Aces when exporting cube maps

[Allen Hemberger]

Hi;

I'm trying to navigate building HDR panoramas for visual effects purposes, and would like to maintain an ACES workflow throughout. I'm feeding ptgui RAW files directly from my Canon 5dmkiii, and exporting a merged/stitched panorama using the ACES-2065-1 option as my Output Color Space. 

This works terrifically, but then I would like to transform the equirectangular image to cube faces, to make it easier to clone out my tripod and impose other paint fixes in Photoshop. The options for image type in the Cube Face export dialog are more limited, and there is no option to choose colorspace. I presumed maybe the tool simply preserved whatever the input colorspace is. But when I do my retouching, then re-assemble and re-export my pano from ptgui, there is a distinct color shift I can see between the original pano and the re-stitched one. 

I'm trying to track down where this color shift is being introduced. It *feels* to me like an unintended color transform is creeping into the workflow, though I'm not sure exactly where. I'm curious if anyone can confirm what the pano-to-cubeface tool is doing with regards to colorspace?

Thank you!
--a

[PTGui Support]

Hi Allen,

I tried but I see no color shift at all:

Stitch bracketed panorama to 32 bit .tif with ACES-2065 color space
Convert to cube faces, 6 faces in tiff format
Load cube faces into PTGui
Stitch to 32 bit tiff
Overlay the reconstructed panorama over the original panorama in
Photoshop, toggle layer visibility

I see no differences and the output is still in ACES-2065 space.

You might run into issues if you use .exr though, perhaps that's what is
causing the problem. EXR doesn't support ICC profiles, instead it has a
'chromaticities' metadata tag. PTGui reads and writes this so it should
preserve color, but it is not widely supported it seems. Photoshop for
example seems to ignore this completely and just use linear sRGB. So if
you do a round trip via Photoshop your colors will be corrupted.

Kind regards,

Joost Nieuwenhuijse
www.ptgui.com

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[Allen Hemberger]

Hi Joost, thank you so much for responding. I am indeed using EXR images, and was digging through the archives here and found mention of this "chromaticities" tag - I'm sure that's exactly what's causing the issue. When I open the cube faces in Photoshop, I can see it seems to want to consider them to be in linear sRGB space, which I hadn't noticed before. 

Hm, what a bothersome gotcha...

[Allen Hemberger]

Hi again;

A followup question to this: I think I’ve figured out how to plumb my ACES chromaticities attribute through my setup, but now I notice that when I pass my panorama - which I can confirm is an ACES full float-32 EXR, with the chromaticities header attribute - through the Cube Faces tool, my cube face images all appear to be half float exrs. My panorama is of noonday sunlight, so I need the full float range of values; the half float faces exhibit clipping. 

This surprises me; what could I be doing wrong? Is there a way to control the bit depth of the output images from the cube face tool?

Thanks;

—a

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[PTGui Support]

Ah yes, the cube face tool currently is hard wired to create half float
EXRs.

As a workaround, I think it should work if you use TIFF instead. If you
still see clipping, try disabling GPU acceleration before running the
cube face tool, the GPU pipeline may be using half float (I"m not sure).

I'll add full float support in the cube face tool to the wish list.

Kind regards,

Joost Nieuwenhuijse
www.ptgui.com

On 19-06-2022 03:01, Allen Hemberger wrote:
> Hi again;
>
> A followup question to this: I think I’ve figured out how to plumb my
> ACES chromaticities attribute through my setup, but now I notice that
> when I pass my panorama - which I can confirm is an ACES full float-32
> EXR, with the chromaticities header attribute - through the Cube Faces
> tool, my cube face images all appear to be half float exrs. My panorama
> is of noonday sunlight, so I need the full float range of values; the
> half float faces exhibit clipping.
>
> This surprises me; what could I be doing wrong? Is there a way to
> control the bit depth of the output images from the cube face tool?
>
> Thanks;
> —a
>
>> On Jun 18, 2022, at 6:20 AM, Allen Hemberger <allenhe...@gmail.com

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