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vec2 mapping_octahedron(vec3 cubevec, vec2 texel_size)
{
/* projection onto octahedron */
cubevec /= dot(vec3(1.0), abs(cubevec));
/* out-folding of the downward faces */
if (cubevec.z < 0.0) {
vec2 cubevec_sign = step(0.0, cubevec.xy) * 2.0 - 1.0;
cubevec.xy = (1.0 - abs(cubevec.yx)) * cubevec_sign;
}
/* mapping to [0;1]ˆ2 texture space */
vec2 uvs = cubevec.xy * (0.5) + 0.5;
/* edge filtering fix */
uvs = (1.0 - 2.0 * texel_size) * uvs + texel_size;
return uvs;
}
vec4 textureLod_octahedron(sampler2DArray tex, vec4 cubevec, float lod, float lod_max)
{
vec2 texelSize = 1.0 / vec2(textureSize(tex, int(lod_max)));
vec2 uvs = mapping_octahedron(cubevec.xyz, texelSize);
return textureLod(tex, vec3(uvs, cubevec.w), lod);
}
vec4 texture_octahedron(sampler2DArray tex, vec4 cubevec)
{
vec2 texelSize = 1.0 / vec2(textureSize(tex, 0));
vec2 uvs = mapping_octahedron(cubevec.xyz, texelSize);
return texture(tex, vec3(uvs, cubevec.w));
}
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