/* InterleavedColumnStereoShader.frag.txt - Shader for interleaved column stereo:
 *
 * Used in the stereo compositing chain of the Psychtoolbox imaging pipeline.
 *
 * Composes an interleaved stereo image by putting the left eye images pixel
 * lines into the even columns of the output buffer and the right eye images pixel
 * lines into the odd columns of the output buffer, thereby interleaving the
 * content of both eyes buffers into one unified horizontally interlaced buffer.
 *
 * This stereo encoding can be used by some auto-stereoscopic displays, e.g.,
 * ones with lenticular sheets or parallax barriers.
 *
 * Written 2010 by Mario Kleiner, part of the Psychtoolbox-3, licensed under
 * MIT license.
 *
 */ 

#extension GL_ARB_texture_rectangle : enable

/* Input image rectangle textures: Image1 = Left, Image2 = Right eye */
uniform sampler2DRect Image1;
uniform sampler2DRect Image2;
uniform vec2 Offset;

void main()
{
    /* Get default texel read position (x,y): x is column, y is row of image. */
    vec2 pos = gl_FragCoord.xy;

    /* oddcol == 1 for odd columns, 0 for even columns: */
    float oddcol = mod(pos.x, 2.0);
    
    /* Update the x component to only read left half of all columns in images: */
    pos.x = (pos.x - oddcol) / 2.0; 
    pos   = pos + Offset;

    /* Read the image pixel from the modified readpos, output it to framebuffer. */
    /* Choose between left- and right- image input buffer, depending if this is  */
    /* an even or an odd output column: */
    if (oddcol < 1.0) {
        /* Even col: Left image */
        gl_FragColor = texture2DRect(Image1, pos);
    }
    else {
        /* Odd col: Right image */
        gl_FragColor = texture2DRect(Image2, pos);
    }
}