Total beginner with a lot of questions.

Discussions - Using ACES
,
1

Greetings, complete beginner here. Until very recently my total understanding of colour management was a gamma curve fixes a legacy problem with crt monitors. Make sure your 3d packaqe knows what kind of texture it is and your golden. That was it. Now a new client has requested that we work in ACES and of out tiny 5 man team I got the short straw of learning and implementing ACES.

The quick start guides are great, and there’s plenty of stuff on the net but that tends to vary in it’s level of confidence and fullness of explanation. My two pronged attack is a/ to learn simply “what buttons to push” so we can carry on working, but more importantly b/ to understand on a much deeper level so we can problem solve.

For background our pipeline is creation in Houdini and or Maya, rendering in redshift, compositing in After effects (I know, not my choice) and delivery is normally some sort of quicktime. Occasionally there might be supplied footage but we never have to wrangle much of it.

Looking at the ACES workflow sample and the VFX quickstart it looks simple but I’m still left with some questions that I can’t find answers too, so if i may, I’ll ask them one at a time…

Simplifying somewhat, i have an image file, it’s a bunch of buckets with r, g and b values that go between 0 and 1, like raw data, is the colourspace simply something in the metadata that gives coordinates for the primaries in relation to the cie diagram? So if my image is sRGB a bucket with a raw value of 1,0,0 the colour will be “here” on the cie diagram (a red that my rgb monitor can display), and If i change the metadata to ACES then it moves the primaries and my raw value of 1,0,0 gives a different part of the cie diagram that my monitor can’t display?

So, basically… in terms of colourspace is my image file a bunch of raw data with some mapping information stored in the metadata? Changeing the colourspace simply changes the metadata mapping values and leaves the raw pixel data as is?

thanks for your patience,

Andi.

1 Like
2

Welcome! Thanks for these questions. I’m sure someone from the community will step up and give you some help. I’ll check back with you too.

Steve
ACESCentral Admin

3

If your image file is in an integer format, that will represent values between 0 and 1, but if it is in a floating point format such as EXR the values will not be constrained to that range. ACES uses EXR files for interchange, normalised such that 0.18 encodes 18% reflectance and 1.0 encodes diffuse white. Values greater than 1.0 encode specular highlights and light sources.

The ACES Output Transform for a given display maps the high dynamic range in the EXR files to the lower dynamic range that display is capable of, rolling off the highlights and altering other characteristics of the image, with the aim of creating a perceptual match between the original scene (real or virtual) and the image on the display.

You are correct that the same RGB values when interpreted as using different primaries will encode different chromaticities. That is why you would never simply change the metadata in a file to tag it as using different primaries. You need to also change the RGB values in the file so that they encode the same colour when interpreted as using the new primaries. In the ACES system that is what an IDT (Input Transform) does. It calculates the ACES values needed to encode the same colour that was encoded by the RGB values in the original source. Think of it like converting between Kilometres and Miles. You don’t just call 100 km 100 miles, you perform a conversion so 62 miles and 100 km both represent the same distance.

4

Hi Nick,

thanks for the response, your last paragraph is what I think I was missing. The raw rgb values do indeed get changed.