How to take screenshot in OpenGL

Question

How to take a screenshot of an OpenGL window in C++ and save it to file.

I found the glReadPixels() function, but I don't know what to do next. Where I can set path to a file, for example?

If not difficult, write code, please.

Answer 1

This piece of code captures the OpenGL window and export to a BMP file. You must have FreeImage library to run it.

// Make the BYTE array, factor of 3 because it's RBG.
BYTE* pixels = new BYTE[3 * width * height];

glReadPixels(0, 0, width, height, GL_RGB, GL_UNSIGNED_BYTE, pixels);

// Convert to FreeImage format & save to file
FIBITMAP* image = FreeImage_ConvertFromRawBits(pixels, width, height, 3 * width, 24, 0x0000FF, 0xFF0000, 0x00FF00, false);
FreeImage_Save(FIF_BMP, image, "C:/test.bmp", 0);

// Free resources
FreeImage_Unload(image);
delete [] pixels;

Answer 2

glReadPixels will copy the bits into a memory buffer that you supply. You have to manually format the data (to the image format of your choice) and write it to disk after glReadPixels returns.

Answer 3

Runnable example

Each time you click with the mouse on the window, a tmpX.ppm file is created with the current screenshot.

You can view this file for example with eog on Linux, and inspect it with a text editor.

To render without showing a window, see: How to use GLUT/OpenGL to render to a file?

#include <math.h>
#include <stdlib.h>
#include <stdio.h>

#define GL_GLEXT_PROTOTYPES 1
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
#include <GL/glext.h>

static GLubyte *pixels = NULL;
static const GLenum FORMAT = GL_RGBA;
static const GLuint FORMAT_NBYTES = 4;
static const unsigned int HEIGHT = 500;
static const unsigned int WIDTH = 500;
static unsigned int nscreenshots = 0;
static unsigned int time;

/* Model. */
static double angle = 0;
static double angle_speed = 45;

static void init(void)  {
    glReadBuffer(GL_BACK);
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glPixelStorei(GL_PACK_ALIGNMENT, 1);
    glViewport(0, 0, WIDTH, HEIGHT);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glMatrixMode(GL_MODELVIEW);

    pixels = malloc(FORMAT_NBYTES * WIDTH * HEIGHT);
    time = glutGet(GLUT_ELAPSED_TIME);
}

static void deinit(void)  {
    free(pixels);
}

static void create_ppm(char *prefix, int frame_id, unsigned int width, unsigned int height,
        unsigned int color_max, unsigned int pixel_nbytes, GLubyte *pixels) {
    size_t i, j, k, cur;
    enum Constants { max_filename = 256 };
    char filename[max_filename];
    snprintf(filename, max_filename, "%s%d.ppm", prefix, frame_id);
    FILE *f = fopen(filename, "w");
    fprintf(f, "P3\n%d %d\n%d\n", width, HEIGHT, 255);
    for (i = 0; i < height; i++) {
        for (j = 0; j < width; j++) {
            cur = pixel_nbytes * ((height - i - 1) * width + j);
            fprintf(f, "%3d %3d %3d ", pixels[cur], pixels[cur + 1], pixels[cur + 2]);
        }
        fprintf(f, "\n");
    }
    fclose(f);
}

static void draw_scene() {
    glClear(GL_COLOR_BUFFER_BIT);
    glLoadIdentity();
    glRotatef(angle, 0.0f, 0.0f, -1.0f);
    glBegin(GL_TRIANGLES);
    glColor3f(1.0f, 0.0f, 0.0f);
    glVertex3f( 0.0f,  0.5f, 0.0f);
    glColor3f(0.0f, 1.0f, 0.0f);
    glVertex3f(-0.5f, -0.5f, 0.0f);
    glColor3f(0.0f, 0.0f, 1.0f);
    glVertex3f( 0.5f, -0.5f, 0.0f);
    glEnd();
}

static void display(void) {
    draw_scene();
    glutSwapBuffers();
    glReadPixels(0, 0, WIDTH, HEIGHT, FORMAT, GL_UNSIGNED_BYTE, pixels);
}

static void idle(void) {
    int new_time = glutGet(GLUT_ELAPSED_TIME);
    angle += angle_speed * (new_time - time) / 1000.0;
    angle = fmod(angle, 360.0);
    time = new_time;
    glutPostRedisplay();
}

void mouse(int button, int state, int x, int y) {
    if (state == GLUT_DOWN) {
        puts("screenshot");
        create_ppm("tmp", nscreenshots, WIDTH, HEIGHT, 255, FORMAT_NBYTES, pixels);
        nscreenshots++;
    }
}

int main(int argc, char **argv) {
    GLint glut_display;
    glutInit(&argc, argv);
    glutInitWindowSize(WIDTH, HEIGHT);
    glutInitWindowPosition(100, 100);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGBA);
    glutCreateWindow(argv[0]);
    init();
    glutDisplayFunc(display);
    glutIdleFunc(idle);
    glutMouseFunc(mouse);
    atexit(deinit);
    glutMainLoop();
    return EXIT_SUCCESS;
}

Compile with:

gcc main.c -lm -lGL -lGLU -lglut

Tested on Ubuntu 15.10, OpenGL 4.5.0 NVIDIA 352.63.

Vulkan

This example just worked: https://github.com/SaschaWillems/Vulkan/blob/b9f0ac91d2adccc3055a904d3a8f6553b10ff6cd/examples/screenshot/screenshot.cpp how to run it: Is it possible to do offscreen rendering without Surface in Vulkan?