blender-archive/source/blender/gpu/shaders/gpu_shader_image_interlace_frag.glsl

/* Keep these in sync with GPU_shader.h */
#define INTERLACE_ROW 0
#define INTERLACE_COLUMN 1
#define INTERLACE_CHECKERBOARD 2

in vec2 texCoord_interp;
out vec4 fragColor;

uniform int interlace_id;
uniform sampler2D image_a;
uniform sampler2D image_b;

bool interlace()
{
  if (interlace_id == INTERLACE_CHECKERBOARD) {
    return (int(gl_FragCoord.x + gl_FragCoord.y) & 1) != 0;
  }
  else if (interlace_id == INTERLACE_ROW) {
    return (int(gl_FragCoord.y) & 1) != 0;
  }
  else if (interlace_id == INTERLACE_COLUMN) {
    return (int(gl_FragCoord.x) & 1) != 0;
  }
}

void main()
{
  if (interlace()) {
    fragColor = texture(image_a, texCoord_interp);
  }
  else {
    fragColor = texture(image_b, texCoord_interp);
  }
}