Drawing with openTK, save to file, show on screen

问题

I am drawing multiple images over earh map, I am using perspective correct texturing link on each image.

I want to store rendered image in to file (sorce file is 1280x760, the rendered images is around 160x90 in most cases rotated). Currently I am doing this with GL.ReadPixels

int width = 1920;
int height = 1080;
using (Bitmap bitmap = new Bitmap(width, height))
{
    System.Drawing.Imaging.BitmapData bits = bitmap.LockBits(new Rectangle(0, 0, width, height), System.Drawing.Imaging.ImageLockMode.WriteOnly, System.Drawing.Imaging.PixelFormat.Format32bppArgb);
    GL.ReadPixels(0, (0, width, height, OpenTK.Graphics.OpenGL.PixelFormat.Bgra, PixelType.UnsignedByte, bits.Scan0);
    bitmap.UnlockBits(bits);
    bitmap.RotateFlip(RotateFlipType.Rotate180FlipX);
    bitmap.Save("output.png", System.Drawing.Imaging.ImageFormat.Png);
}

The problem occurs when I move the map, rendered images are not visible anymore on the screen and it looks like that in this case GL.ReadPixels returns empty pixels.

How to get rendered image even if this is not currently rendered on screen?

Extra question, if I use framebuffer the result is the same but using the frame buffer I never see image on the screen, it looks like framebuffer is not drawn on the screen, but GL.ReadPixels can get image out. Which lines of code are needed to draw framebuffer also on the screen?

Any idea?

来源：https://stackoverflow.com/questions/65250801/drawing-with-opentk-save-to-file-show-on-screen