How can we see the non-texture surfaces on the depth map?

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I suppose you have noted a lack of depth solution in low-contrast areas of your object. Some ideas for adding unique registration points to the surface, under ambient lighting conditions:

• Tune your stereo depth algorithm settings, especially block size, and noise filtering. If you can tune illumination for best contrast, that's best. Try a semiglobal block match algorithm.
• Affix pigment to the surface, ideally high contrast and matte. This works in a wide range of ambient lighting conditions. This is similar to 3d motion capture techniques used for actor's performance to be used to animate a model.
• Illuminate the surface with a random high-contrast pattern projector. This works best you have control over the general illumination of the item, output power of the pattern proejctor, the surface is non-reflective, high-albedo, and at a range of distances so that illumination light doesn't saturate at far or near range.
• Combine multiple depth solutions illuminated by different frame-synchronized light sources surrounding the object. This has some features in common with optical flow techniques and optical image scanners with moving illumination source.