OpenGL mini framebuffer display example

example.c

// cc example.c -lX11 -lGL
// This is free and unencumbered software released into the public domain.
#define GLX_GLXEXT_PROTOTYPES
#include <stdio.h>
#include <GL/glx.h>
#include <X11/Xlib.h>

#define WIDTH   256
#define HEIGHT  128
#define SCALE   4

int main(void)
{
    Display *d = XOpenDisplay(0);
    if (!d) {
        fputs("XOpenDisplay\n", stderr);
        return 1;
    }

    static const int attribs[] = {
        GLX_X_RENDERABLE, True,
        GLX_DRAWABLE_TYPE, GLX_WINDOW_BIT,
        GLX_RENDER_TYPE, GLX_RGBA_BIT,
        GLX_X_VISUAL_TYPE, GLX_TRUE_COLOR,
        GLX_RED_SIZE, 8,
        GLX_GREEN_SIZE, 8,
        GLX_BLUE_SIZE, 8,
        GLX_ALPHA_SIZE, 8,
        GLX_DEPTH_SIZE, 24,
        GLX_STENCIL_SIZE, 8,
        GLX_DOUBLEBUFFER, True,
        GLX_SAMPLE_BUFFERS, 1,
        GLX_SAMPLES, 4,
        None
    };
    int fbcount;
    int screen = DefaultScreen(d);
    GLXFBConfig *fbcs = glXChooseFBConfig(d, screen, attribs, &fbcount);
    if (!fbcs || !fbcount) {
        fputs("glXChooseFBConfig\n", stderr);
        return 1;
    }
    GLXFBConfig fbc = fbcs[0];
    XFree(fbcs);

    XVisualInfo *vi = glXGetVisualFromFBConfig(d, fbc);
    XSetWindowAttributes swa;
    swa.colormap = XCreateColormap(d, RootWindow(d, vi->screen), vi->visual, AllocNone);
    swa.background_pixmap = None;
    swa.border_pixel = 0;
    swa.event_mask = StructureNotifyMask;
    Window win = XCreateWindow(d, RootWindow(d, vi->screen),
                               0, 0, WIDTH*SCALE, HEIGHT*SCALE,
                               0, vi->depth, InputOutput, vi->visual,
                               CWBorderPixel|CWColormap|CWEventMask, &swa);
    if (!win) {
        fputs("XCreateWindow\n", stderr);
        return 1;
    }
    XStoreName(d, win, "MCU");
    XMapWindow(d, win);

    GLXContext ctx = glXCreateContext(d, vi, 0, True);
    XFree(vi);
    XSync(d, False);
    if (!ctx) {
        fputs("glXCreateContext\n", stderr);
        return 1;
    }
    glXMakeCurrent(d, win, ctx);
    glXSwapIntervalEXT(d, glXGetCurrentDrawable(), 1);

    GLuint tex;
    glEnable(GL_TEXTURE_2D);
    glGenTextures(1, &tex);
    glBindTexture(GL_TEXTURE_2D, tex);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, WIDTH, HEIGHT, 0, GL_RGB, GL_UNSIGNED_BYTE, 0);

    unsigned long long rng = 1;
    unsigned char framebuffer[HEIGHT][WIDTH][3];
    for (;;) {
        // Fill framebuffer with random data for demonstration
        for (int y = 0; y < HEIGHT; y++) {
            for (int x = 0; x < WIDTH; x++) {
                rng = rng*0x3243f6a8885a308d + 1;
                framebuffer[y][x][0] = rng >> 40;
                framebuffer[y][x][1] = rng >> 48;
                framebuffer[y][x][2] = rng >> 56;
            }
        }
        glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, WIDTH, HEIGHT,
                        GL_RGB, GL_UNSIGNED_BYTE, framebuffer);
        glBegin(GL_QUADS);
        glVertex2f(-1, -1); glTexCoord2f(0, 0);
        glVertex2f(-1, +1); glTexCoord2f(1, 0);
        glVertex2f(+1, +1); glTexCoord2f(1, 1);
        glVertex2f(+1, -1); glTexCoord2f(0, 1);
        glEnd();
        glXSwapBuffers(d, win);
    }
}