# lights sum, linear vs srgb

 06 June 2013 sandidolsak Expert   portfolio Sandi Dolšak CEO / TD Ratata studio Ljubljana, Slovenia lights sum, linear vs srgb Why don't two 50% lights sum up as 100% in linear mode, but do in srgb, simply making two overlaping spots to test shows each light at 50% in both cases, but the sum is different... I have read hellolux article but there is no obvious answer there, or atleast not for me, can anyone explain? Thanks in advance =) edit: to elaborate... __________________ Twitter | Ratata Last edited by sandidolsak : 06 June 2013 at 11:23 AM. Reason: image share quote
 06 June 2013 ChrisCousins Lance for Hire   portfolio Chris Cousins Lance For Hire Brighton, United Kingdom I've dug up this graph showing the sRGB values vs sRGB intensities, from here. The red line is the one. 50% on the x is close to 35% on the y. So it looks like there's a LWF conversion going on in calculating the combined brightnesses of two lights. Not going to pretend to understand if this is 'correct' in physical terms - or if that's even a valid question. What is '50% brightness' for a light? Is a 50W bulb half as bright as a 100W bulb? If you overlap two 50W spot lights, do you get brightness equivalent to a 100W bulb in the space where they overlap? But the maths seems to point to the answer being here somewhere: __________________ Behance | Twitter - - - - - - Last edited by ChrisCousins : 06 June 2013 at 01:04 PM. share quote
 06 June 2013 sandidolsak Expert   portfolio Sandi Dolšak CEO / TD Ratata studio Ljubljana, Slovenia Thanks Chris, yeah the math, I wonder too if this is correct, and if it is, an explanation in help files wouldn't hurt (I hope I didn't miss it) at least to know what to expect. __________________ Twitter | Ratata share quote
 06 June 2013 madwycliffe Earthling   Lindsey Vereen Bellingham, USA Falloff? Where the lights intersect would be some distance from the origin, and so I would think falloff would be a factor, so the sum of the brightness where they intersect would be less than the sum of the brightness at the source. Just a guess. . . share quote
 06 June 2013 sandidolsak Expert   portfolio Sandi Dolšak CEO / TD Ratata studio Ljubljana, Slovenia nop, falloff is not the culprit, does the same with infinite light... __________________ Twitter | Ratata share quote
 06 June 2013 bobtronic aka bobtronic   portfolio Matthias Bober 3D monkey Frankfurt, Germany It's a LWF thing. What happens is that when working in sRGB space the rendered pixel is gamma corrected. This is what the renderer is doing: sRGB gamma = ca. 2.2 light1 and light2 = 50% degamma everything to bring it into linear space for rendering 50% ^ 2.2 = 21.783% light1 and light2 = 21.763% after degamma sum the lights light1 + light2 = 43.527% gamma correct the result 43.527% ^ 1/2.2 = 68.517% When using linear color space there is no need for gamma correction. The problem with sRGB is that 50% are not 50% To get both lights added up to 100% you need to set the lights to 72.974% (50% ^ 1/2.2). cheers, Matthias __________________ http://www.bobtronic.com share quote
 06 June 2013 sandidolsak Expert   portfolio Sandi Dolšak CEO / TD Ratata studio Ljubljana, Slovenia Ok, thanks for this, now about "When using linear color space there is no need for gamma correction." does that mean setting "Input Color Profile" to linear? cause if I do that 50% intensity gives 72% grey... and how are we suppose to compare this to real light behaviour? thanks __________________ Twitter | Ratata share quote
 06 June 2013 bobtronic aka bobtronic   portfolio Matthias Bober 3D monkey Frankfurt, Germany Originally Posted by sandidolsak: "When using linear color space there is no need for gamma correction." does that mean setting "Input Color Profile" to linear? No, it must be a complete linear pipeline, e.g. linear input and 32bit output, which has linear color profile too. It looks like the picture viewer always shows gamma corrected values, even for linear images. If you import the 32bit rendering into Photoshop you get the correct values. Quote: and how are we suppose to compare this to real light behaviour? It's simply adding values, same as real lights. It's just the gamma correction that makes it confusing. It would be easier if everything would work in linear color space, e.g. displays, textures etc. There seems to a flaw in the degamma step for light sources though. It looks like it is always treating like linear input. I'll bring this to the attention of our developers. cheers, Matthias __________________ http://www.bobtronic.com share quote
 06 June 2013 sandidolsak Expert   portfolio Sandi Dolšak CEO / TD Ratata studio Ljubljana, Slovenia Oh my, this clears it out then, picture viewer color picker got me thanks for all the info, I am glad it is clear what to expect now. __________________ Twitter | Ratata share quote
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