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Super ghetto cross polarised photos for albedo reference!

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So a lot of people are asking what resources there are to find both reflective values, and albedo values for materials. can we make libraries from this information, and can we ensure "physical accuracy" for them.

I'll answer that last question first with a resounding "NO".

And here's why:
Outside of a laboratory there will always be some form of interference, whether it's dust in the air, particles of "x" on your surface, or whatever, or even the calibration of your equipment itself, there is always going to be some variance that will cause minor or major inaccuracies.

That is why when people talk about material libraries or measured values, they quite firmly state; "this information is a guideline".

So what can you do as an artist, that's cheap, quick and gives you a solid baseline to start from with your texturing approach?

step 1.
Get a camera! in my case i'm using a dSLR, a canon 500d to be exact. you don't have to have an SLR, any point and click will do as long as it has a flash.

if you do have an SLR, a tripod and a remote trigger (below) are really useful as it means you don't accidentally move the camera between shots.

step 2.
Get some linear polarized film. this can be expensive (i've ordered some so i can make a custom setup for my camera), so in the ghetto we use these bad boys!

that's right, a pair of cheap ass PASSIVE 3d glasses (they cost like, 2 bucks from any cinema).

step 3.
remove the lenses, it doesn't matter which side goes where, just make sure they're clean.

position the film on your camera, you can tape it down if you like (on an SLR you can't really do this to the lens), but here is how you want to do it:
horizontally across the flash.

vertically over the lens.

Now for the pictures!

If you want to separate the reflective value (so you can use a full specular/reflectivity map) then you'll need to take two shots, one with the filters on, and one with the filters off, you then need to convert these images to linear space (as they're saved in sRGB) and simply subtract the filtered from the non-filtered to give you your reflective value.

If you just want albedo values (and then use the metalness workflow) this works really well for non-metallic surfaces - simply take a shot with the filters on, and it "should" cut out almost all of the reflective values. which means you can either project your photo, or use it to take values for your texture.

NOTE - it's very important that you take your photos in as dark a room as possible to avoid contamination from other light sources that can't be polarized by one or both of your filters.

here are some examples!

stinky shoe without filters:

with filters:

a keychain (multiple material types) without filters:

with filters:

Notice how the filtered images are much more vibrant, and saturated? this is awesome, these are the raw albedo values we need, and once the reflectance is applied, due to energy conservation they should appear slightly duller in final composition.

hope you enjoyed this ghetto tutorial from ghettoville!


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