omino pixel blog

This is a rendering simulating an image projected onto a simple 3d scene. There’s nothing too exotic about it, except that the projection was done in 2d, in After Effects.

And… here’s a moving sequence of the same scene. With old family movies.

The usual way you’d do this is by rendering a movie, taking it into your 3d program, and projecting it onto a scene through a light source.

For my current project, I needed this effect, but wanted a way to work in After Effects. I did this for workflow reasons: my projection content was all AE authored. But it also turns out to save render time. More than a little.

The trick was to render the scene in 3d as a coordinate-map, in Red and Green. I used Modo, and the projection mode is called “Front Projection”, from a spotlight or camera. I projected a red-green ramp onto the scene.

But we’re going to use the red and green values as a projection function. Changing the shader to use unattenuated (“luminous” @ 1W/steradian in modo) accomplishes this. We’re rendering geometry, not optics.

Then, I wrote a simple Pixel Bender kernel to use the red and green pixel values as coordinates to draw from the source projected image.

// by david van brink / poly@omino.com / 2009
// https://omino.com/pixelblog/
kernel rg_projection
<  namespace : "omino";    vendor : "omino";    version : 1;    
   description : "uses red/green from one image to map another"; >
{
    input image4 src; // Red-green mapping image
    input image4 ref; // image to project
    output pixel4 dst;

    parameter float imageXSize;
    parameter float imageYSize;
    parameter float imageXOffset;
    parameter float imageYOffset;

    void
    evaluatePixel() {
        pixel4 a = sampleLinear(src,outCoord());
        float r = a.r * imageXSize + imageXOffset;
        float g = a.g * imageYSize + imageYOffset;

        dst = sampleLinear(ref,float2(r,g));
    }
}

To test the Pixel Bender kernel, I ran a reference grid through the red-green ramp.

The first one shows gamma correction on the ramp image in After Effects. Under “Interpret Footage”, be sure to turn on “Preserve RGB”.

The second one reveals that Photoshop’s default ramp is some sort of s-shaped modified linear function. Creating a custom gradient with more stops along the way helped get the third, which is Good Enough. I went back and redid the scene with the improved red-green gradient.

And of course, use 32-bit color throughout. 16-bit works ok, too. 8-bit showed definite Squarifying in the projection, though.

To reintroduce the optical nature of the projection, back in After Effects it’s multiplied by a normally rendered image and, because I still think it’s pretty neat idea, an ambient occlusion pass as well.

And there you have it! 3d projections done all in 2d.

Download the AE Project