Tag Archives: Exposure

Exposure/Offset/Gamma photoshop tool to Shaders

The goal here was to understand how works the Exposure feature of Photoshop for my research on lift/gamma/gain

exposure-photoshop-panel

NB : I wouldn’t say I’m an 100% sure about how it works, but that’s how I understood it worked so far


Exposure :

I though exposure was more complicated than that (maybe it is) but the way I’m reproducing it, is only by adding a percentage of the pixel by himself. nothing more complicated than that.


Offset :

As seen in my previous post, the Offset is just an Add of a value. My first approach on it was saying Offset is controlling the luminance of it, so I guess the correct way would be to convert the pixel in the YCbCr color space, add a value to the Y channel, and reconverting back to RBG color space. But this is doing a lot of calculation for nothing, while adding just a value to the entire picture would probably do the same, and actually would correspond to the Lift (or Offset of the CDL).
Plus switching between color space makes you loose some tiny precision in the color.

Anyway, I’ll give the two solutions, please let me know if you think one is better than the other


Gamma :


As usual, power(1/gamma)

The right order :

If all this was actually all pretty simple, at least I learned one thing (obvious as well I must say). Don’t forget to do the operation in the right order. In my first try I had a small bug, and couldn’t have the same result in Photoshop & in my shader, because I was doing the offset before the exposure. Which is pretty silly, since if you do that, you are loosing all the dynamic range of your picture when you actually want to have as much information as you can for your exposure.

The order is Exposure -> Offset -> Gamma.


Formula :

output = ((input*exposure) + offset)^(1/gamma)

hum… looks a lot like the CDL formula doesn’t it ? 🙂 Except exposure is a single float value between 0 and 20 ? (maybe more, maybe less depends of your picture)and the default value should be 1.The offset should be between -10 ? and 1 I think.


Shaders (HLSL in Media Player Classic) :

http://www.pasteall.org/8868/c

Here I’m giving the version with YCbCr conversion, but I’m pretty sure it is not doing anything more than the above one, except loosing information in conversion. But at least it shows the YCbCr conversion 🙂

http://www.pasteall.org/8869/c