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Color space transform bidirectionality
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Jeremy Selan <jeremy...@...>
Alan,
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OCIO is being a bit clever under the hood, though in my opinion not in a dangerous way. OCIO's currently approach is that to make colorspace definitions "convenient", we assumes inverse color transforms are allowed when the round-trip can automatically be made 100% accurately. I.e, if your colorspace definition is only built from cleanly invertible ops (simple cc math, matrix ops, log, 1D lookup tables), and is round-trip safe, the inverse transform is allowed. If your colorspace definition relies on 3D LUTs, any attempt to run the inverse transform will fail during the getProcessor call. (If you try to add a 3d lut to nuke and try to perform the inverse you'll see this). I'd love to figure out a way to make this more explicit in the API. Suggestions? The reason we defer the inverse 3d lut check to this late part in the API is to allow for the following interesting possibility. Say you have 2 display emulation colorspaces defined as follows. srgb: (A normal filmlook) lin_to_log.lut1d log_to_srgb.lut3d srgb_warm: (A warmer filmlook varient). lin_to_log.lut1d log_to_srgb.lut3d warm.mtx Say you have pixels in a baked srgb representation, and you wanted to view them with the warm look. Internally, our processing chain would look like: INPUT -> inverse log_to_srgb.lut3d -> inverse lin_to_log.lut1d -> lin_to_log.lut1d -> log_to_srgb.lut3d -> warm.mtx -> OUTPUT But, OCIO ( will soon) optimize this to a simplified conversion: INPUT -> warm.mtx -> OUTPUT This particular color space transformation, although conceptually requiring an inverse 3d lut, doesnt actually end up using it! Which is pretty cool, and really convenient in practice. This is why in Nuke we just blindly added all color spaces to both the input and output side, to allow for these circumstances. I can see this is subtle though, and potentially misleading from a new user perspective. Thoughts? -- Jeremy
On Mon, Aug 23, 2010 at 1:53 PM, Alan Jones <sky...@gmail.com> wrote:
Hi All,
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Alan Jones <sky...@...>
Hi Jeremy,
On Mon, Aug 23, 2010 at 4:30 PM, Jeremy Selan <jeremy...@gmail.com> wrote: OCIO's currently approach is that to make colorspace definitionsFor practical purposes those are all probably invertible, but even a 1D lut can lose information (such as everything below black point getting pushed to 0 - though seeing that information isn't required it doesn't really matter). I'm not a fan of putting too much effort into protecting users from their own stupidity ;) An uninvertible matrix could also happen, but again, probably not in practice. I'd love to figure outThat depends - you make a good usage case that somewhat goes out the window. The only real solution (which unfortunately cuts down your options for processing shortcuts and also limits what LUTs you can bring in) I can think of is something like follows. A color space is defined and it is always defined relative to scene referred linear. This way for each space it knows how to convert to or from scene referred and nothing else. Then when it registers it can say which directions it can handle. You kind of also need to define types of LUTs at this point though. I'm thinking of three. Storage LUTs (which pull data two and from representations used for storage, such as S-Log etc), Grading LUTs (which are designed to apply a particular look whether that's a color grade or a film transfer representation), and Display LUTs (which modify values to ensure that it displays correctly on a given device - i.e. sRGB, Rec709 etc). With this storage LUTs would go from whatever their storage form across to scene referred linear usually (though you'd use the reverse for saving perhaps) then the grading LUTs would make any changes to the color (if you had some magical linear display then this data would look correct on that display, but the grading LUTs wouldn't be used until the end as not to screw the radiometric linearity of your data). The finally with that data you apply the display LUT for your output device. So it'd be 1 Storage LUT 0+ Grading LUTs 1 Display LUT as the common workflow. I've got no idea how well this aligns with the design goals of OCIO as I'm just getting familiar with it, but that's how I'd look at structuring things to help define the process so it's more accessible to less technical users. It's pretty much what I'm planning on putting in place in the pipeline I'm building here, but on a pipeline wide scale where at ingest everything is shifted to radiometrically linear space and stored in float (well probably half-float for most cases) on disk. Then people working in lighting/comp would have grading+display LUTs applied on the fly so they're looking at as close to final as possible. I'd be interested to hear your thoughts. Cheers, Alan.
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Jeremy Selan <jeremy...@...>
Alan,
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Sorry for the delay in responding. (Had to attend to non-OCIO responsibilities this week). The approach you describe is exactly in line with what I've been thinking. Excellent. A few additions: You refer to the different LUT types as "storage, grading, and display". In the currently library we dont draw hard lines between the different transform uses. All three of these would be color spaces, and are treated equally in the code. Adapting your terminology to OCIO, "storage" color spaces would always provide transforms both to and from scene linear, display color spaces typically would only define a transform from scene linear, and grading spaces would be defined dynamically at runtime. But there's no code-level distinction between the types. Is there any benefit to doing so? I've been thinking about adding metadata (tags) that will let the user tag color spaces as belonging to different categories. (such as 'display', 'IO', etc). This would probably help in the UI (you could filter by tag), but I cant think of any other uses so it's a bit low on the priority list. * You refer to these things all as LUTs. I'd prefer to just call them "transforms" to leave open the possibility of other correction math. (For example, the grading lut could just as easily be a 1D lut, a 3D lut, or a ASC-CDL). Your processing pipeline is already expressed in the DisplayTransform. You specify the storage LUT (inputColorSpace), the displayLut (outputColorSpace), and also optional color correction(s). (Which can occur in any color space, including scene linear or a log-like DI space). The Nuke OCIODisplay node provides an example of using this API. -- Jeremy
On Mon, Aug 23, 2010 at 2:59 PM, Alan Jones <sky...@gmail.com> wrote:
Hi Jeremy,
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Alan Jones <sky...@...>
Hi Jeremy,
On Fri, Aug 27, 2010 at 4:18 PM, Jeremy Selan <jeremy...@gmail.com> wrote: Sorry for the delay in responding. (Had to attend to non-OCIONo worries :) The approach you describe is exactly in line with what I've beenThis is great news :) But there's no code-level distinctionI agree, the only benefit is from a UI perspective to assist the user in not doing something stupid. Downside to this is sometimes there is a good reason to do something stupid. So perhaps it would be best to go for some middle road (listing the logical ones first and including the type of all transforms for instance). I'd prefer to just call them "transforms" to leave open the possibility of other correctionSounds like a logical choice of terminology to me. Your processing pipeline is already expressed in the DisplayTransform.Great stuff :) thanks for sharing. Slowly getting my head around how OCIO works. Cheers, Alan
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