Estimating incident illumination in outdoor scenes
In the paper named "Estimating Surface Reflectance Properties of a Complex Scene under Captured Natural Illumination", it explains the way to estimate the incident illumination in outdoor scenes.
- diffuse gray ball
- The reflectivity is (0.30, 0.31, 0.32).
- mirror ball
- This ball is used to capture the sky image. Its reflective is 0.52.
- black glossy ball
- This is used to estimate the direction of the sun.
- It is difficult to get the value of the sun's bright intensity, because it tends to saturate the mirrored sphere image.
- So, this paper uses these steps to get the sun's bright intensity.
- Estimate the sun direction from the black glossy ball.
- Convolve the mirror ball value and get the irradiance map as D'.
- Render the sun's diffuse lighting of sphere as D*.
- If we assume the sun intensity as α and the diffuse gray ball value as D, then D' + α D* should be D.
- So, by changing the value of the α, we can estimate the sun intensity α.
- If you write down the equation, it becomes like this.
convolve ( mirror_ball_pixel_value / ( 0.52, 0.52, 0.52 )) + sun_intensity * dot( sun_direction, surface_normal ) = diffuse_gray_ball_pixel_value / ( 0.30, 0.31, 0.32 )
- Reference
- "Estimating Surface Reflectance Properties of a Complex Scene under Captured Natural Illumination"