OpenGL for AviSynth [Update: now w/code]


I had a little project at work recently, that involved creating movie clips using AviSynth.
And I was appalled by the shabbiness of existing transition plugins available freely for AviSynth, they always reminded me of 80s-like video editing...
So I set out to integrate AviSynth with OpenGL to create a nice 3D transition effect for our movie clips.

I had 2 major bases to cover:

  • AviSynth plugin API
  • OpenGL rendering

AviSynth API is not so well documented, but they have very good ground-up examples on how to DIY plugin. Here is the one I used, that basically does nothing but copy the input frame to the output frame.
Open GL on the other hand is very well documented and "tutorialed". I based my code on this example from NeHe.

So basically what I wanted to achive is:

  1. Read input frame (AviSynth)
  2. Paint frame as texture over 3D model (OpenGL)
  3. Draw rendered 3D image to output frame (OpenGL+AviSynth)

Reading the frame is pretty straightforward. Frames come encoded as RGB 24bit, with a little twist: rows size in bytes is not width*3 as you'd expect it be, but AviSynth use a parameter called "Pitch" to determine row size in bytes.

Update (14/9/09): source is now available in the repo: browse download

So when I extract the input image I do:

uchar* texBuf = (uchar*)malloc(width * height * 3); //"pure" RGB24 encoding
const unsigned char* srcp = src->GetReadPtr(); //strange AviSynth RGB24 encoded data

int line_length = width*3;
for(int y=0;y [lt] height; y++)
const unsigned char* line_srcp = srcp + y*src_pitch;
unsigned char* line_dstp = texBuf + (height - 1 - y)*line_length;

OK, now I have the input frame in my memory space and I can move on to make an OpenGL texture out of it:

glBindTexture(GL_TEXTURE_2D, tex);
gluBuild2DMipmaps( GL_TEXTURE_2D, 3, width,height,


This is when I ran into the first wall. In order to render anything, OpenGL must have a rendering context, for a rendering context you need an OS drawing context. In Win32 case a drawing context can be either a bona fide actual window, or a memory-based BITMAP...

I burned many hours trying to make BITMAPs work as offscreen redering contexts for OpenGL but with no avail... So I went with the "dirtier" solution of creating a window for the sake of off-screen redering (which kinda takes the "off" out of "offscreen"), but it works..
For easier setup I used GLUT (with an OpenGLUT impl.) and GLEE:

glutInitDisplayMode ( GLUT_RGB | GLUT_DOUBLE );
glutCreateWindow( "offscreen" );

glutDisplayFunc ( display );
glutReshapeFunc ( reshape );
glutKeyboardFunc ( keyboard );
glutIdleFunc ( idle );


This is done in each frame, sadly, but I wasn't able to make it work otherwise.
Now, another hack I had to do is have the GLUT main loop render only one frame... Because the whole thing is setup again in the next frame.
So I enter the main loop after all settings are done:


And in the end of the display() function I exit the main loop: (this can only be done in OpenGLUT, regular GLUT does not support this)




But, not before I grab all the pixel data from the rendered image using glReadPixels. This will go into the output frame of the AviSynth session:

for(int y=0;y [lt] height; y++)
unsigned char* line_dstp = dstp + y*dst_pitch;
unsigned char* line_srcp = img_data + y*width*3;

OK, we're pretty much done. Just need to draw some polygons in the display() function, use the tex texture and the input movie will appear frame-by-frame on the polygons - sweet!

Some results
This is the .avs file:
A = ImageSource("A.jpg",end=70)
B = ImageSource("B.jpg",end=50)

This is the result:

Another result, this time with actual video as an input:
A = AVISource("MVI_6130.avi.AVI").ConvertToRGB24()