Create a video from OpenGL rendered frames - opengl

Question

I am trying to create a video file from an animation I have in OpenGL.
I have been reading on how to do that and to my understanding there are two options:
Save each rendered frame in OpenGL to an image file and then create a video file from those
Get the frame data using glReadPixels() and on the fly write those to a video file
The second approach is what I believe would work best for me, however, I cannot find info on how to achieve the second part (write to a video file).
Can anyone point me out to some web sites where I learn how to do that? What kind of libraries are out there that I can use to encode(?) a video from the frames I am rendering in OpenGL?
EDIT
After searching a bit more about this, I believe ffmpeg is the way to go. I found this blog that has a code that apparently works on windows.
I have downloaded ffmpeg from the website so that I can execute the command just as in the example. Unfortunately, my application crashes and no video is being created. I checked for the file pointer to be valid but it is not, so I believe the error comes from the execution of the function popen.
I am passing the exact same arguments as the command but still no valid file pointer, any idea on what could be happening?
The thing is, I don't want to spend much time coding the video encoding since I have other projects to work on.

Answer 1

Since I couldn't use ffmpeg directly from my c++ code, a possible solution is as follows. In Qt5 you have the function paintGL where you update the frame to be rendered. After it, get the pixels with glReadPixels and then just save the frame as a png image using QImage
void OpenGLViewer::paintGL()
{
// Clear screen
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// Update array attached to OpenGL
glDrawArrays(GL_POINTS, 0, _points);
update();
glReadPixels(0, 0, this->width(), this->height(), GL_RGBA, GL_UNSIGNED_BYTE, _buffer);
std::stringstream name;
name << "Frame" << _frame++ << ".png";
QString filename(name.str().c_str());
QImage imagen(_buffer, this->width(), this->height(), QImage::Format_ARGB32);
imagen.save(filename, "PNG");
}
This will leave a bunch of images in your working directory that you can encode in a video using the following command from the console
ffmpeg -framerate 30 -start_number 0 -i Frame%d.png -vcodec mpeg4 -vf vflip test.avi
I still have to check why the colors are inverted but for now this works fine since the animation is the important thing and not the colors.

Answer 2

You can do in this way after installing libpng:
uint8_t pixels = new uint8_t[wh*3];
glReadPixels(0, 0, w, h, GL_RGB, GL_UNSIGNED_BYTE, (GLvoid *) pixels);
for (int j = 0; j * 2 < h; ++j) {
int x = j * w * 3;
int y = (h - 1 - j) * w * 3;
for (int i = w * 3; i > 0; --i) {
uint8_t tmp = pixels[x];
pixels[x] = pixels[y];
pixels[y] = tmp;
++x;
++y;
}
}
png_structp png = png_create_write_struct(PNG_LIBPNG_VER_STRING, nullptr, nullptr, nullptr);
if (!png)
return false;
png_infop info = png_create_info_struct(png);
if (!info) {
png_destroy_write_struct(&png, &info);
return false;
}
std::string s = "IMAGE/"+string(filename);
FILE *fp = fopen(s.c_str(), "wb");
if (!fp) {
png_destroy_write_struct(&png, &info);
return false;
}
png_init_io(png, fp);
png_set_IHDR(png, info, w, h, 8 , PNG_COLOR_TYPE_RGB, PNG_INTERLACE_NONE,
PNG_COMPRESSION_TYPE_BASE, PNG_FILTER_TYPE_BASE);
png_colorp palette = (png_colorp)png_malloc(png, PNG_MAX_PALETTE_LENGTH * sizeof(png_color));
if (!palette) {
fclose(fp);
png_destroy_write_struct(&png, &info);
return false;
}
png_set_PLTE(png, info, palette, PNG_MAX_PALETTE_LENGTH);
png_write_info(png, info);
png_set_packing(png);
png_bytepp rows = (png_bytepp)png_malloc(png, h * sizeof(png_bytep));
for (int i = 0; i < h; ++i)
rows[i] = (png_bytep)(pixels + (h - i - 1) * w * 3);
png_write_image(png, rows);
png_write_end(png, info);
png_free(png, palette);
png_destroy_write_struct(&png, &info);
fclose(fp);
delete[] rows;