QT solution for this? Also I've got borders around my GLWidget. How do I make the widget cover the entire window?

I'm just going to post the code:

#include <QtOpenGL>

class GLWidget : public QGLWidget
{
public:
    void initializeGL()
    {
        glClearColor(0.0f, 0.0f, 1.0f, 0.0f);
        glClearDepth(1.0f);
    }

    void paintGL()
    {
        glClear(GL_COLOR_BUFFER_BIT);
    }

    void resizeGL(int width, int height)
    {
        int side = qMin(width, height);
        glViewport((width - side) / 2, (height - side) / 2, side, side);
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        glOrtho(-0.5f, +0.5f, -0.5f, +0.5f, 4.0f, 15.0f);
        glMatrixMode(GL_MODELVIEW);
    }
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QDesktopWidget *desktop = app.desktop();
    QWidget window;
    GLWidget *glWidget = new GLWidget;
    QHBoxLayout *mainLayout = new QHBoxLayout;
    mainLayout->addWidget(glWidget);
    window.setLayout(mainLayout);
    window.setWindowTitle("Hello GL");
    window.resize(QSize(640, 480));
    window.show();
    window.showFullScreen();
    return app.exec();
}

Answer 1

You can use xrrsetscreenconfigandrate, as explained here:

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
#include<X11/Xlib.h>
#include<X11/extensions/Xrandr.h>

Display                 *dpy;
Window                  root;
int                     num_sizes;
XRRScreenSize           *xrrs;
XRRScreenConfiguration  *conf;
short                   possible_frequencies[64][64];
short                   original_rate;
Rotation                original_rotation;
SizeID                  original_size_id;

int main(int argc, char *argv[]){
 //
 //     CONNECT TO X-SERVER, GET ROOT WINDOW ID
 //
 dpy    = XOpenDisplay(NULL);
 root   = RootWindow(dpy, 0);
 //
 //     GET POSSIBLE SCREEN RESOLUTIONS
 //
 xrrs   = XRRSizes(dpy, 0, &num_sizes);
 //
 //     LOOP THROUGH ALL POSSIBLE RESOLUTIONS,
 //     GETTING THE SELECTABLE DISPLAY FREQUENCIES
 //
 for(int i = 0; i < num_sizes; i ++) {
        short   *rates;
        int     num_rates;

        printf("\n\t%2i : %4i x %4i   (%4imm x%4imm ) ", i, xrrs[i].width, xrrs[i].height, xrrs[i].mwidth, xrrs[i].mheight);

        rates = XRRRates(dpy, 0, i, &num_rates);

        for(int j = 0; j < num_rates; j ++) {
                possible_frequencies[i][j] = rates[j];
                printf("%4i ", rates[j]); } }

 printf("\n");
 //
 //     GET CURRENT RESOLUTION AND FREQUENCY
 //
 conf                   = XRRGetScreenInfo(dpy, root);
 original_rate          = XRRConfigCurrentRate(conf);
 original_size_id       = XRRConfigCurrentConfiguration(conf, &original_rotation);

 printf("\n\tCURRENT SIZE ID  : %i\n", original_size_id);
 printf("\tCURRENT ROTATION : %i \n", original_rotation);
 printf("\tCURRENT RATE     : %i Hz\n\n", original_rate);
 //
 //     CHANGE RESOLUTION
 //
 printf("\tCHANGED TO %i x %i PIXELS, %i Hz\n\n", xrrs[1].width, xrrs[1].height, possible_frequencies[1][0]);
 XRRSetScreenConfigAndRate(dpy, conf, root, 1, RR_Rotate_0, possible_frequencies[1][0], CurrentTime);
 //
 //     SLEEP A WHILE
 //
 usleep(6000000);
 //
 //     RESTORE ORIGINAL CONFIGURATION
 //
 printf("\tRESTORING %i x %i PIXELS, %i Hz\n\n", xrrs[original_size_id].width, xrrs[original_size_id].height, original_rate);
 XRRSetScreenConfigAndRate(dpy, conf, root, original_size_id, original_rotation, original_rate, CurrentTime);
 //
 //     EXIT
 //
 XCloseDisplay(dpy); }
//
//      gcc -o Xrandr Xrandr.cc -lX11 -lXrandr -lstdc++
//