/* InterleavedLineStereoShader.frag.txt - Shader for interleaved line 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 rows of the output buffer and the right eye images pixel
 * lines into the odd rows of the output buffer, thereby interleaving the
 * content of both eyes buffers into one unified interlaced buffer.
 *
 * This stereo encoding can be used by some stereo goggles, e.g., the
 * "iGlasses 3D Video Goggles".
 *
 * Written 2007 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;

    /* oddrow == 1 for odd lines, 0 for even lines: */
    float oddrow = mod(pos.y, 2.0);
    
    /* Update the y component to only read upper half of all lines in images: */
    pos.y = (pos.y - oddrow) / 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 row: */
    if (oddrow < 1.0) {
        /* Even row: Left image */
        gl_FragColor = texture2DRect(Image1, pos);
    }
    else {
        /* Odd row: Right image */
        gl_FragColor = texture2DRect(Image2, pos);
    }
}