Allow full user control of the SDR rendition in an HDR gain map export

[gregbenz]

The HDR gain map export capabilities in LR and ACR are very helpful to share HDR images which still look great on SDR displays (https://gregbenzphotography.com/hdr-images/jpg-hdr-gain-maps-in-adobe-camera-raw/). However, they do not offer the highest quality SDR experience possible due to limited artistic control over the base image (ie the gain map specification itself is fine, but we lack full control over the SDR rendition when exporting the image).

The 7 sliders in the "preview for SDR display" controls in LR / ACR do not enable artists to fully optimize the SDR rendition in a gain map. In many cases, higher quality SDR results can be obtained through other tools - but we still need a way to use a user-generated SDR image in an HDR gain map.

Having full control over both the versions of the image in a gain map provide several benefits:

• Superior and more consistent resuls when the HDR is created by upgrading an SDR edit. This is a crucial workflow to help upgrade existing edits to HDR, support workflows which are designed to optimize for print but also support HDR, or avoid limitations of 32-bit Photoshop (such as using liquify and frequency separate for portrait retouching, as they are not available in 32-bits). In all these cases, a perfect and optimal SDR has already been created and a beautiful HDR conversion is easy to create. However, the existing tools result in a degraded SDR after the round trip to HDR and then back through the preview for SDR display. This loss of quality is entirely avoidable by simply using the original SDR edit in the gain map.
• Full use of all ACR controls for tone mapping original HDR edits to SDR (ie those available in 32 to 16-bit conversion). This includes local adjustments with sky masks, color-specific corrections, etc. These SDR results are often superior than those available with only the preview for SDR controls.
• As HDR support increases in the future, it may be optimal for an artist to encode a gain map built with a base image which is slightly more capable than just SDR. For example, it may be better to encode the image with a base image using 1 stop of headroom (rather than SDR) when exporting an image with 4 stops or more of headroom in the full HDR version. This would require gain map formats beyond JPG, but demonstrates that such user control may become even more useful in time.

Adding an option to specify an SDR source layer in Photoshop's native save dialog or "Export As" panel would of course be nice to have, but is not necessary that Adobe does all that work. A simpler development effort focused only on back end support would still offer significant value (by relying on independent developers). It would be very useful just to add a UXP BatchPlay API which allows 3rd-party developers to specify a layer to use as the base image when exporting an HDR gain map.

The interface and most input validation / edge case management could be left to developers. This would be fairly straight-forward as the base/SDR layer would already have the same document dimensions and color space as the full HDR image. Transparency in the base layer should be ignored. Invalid gain scenarios (such as requesting a bright HDR result from a dark SDR base pixel) could simply clip to the maximum gain to manage edge cases that cannot be encoded.

This request is of course predicated on either Photoshop adding support to save HDR gain maps or enabling an API for developers to request such an export through ACR. Native support in PS is probably ideal for several reasons, including performance. Such support would also add significant value by enabling batch exporting of HDR images for the web (through scripts or actions leveraging a native PS save capability).

Gain maps are already a tremendous achievement, and providing the artist full control over both versions of the image would help fully realize the vision to optimize these images for display on any monitor.

[gregbenz]

Another possible solution would simply be to use a layer with a unique name which designates that it should be used for the gain map. This would support scripting, actions, and manual use without any additional Adobe UI. It would simply be used when saving an HDR image in a file format which supports gain maps.

The name could be something like "userGainMap" or "customGainMap". Such a name should be memorable, clear, and unlikely to ever be used accidentally. Note that I am explicitly not referring to "SDR" in the layer name (as brighter than SDR versions might become useful in future gain maps). I'm also avoiding referring to it as a "base map" (as either the SDR or HDR might be the base depending on file format, a separate file option in the future, or simply change over time as we move from a time where an SDR base is desireable for compatibility to a future where an HDR base is desirable for higher-quality HDR).

I do not believe such a name needs to be limited to English as proposed here, but it may be preferrable to use a single, global name to simplify development, 3rd-party support, and actions.

If local languages are suppoted for a custom gain map layer name... Localized strings could be added for equivalent names which are memorable in other languages. This would help make it easier to use such a feature manually (but at the potential cost of complexity for development, scripting or actions). Photoshop could simply accept the first layer with a match in any language (to manage an edge case with multiple possible matches). Matching should not be tied to the language used in Photoshop, so that a given image behaves consistently when opened anywhere (such as if someone edits an image in one language and shares it with someone who uses a different local language). Being able to detect a user-generated gain map input could enable simpler or more powerful plugin and script support, so there should be a mechanism in Photoshop or official list of names to test so that developers can detect the presence of such gain map input to Photoshop. 

[afen261243440837ddb]

Exporting AVIF and JPEG XL with gain map is currently an experimental feature (not fully tested or tuned).  Therefore, it is not enabled by default.  However, you may access this feature by holding the Shift key while clicking on the Save button in Camera Raw.  (Be sure to press Shift before clicking the Save button.)  Then the Save dialog will have a new checkbox in the Color Space section called "Embed HDR Gain Map."