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HDRI Neutralization method


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Hi guys.

I'm a HDRI photographer and i use Affinity Photo most the time to edit my panos removing nadir and adjusting exposure. The HDRI spheres (32bit float) will then be used for CGI works.

Now to my question: i have been asking myself for quite some time how i can guarantee correct colours and also how to neutralize the colors in order to reset reflection saturation values.

For this reason, i position an x-rite colorchecker in each of my scenes (used to generate a dcp color profile) but during hdri development process, colors tend to change intensity so that some color cast is introduced.

Till now my method was to correct WB, but some times, especially with indoor conditions where lightsources could be many, adjusting WB is not enough.

So my question is: what is your approach in neutralizing colors of the HDRI sphere?

Please remember that i'm not talking about HDR images 16 bit but HDR spheres in 32 bit full float.

 

Thanks for any help.

Bye

Raf

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Hi @cgiout,

Welcome to the forums and my apologies for the delayed response here :)

I have forwarded this question to our resident Photo expert, who recommended the following - 

If you're using a colour-checker passport that has grey cards, you could use a white balance adjustment and pick from that, to adjust the colours of your image.
Alternatively in 1.9 you'll have a Divide blend mode, so just add a fill layer, hide it, colour pick off what should be neutral white, then show the layer and set its blend mode to divide.

 

I hope this helps!

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@cgiout - How much control do you have over the scene you are photographing and how long do you have to acquire all of the source images that you ultimately use to make your HDR composite?  The reason I ask is because, when you use a full 32bit per channel workflow, you maintain the physical integrity of the lighting in your scene, making it easy to manipulate that lighting after capture.  However, to give you the most flexibility, you want to sample the scene one light at a time and then add all of the lighting together in post.  That is, let's say your scene has 3 lights in it.  Ideally, you would want to shoot the entire scene with each light illuminated individually, with all of the other lights off.  In post, you can combine your HDR exposure sequence for each light into its own HDR file (32bit per channel) and then bring each HDR file into a working document and add the light sources together in your 32 bit per channel working environment.  In this scenario, you can add an exposure adjustment layer and a color filter adjustment layer clipped to each light layer and use these controls to change the intensity and color of the contribution of that light to the scene.  This gives you the power to recolor each light and adjust is contribution to the scene as you see fit.  Not only can you neutralize the color temperature of each  light, if that is what you want to accomplish, but you can add any color filter, completely relighting the scene with some look or mood.

Essentially, you stack the light layers and set the blend mode of each one that is above the background layer to "Add" - because you are working in a 32 bit per channel document, the light will add linearly, just as it does in "real life."  

Attached are a few images of the process based on an example I wrote up several years ago, but it is no different now (the example is in Photoshop, but it is the same in AP).

The first three images show the three light sources in the scene, each one illuminating the scene without the other two.  An HDR sequence was shot for each light.  A color checker card is included near the light source that is being imaged.  The color checker can also be cloned out or the image sequence can be shot again without the color checker.

Next, the layer stack that is constructed to mix the lighting - note that each image of a light has a color filter ("Gel") and an Exposure control to modulate the properties of the light.  It is like having a graphic equalizer for the scene!  Also note the Master Exposurecontrol at the top of the stack, giving you control over the overall intensity of the scene (you could add a master color filter layer too).

The next image demonstrates how a local white balance for one of the lamps is accomplished to bring its CCT (correlated color temperature) into line with the other lamps in the scene.  In this scene, two of the lamps were LEDs with a daylight CCT, and one lamp was a tungsten filament light bulb with a much warmer CCT.  I balanced the warmer lamp to bring its color into line with the other LED lamps by adjusting the warmer lamp's color filter layer.

Finally, the rendered results for a "literal" tone mapping of the scene, and then a moody, funky relighting of the scene using the exposure and color filter layers for each image.  Note that the scene is rendered "correctly" when you make large and extreme changes to the lighting because you spent the time to capture each light's contribution to the scene (for example, the mixing of colors and reflections within the scene).  You can also add ambient lighting to the scene by acquiring a separate HDR sequence taken in that ambient lighting condition (daylight from outside, for example) and mix that into the scene as well.  You just need to keep your tripod set up locked down within the scene and wait for the ambient lighting conditions you want.  For example, set up your tripod and camera and shoot the ambient scene during the time of day you want (or several different times of day) and then shoot the individual lamps in the scene at night, where there is no ambient light in the scene.

This process take a lot of time to sort out and acquire the image sequences, but it gives you an incredible amount of data to work with when compiling your HDR image.  It sounds like you also are acquiring spherical panoramic HDRs for image-based lighting - the process is no different, but it will take time and diligent management of the workflow.  You can mix your scene in 32 bits per channel and then export a 32 bit per channel flattened EXR to use for your CGI rendering.

 

Have fun!

Kirk

Light01.jpgLight02.jpg

Light03.jpg

Layers.jpg

ColorTempCorrect.jpg

Final01.jpgFinal02.jpg

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Hi, thank you very much for your reply.

Yes, my main job is acquiring HDRI pano sphere to be used for CGI, and most of my productions are outdoor, so in best conditions with sun and clear sky.

My concern now is, how to choose the right "Gel" for a scene, because WB varies from scene to scene.

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  • 2 months later...

I would love to see a tutorial on this.  I am getting into CG work and am a photographer and videographer and I own a nice GH5 for video and a Canon 5D mark 3 for photo, but I am not sure how to stich the output using affinity photo. Any recommendations on this would be great.  I use Affinity Photo, ACDSee for photo organizing, DaVinci Resolve for video editing, and Blender for 3D graphics modeling, texturing and rendering.

Having nothing to do with HDRi (but fun to share nonetheless) - I have attached a model I created and textured in a day between meetings, and it would be fun to fly that around my yard with proper reflections and lighting.

airplane-3.png

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