Ring Flash vs Hotshoe Speedlite? (with bonus question... TTL vs. manual?)

[Richard House]

Hello,

My most recent attempts for Kintsugi were done with some shiny brass objects using a Canon 580EXII flash on a Canon DSLR. I wanted to test out the workflow when it requires recalibrating the shadow distance in Kintsugi. I came upon issues in Metashape's attempt to align the photos as these photos weren't ultimately lit evenly enough to match pixel data. One object is a brass box and the other is a brass lamp. These objects are less than one foot along their maximum dimension which means the camera and flash were somewhat close to the turntable during capture. Shooting horizontal, rotating left and rotating right with the hotshoe flash result in very different looks on the objects. The thing that I believe is happening is that the flash distance from the lens is significant enough that the lighting changes the look of the object too much to match pixels. With the lamp, there is an internal area that goes into shadow from these different rotations. I am thinking a ring flash is going to be necessary to get the good underlying model for these two objects and obviously will work well for Kintsugi. Is anyone else working with hotshoe flash distances that have shadows interfering with getting a good model?

I think going back to a ring flash will be the best solution here. I could try using 4ft long tube led tube lighting to get soft lighting close in to the lens. I suspect my 4 ft led tubes would be too soft of a light source for Kintsugi even if they are very close to the lens. (??) I may just need to be further away and with somewhat larger objects to use an on camera flash without the shadow distance getting exaggerated by being too close in.

 

The other concern with these objects is that when a surface is angled away from the lens, it goes quite a bit darker. When the flatter surfaces aim back to the camera, I get direct glare/flare which need to be underexposed to make out much of the surface. Will Kintsugi work with mapping textures if the light source varies in power... for example if it was set to TTL for controlling the light output?

 

Thanks!

Rich

[Charles Walbridge]

Hey Rich - some thoughts for you:

- At Mia we've had good luck with flash-on-camera and smaller objects: we often use longer lenses for smaller objects and try to get the camera 2 or 3 feet (60-100cm) away from the object.

- I think Kintsugi will lose its specularity visualization effectiveness at some combination of distance and light size - when the light source is so close to the object that the highlight is always huge on the surface that's facing the camera, Kintsugi can't tell what part of the surface is really specular. (Michael will have a better way to explain this.) I wouldn't trust Kintsugi to make empirical highlights with a macro lens and close focus and a ring light, because the light would be so big compared to the object. But we've had great luck with a ring light that's about 30 cm / 10 inches in diameter and the light and camera about a meter from the object.

- I absolutely think you should try the ringlight to get into the shadowed areas on the lamp and box. But then I think you would want the whole image set to be with the ringlight.

- For the pixel matching and uneven lighting: we often lighten the shadows and darken the highlights in photogrammetry data sets so Metashape has an easier time matching areas of interest on the object - would that work in this case? And the grey tone (re)calibration step in Kintsugi 3D Builder will re-map your light and dark tones on the surface of the object when it's making a new diffuse map for you.

- And about TTL: don't do it! Kintsugi expects your light output to be the same for all your images and therefore to fall off in a consistent way. So in photo terms, if your light was f16 at one meter, Kintsugi expects that light to be f8 at two meters (a two-stop / one-quarter of the light difference).

[Michael Tetzlaff]

Hi Rich, to add to what Charles said:

• As Charles said, definitely don't do TTL.  You want all exposure related settings (ISO, f-stop, exposure time, flash power) set to manual and locked down, otherwise Kintsugi doesn't know if the difference is due to the material of the object or the lighting (and will assume it's from the object's material). 
  • Don't worry too much about if the image is underexposed, especially if the object is oriented away from the camera and flash -- those angles are less important for Kintsugi since they don't have much highlight information (and it aggregates the texture information from all images -- unlike Metashape which stitches together what it things are the "best views").  
  • If the images seem too underexposed for Metashape, you can always tonemap them differently for Metashape than for Kintsugi -- so long as you are working from the same raw images and don't do any cropping, the camera calibration and 3D model will still be valid.
• As Charles noted, the tone calibration step in Kintsugi CAN undo tonemapping applied consistently across all images -- the key is that you need to do the same exposure / tonemapping for all images in the set.
• I think what Charles was trying to get at with the distance / light size combination thing is that you want the light to act as small as possible, so that it's as close to a "point light" as possible.  This isn't the biggest deal if it doesn't happen -- it's what you see is what you get; if you use a larger light or have it closer to the object, you'll get more diffuse highlights that can't be sharpened easily in software (whereas conversely, it is possible in theory to simulate softer/ larger lights if you capture the "point light" reflectance).
• For the hotshoe flash, you'd want to use the "light calibration" task in the Kintsugi workflow, which was in the 2and3d workshop and the documentation... but is admittedly still not intuitive at all from a user experience perspective.  But if you can learn how to do that, you can compensate for the offset and mask out the self-shadows.  Of course, if the hotshoe flash is causing problems in Metashape, this won't help with that.
• Go for a ring light if you feel more comfortable with that!  We'd like to get more feedback on how the ring light workflow works.

[KurtH]

Hi Rich, I've been working with some smaller objects that require getting close. A few things that I've been doing.

• Like Charles said try to get back a little bit. Not only will you have light fall off issues, you'll also have DoF issues. So we've been capturing sets with 24mm but backed up 2 feet which leaves a lot of empty space in the frame (but gives a solid basis for alignment and scale accuracy) and then following up with a 50mm.
• I'm trying to fashion a nice potato masher for a picolite like Charles has but in the mean time we've been using 580EX flashes, which at 24mm aren't the most perfectly even illumination. So once I fix the distance of the lens, I tape a color checker to the a white wall, I square up and focus on the color checker, take a shot, and slide to the left/right and take a shot of the white wall (same distance, still squared up). I now have a file I can create an LCC in C1 (or flat field in Lightroom) along with the color checker file. I think that helps a bit.
• (CHI's guides recommend this for turntable work) If the object is not filling the frame, mask. Either create a low res mediocre model and have Agisoft create masks of the object, or make some photoshop actions. If on a turntable, if agisoft is set to "ignore static tie points" it will still waste its key-point count on areas in the static background that won't be used.
• I have been rotating with Camera Flash, particularly to fill-in areas of occlusion or to have some more lighting angles and they seem to align alright
• You can have a 2nd set of images to aid with alignment and then disable them before texturing. We shot a set with cross polarized ring flash along and another set with a non-polarized speed light. We were able to align everything, but didn't find the polarized gave us substantially more so for subsequent objects we've been using the cross polarized ring flash less.

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The thing that I believe is happening is that the flash distance from the lens is significant enough that the lighting changes the look of the object too much to match pixels.

That can be the case, but working about 2 feet back I haven't had as much issue. When processing the RAWs, try to use a flatter/linear profile or tone curve. Don't be afraid if you get some small areas of blown out speculars, but you may need to capture a few more images to have those areas covered at more of an angle where it doesn't blow out.

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With the lamp, there is an internal area that goes into shadow from these different rotations.

I've had some things where occlusions put areas in completely shadow. I took additional photos with extra rotations and positions. Keep in mind that every point needs to be (LIT) in at least 3 (preferably 9+) images from different positions. Areas that are in shadow, that image doesn't help the build.

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I think going back to a ring flash will be the best solution here.

I've had some promising results with some ring flash use, but I haven't tested enough on SUPER shiny objects (if the specular shows up as a donut, what happens? I don't know). The nice thing with the ring flash is the lighting calibration step can be skipped as the light is basically at 0-0.

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When the flatter surfaces aim back to the camera, I get direct glare/flare which need to be underexposed to make out much of the surface

Use as flat a profile as you can to minimize the contrast, accept that there will be some blown out areas and nearly blocked up areas.

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I could try using 4ft long tube led tube lighting to get soft lighting close in to the lens

I really think that would be problematic the way things are working, but the ring flash would be a better option.

Don't hesitate to give me a call at some point, you helped me out a ton early on with RTI and Photogrammetry. If you want me to take a look at anything I'm happy to repay the favor if I can.

[Richard House]

Thank you all for the replies. This is all very helpful.

Michael, I did use your data set that was shared in the workshop to do the correction of the lighting offset. That step wasn't too difficult to dial in for me. I was thinking that for the purpose of Beta testing, I should try to do a capture set with a hot-shoe flash. The shiny bronze subjects I am testing obviously are not a good fit for that so yes, I will redo these objects from scratch using a ring light flash on Manual output. I just ordered one of the Canon Macro Ring lite flashes since my other option has been a Profoto Ring light which is larger in diameter and quite difficult to dial down the power enough when close in on small subjects.

To clarify the processing of images for Metashape and then for Kintsugi 3D... You suggest I pull the highlights down and open up the shadow info for Metashape? I'm guessing it would be better to reset the adjustments to a linear response curve with no highlight/shadow adjustments and re-export the same image set for Kintsugi 3D to work with. I understand that the tone calibration would reset the values on the greyscale chart to correct the tone mapped images but that's probably better done by re-exporting the from the original raw files.

Am I understanding this correctly that the best workflow would be to process out two separate image sets without any cropping applied between them.

Fingers crossed the ring light I ordered arrives soon so I can get these shot again soon. I will keep updating this forum and share the results when I have them.

Thanks again,

Rich

(Kurt, I might reach out to you about your Blender render steps once I have good models to import. Using Blender to output videos is something I haven't done yet.)

[Richard House]

Brass Box Photogrammetry- Ring Flash Update:

The Canon MX-14 Macro Ring light solved my issues for Metashape being able to build the model... (381 photos/ 381 aligned). I have posted two models to Sketchfab.

One model is directly exported from Metashape:

https://skfb.ly/p6uJ6

The second model is textured using Kintsugi 3D:

https://skfb.ly/p6tvQ

The main inaccuracy I encountered is that there are a few holes in this box along the front. The larger open holes along the proper left side wound up reflecting the interior bronze surface from the ring light direction and makes it appear to be a solid surface. I would need to line the interior with something black so that it doesn't reflect the bronze color back to the lens for that particular detail to read accurately. Otherwise, Kintsugi 3D does a very nice job rendering this surface. It has a dull quality to it but will appear more glossy with more direct light angles. The Kintsugi rendering reads rather true to the actual box. Thanks again for the feedback on how best to approach this one.

-Rich

 

Example where hole in front reads as a hole

Ring Light flash angle reflecting the bronze interior. This occurs in many of the captures and makes it appear to be solid in the rendering.

 

 

 

[Michael Tetzlaff]

On 7/26/2024 at 8:45 AM, Richard House said:

To clarify the processing of images for Metashape and then for Kintsugi 3D... You suggest I pull the highlights down and open up the shadow info for Metashape? I'm guessing it would be better to reset the adjustments to a linear response curve with no highlight/shadow adjustments and re-export the same image set for Kintsugi 3D to work with. I understand that the tone calibration would reset the values on the greyscale chart to correct the tone mapped images but that's probably better done by re-exporting the from the original raw files

You can either suppress the highlights with tonemapping and then try to bring them back in Kintsugi using tone calibration -- or redo the tonemapping after processing in Metashape (and still do tone calibration in Kintsugi but it will have less of an effect).  The one caveat about Kintsugi's tone calibration is that it doesn't have any data from the ColorChecker beyond "diffuse white" -- so it has to assume a linear sRGB tonemapping curve beyond that point.  We haven't done a good head-to-head experiment between using Kintsugi's tone calibration vs just going back to a linear sRGB encoded image -- so I can't say definitively how much this matters.  (@Charles Walbridge, maybe we could look into this next time you have a dataset that might be good for testing this?)

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Am I understanding this correctly that the best workflow would be to process out two separate image sets without any cropping applied between them.

Regarding cropping, I believe that Carla recommends (in general, for all photogrammetry projects) not to do cropping.  As mentioned above, I'm not sure if one set or two sets is better -- Kintsugi will do its best if you have heavily tonemapped images, but isn't necessarily perfect -- and I'd love to hear about the results if anyone wants to do a head-to-head comparison.

For the box with holes, I think that's going to be tricky even with conventional photogrammetry.  You could try using some black cloth in the interior, but it might also just be something that requires some manual editing.  I have less experience with the actual photogrammetry process as many on this forum, but I've always found holes like that to be extremely difficult to get Metashape to model correctly (it likes to fill holes whether you want it to or not).

I think I've answered all your questions, Rich -- let me know if I missed something.

[Richard House]

I'll keep this thread going since I'm still working on the same subject here. I tried using a ring light from slightly further back hoping it would record as a smaller, more specular light source. This time around, I am seeing some strange artifacts from Kintsugi 3D's texture mapping... specifically in the specular render.

Some screenshots of the Sketchfab model renderings... Not sure where the bright lime green/yellow artifacts are coming from. This version is a second attempt at rendering the same model in Kintsugi. I got the same result from both the first and second renderings so perhaps not a glitch in that process.. 

Metalness render:

 

Specular render:

 

The Metashape exported model: https://skfb.ly/ppSXy

 

The Kintsugi modified textures: reprocessed a second time: https://skfb.ly/ppSUA

Thanks,

-Rich

 

 

[Michael Tetzlaff]

I think the issue here is that Kintsugi never gets a good head-on shot of the backside of the rings, so it doesn't have good specular data there.  I think it's probably overfitting to the images that is has there.

One more thing you could try would be to reduce the "specular count" when doing the process textures step, to maybe 4 or even 2.  That will force it to use fewer distinct materials when generating the textures, which might be enough to make that bright yellow one go away.  Note that we haven't tested that feature much recently, so if you run into any strange issues (i.e. it gets a lot worse after doing that), check in here as there might be a bug to fix.

In the future, I want to add a feature to let you manually edit the materials Kintsugi "discovered" so that you can discard ones that you know are problematic... but that's probably a 2.0 feature that's a couple years away at least to ensure that we do it right with a good user experience.

[Richard House]

@Michael Tetzlaff

Hi Michael, I appreciate your reply. I have re-processed both options... Specular Value at 4 and Specular Value at 2. They both resolved the bright yellow color issue in the Sketchfab render. Specular setting at 4 seems to deal with the open hole area on the front surface a bit better than Specular setting  2. Here are links to these two most recent versions: 

Specular 4 version: https://skfb.ly/p7FFu

 

 

Specular 2 version: https://skfb.ly/p7FGM

 

Thanks!

Rich

[Michael Tetzlaff]

Thanks for sharing!  Glad that was able to resolve the issue, and I agree that the specular count = 4 version looks the best.