# Adding Ozone to my sky simulation

I implemented a simulation for the colour of the sky a while ago by following the scratch a pixel tutorial: https://www.scratchapixel.com/lessons/procedural-generation-virtual-worlds/simulating-sky

I adapted it for the actual sun position and am able to get realistic sky colours during the day. However, I noticed that after sunset/ before sunrise, the colours are greyish when they should be deep blue. After researching about this, I read that this is due to the ozone absorption not being present in my model.

I used extinction coefficients : (3.426,8.298,0.356) * 0.06e-5 -> found on https://media.contentapi.ea.com/content/dam/eacom/frostbite/files/s2016-pbs-frostbite-sky-clouds-new.pdf

and also read that since the ozone does not scatter, it should only be added to the transmittance value.

Therefore, I modified the code from scratchapixel as follows:

        for (uint32_t i = 0; i < numSamples; ++i) {
vec3 samplePosition = ray_in2.origin() + (tCurrent +
segmentLength * 0.5f) * ray_in2.direction();
float height = samplePosition.length() - atmosphere.earthRadius;

// compute optical depth for light
float hr = exp(-height / atmosphere.Hr) * segmentLength;
float hm = exp(-height / atmosphere.Hm) * segmentLength;
float ho = exp(-height / atmosphere.Hr)* segmentLength*(6e-7);
opticalDepthR += hr;
opticalDepthM += hm;
opticalDepthO += ho;

// light optical depth
float t0Light, t1Light;

...
for (j = 0; j < numSamplesLight; ++j) {
vec3 samplePositionLight = samplePosition + (tCurrentLight +
segmentLengthLight * 0.5f) * sunDir;
float heightLight = samplePositionLight.length() -

if (heightLight < 0) break;
opticalDepthLightR += exp(-heightLight / atmosphere.Hr) *
segmentLengthLight;
opticalDepthLightM += exp(-heightLight / atmosphere.Hm) *
segmentLengthLight;
opticalDepthLightO += exp(-heightLight / atmosphere.Hr) *
segmentLengthLight*(6e-7); ;
tCurrentLight += segmentLengthLight;
}
if (j == numSamplesLight) {
vec3 tau = (betaR) * (opticalDepthR + opticalDepthLightR) +
betaM * 1.1f * (opticalDepthM + opticalDepthLightM)+ betaO*
(opticalDepthO + opticalDepthLightO);
vec3 attenuation(exp(-tau.x()), exp(-tau.y()), exp(-
tau.z()));


summary : -I added variable opticalDepthO and opticalDepthLightO which are calculated same as the optical depth for Rayleigh, but multiplied by 6e-7.

-Then, the sum of opticalDepthLightO and opticalDepthO is multiplied by the extiction coefficient for ozone and added to variable tau.

Problem is, I see no difference in my sky colour before and after adding ozone. Can someone guide me to what it is that I'm doing wrong? Thanks in advance!