Rendering Oudoor Light Scattering in Real Time实时渲染室外光散射

1、makebettergamesrendering outdoor light scattering in real timenaty hoffmanwestwood sarcot j preethamati r makebettergamesoutline basics atmospheric light scattering radiometric quantities from radiance to pixels scattering theory absorption, out-scattering, in-scattering rayleigh and mie scattering

2、implementation aerial perspective, sunlight, skylight vertex shader future workmakebettergames is caused by a variety of particles molecules, dust, water vapor, etc. these can cause light to be: scattered into the line of sight (in-scattering) scattered out of the line of sight (out-scattering) abso

3、rbed altogether (absorption)atmospheric light scattering illuminates the skyatmospheric light scattering attenuates and colors the sunatmospheric light scattering attenuates and colors distant objectsatmospheric light scatteringatmospheric light scattering varies by time of day weather pollutionatmo

4、spheric light scattering varies by time of day weather pollutionatmospheric light scattering varies by time of day weather pollutionatmospheric light scattering varies by time of day weather pollution varies between planetsatmospheric light scatteringmakebettergames extinction (absorption, out-scatt

5、ering) phenomena which remove light multiplicative: in-scattering: phenomenon which adds light additive: combined: atmospheric light scattering0exextinctionlflinlin0exscatteringllflmakebettergamesradiometric quantities radiant flux radiance irradiancemakebettergamesradiometric quantities radiant flu

6、x quantity of light through a surface radiant power (energy / time) wattmakebettergamesradiometric quantities radiance l quantity of light in a single ray radiant flux / area / solid angle watt / (meter2 * steradian)makebettergamesradiometric quantities irradiance e quantity of light incident to a s

7、urface point incident radiant flux / area (watt / meter2) radiance integrated over hemispheremakebettergamesfrom radiance to pixels compute radiance incident to eye through each screen pixelmakebettergamesfrom radiance to pixels pixel value based on radiance but radiance is distributed continuously

8、along the spectrum we need three numbers: r, g, bmakebettergamesfrom radiance to pixels spd (spectral power distribution) to rgb fast approach: do all math at r, g, b sample wavelengths correct approach: use spds, convert final radiance to rgb 400nm 500nm 600nm 700nmsmlmakebettergamesabsorption abso

9、rption cross section absorbed radiant flux per unit incident irradiance units of area (meter2)abeabmakebettergamesabsorption absorption cross sectionabeabeababmakebettergamesabsorption absorption coefficient particle densitytimes absorption cross section units of inverse length (meter-1)abababmakebe

10、ttergamesabsorption total absorption cross section: probability of absorption:adsdsa aabababababababab aapdsmakebettergamesabsorption attenuation of radiance from travel through a constant-density absorptive medium: selslab0l0l(s)smakebettergamesout-scattering exactly as in the absorption case scatt

11、ering cross section scattering coefficient attenuation due to out-scattering in a constant-density medium: selslsc0scscscscscmakebettergamesextinction both absorption and out-scattering attenuate light they can be combined as extinction extinction coefficient total attenuation from extinction selsle

12、x0 sesfexexscabexmakebettergamesin-scattering light is scattered into a view ray from all directions from the sun from the sky from the ground we will only handle in-scattering from the sunmakebettergamesin-scattering where does a scattered photon go? scattering phase function if a photon is scatter

13、ed, gives the probability it goes in direction most atmospheric particles are spherical or very small:,f, ff,makebettergamesin-scattering how do we usefor in-scattering? in-scatter probability: in-scatter radiance :eye raysun f sunf sunsunsuneflfsunmakebettergamesin-scattering in-scattering over a p

14、ath radiance from a single event: over a distance ds: angular scattering coefficient in-scattering over ds: units of: meter-1 * steradian-1 fscsc sunef dsefscsun dsescsun scmakebettergamesin-scattering added radiance from solar in-scattering through a constant-density scattering medium: seeslex11,sc

15、sunexins,inslmakebettergamesextinction and in-scattering ,f,inex0slslsl sesexexf seeslex11,scsunexins, sl0lmakebettergamesextinction and in-scattering ,f,inex0slslsl compare to hardware fog: monochrome extinction added color completely non-directional scslslff-1fog0makebettergamesrayleigh scattering

16、 small particles is proportional to05. 0rsc4makebettergamesrayleigh scattering phase function: 2rcos1163frayleigh scatteringmakebettergamesmie scattering larger, spherical particles phase function approximation: henyey-greenstein 2/322hgcos2141gggf0-0.5-0.750.50.75makebettergamesmie scattering wavel

17、ength dependence complex and depends on exact size of particle in practice, air usually contains a mix of various sizes of mie particles in the aggregate these tend to average out any wavelength dependencemie scatteringmakebettergamescombined scattering in practice, air contains both rayleigh and mi

18、e scatterers absorption is usually slight we will use:miescrayleighscex hgmiescrrayleighscsc ffmakebettergames atmospheric parameters: constant? affected by extinction constant:parametersmiescrayleighschggsune0sunewith scatteringwithout scatteringhow ?implementation aerial perspective extinction &am

19、p; inscattering rays low in atmosphere constant density good approximation implementations sunlight is white density is not constant! use a more accurate model for fex?implementationex0sungroundsunfee0sunegroundsune0sune sunlight: virtual sky dome, use simple modelimplementationdensitydistance r r s

20、 sky color: density is not constant! more accurate model too expensive many computations needed per frame sky geometry virtual sky domeimplementation,inskyflmakebettergamesimplementation compute: can be done with textures 1d texture for texture coordinate is a function of s 2d texture for texture co

21、ords are functions of s, combine in pixel shader we decided on a different approach ,f,inex0slslslexfinlmakebettergamesimplementation compute: use vertex shader to compute apply as vertex interpolated colors in pixel shader, or even fixed pipeline pros: doesnt use valuable texture slots can change a

22、tmosphere properties cons: somewhat dependent on tessellationinex,fl ,f,inex0slslslmakebettergamesvertex shaderrhggextf od0positionmin od1lsun directioneye positionvertexshadertransform matrixconstants:suneoutputs:makebettergamesvertex shader sesmrexf seeslmr1,sunmrmrin ,f,inex0slslsl 2rrcos1163 2/3

23、22mmcos21141gggmakebettergamesvertex shader current implementation: 33 instructions not including macro expansion could probably be optimized 8 registersx=l0fexl0 * fexl = l0 * fex + linpixel shader+=l0 * fexlinl = l0 * fex + linl = l0 * fex + linpixel shaderresultsrayleigh scattering - highmie scat

24、tering - lowsun altitude - highrayleigh scattering - lowmie scattering - highsun altitude - highrayleigh scattering - mediummie scattering - mediumsun altitude - lowplanet mars like scatteringdemomakebettergamesconclusion scattering is easy to implement. easy to add to an existing rendering framewor

25、k compute fex and lin apply these to existing color to get final colormakebettergamesfuture work in-scattering from sky clouds (scattering and extinction) volumetric cloud shadows non-uniform density distributions full-spectrum math?makebettergamesacknowledgements we would like to thank kenny mitchell for the terrain engine used in our demo solomon srinivasan for help with the demo m