Tutorial #12 – masking for turntable

Welcome to the 3DF Zephyr tutorial series.

In this recipe, you will learn basic and advanced masking techniques that applies to a turntable scenario in seven steps to Turbocharge your Turntable.

While we have already introduced masking with Tutorial #3, this tutorial is focused on turntable settings.

Using a turntable can help a lot when scanning a whole subject. There are many different ways to approach turntable scanning. In this tutorial we will go over the fundamental principles.

1. Background Colour

Choosing your background colour is very important. Typical backgrounds are either black or white. There are pros and cons to either approach.

Black can be used for subjects with lighter features. Black absorbs light, stopping reflections from bouncing across the subject whilst also obscuring shadows. Sometimes you don’t have enough light, or your subject has lots of very dark features, in these cases a black background may not be ideal. Choose a black material which doesn’t crease and is as non-reflective as possible. The ideal black material for black backdrops is Vantablack, though black muslin works for most applications. Explore the different fabric and paper options available to learn which works best for you.

White can be used for subjects with darker features. It reflects light, further illuminating your surface with bounce light. However, white backgrounds can potentially introduce shadows onto turntable surfaces and the backdrop. Careful positioning of your lights will help to reduce this problem. A light directed to your backdrop overexposing the surface removes any shadows otherwise cast. Glare free canvas is a great option for a white backdrop. Alternatively, a wide roll of matte paper can be used to similar effect.

Shades of grey & different colours can be used for special cases, depending on your subject. Be careful because colored backgrounds will bounce colored light onto your subject, contaminating the texture of your model with false colours.

Background key notes:
  • Black can be used for subjects with lighter features. Black absorbs light, stopping reflections from bouncing across the subject and obscuring shadows.
  • White can be used for subjects with darker features. White reflects light, illuminating your surface with bounce light, potentially introducing shadows on the turntable surface and backdrop.

2. Lighting setup

There is a wide variety of lighting options available. You can spend a lot of money on lights, but you can still get great results with a less expensive light setup. The two main types of lighting we shall focus on are continuous and flash.

Continuous:
  • Incandescent: Typically high CRI*, low cost. They have a high power requirement and can get very hot when left on for a long period of time (and eventually break). Colour temperature** is usually between 2700-3000k.
  • Fluorescent: Domestic fluorescent lights are low CRI and low cost. Typically they have a low power requirement and stay cool to the touch. Flicker may be a problem, especially with fast shutter-speeds. Colour temperature is usually between 2700-6500k.
  • LED: Typically high CRI, low heat and long life. High cost to light output ratio compared to other options, but they are becoming less expensive over time. Colour temperature* is usually between 2700-7000k.

*CRI is a quantitative measure of the ability of a light-source to reveal the colors of various objects faithfully in comparison with an ideal or natural light source. **Colour Temperature is a characteristic of visible light, it is measured in Kelvin.

*Flash:
  • Strobe Light:- Can be quite expensive, not very portable, but typically powerful lights. A lot of strobe lights are mains powered, so there’s no need to keep replacing battieres. Vistro strobes are the budget conscious strobe choice, though alternatively Profoto strobes are powerful fast and consistent but carry a hefty price-tag.
  • Speedlites: Less expensive than strobes, highly portable, and still quite powerful. They have a slower recycle time, so can take longer to capture lots of photographs than strobe alternatives. Having to replace batteries can be a pain, so invest in some battery packs to keep your flash going for longer periods of time. Off-brand speedlites are much cheaper but be careful to ensure camera compatibility. Yongnuo and Godox are popular speedlite choices, as they are cheap and fairly reliable (but if you push them too hard they are prone to failure).
  • Ring flash: Highly portable, and range in price. The more you pay, the more powerful your ring flash. The main advantage of a ring flash is it’s shadow suppressing power, flooding your subject with light from all sides of the lens. If you’re looking to get a ring flash on a budget, the Apurture Amaran halo range is a great inexpensive alternative to the expensive and powerful Godox AR400, a ring-flash commonly used for high end photogrammetry.

*Flash lighting usually sits around 5600k, which is close to sunlight at midday, though this varies between manufacturers and flash models.

Lighting is a very in-depth subject with many exceptions to the rules. The main targets are even illumination of your subjects surface and minimisation of cast shadows.Test different light setups to get a better understanding of how they work.


3. Camera Setup

There are lots of different camera setups, but here are a few concepts which will help you achieve better results:

  • Choosing a file type: RAW is ideal because it gives you more flexibility in post production stages, JPEGs are smaller files but are less flexible.Remember that most professional cameras allow to save as both RAW+JPG
  • Selecting a Focal Length: Smaller focal lengths have greater distortion but cover a larger field of view, while larger focal lengths limit the visible area of your subject but typically are less distorted. 50mm is commonly used as it’s well balanced, but you can use almost any focal length and still get results.
  • Setting the Aperture: Higher apertures (F.stop) have a larger DOF, but also limit light into the camera, therefore requiring more light for a good exposure. If your F.stop is too high diffraction is introduced, reducing overall image sharpness, but don’t worry about this too much – it’s more important to have a larger DOF rather than ULTRA sharp thin focus.
  • Shutter Speed: For syncing with strobes, standard shutter speed is between 1/160 and 1/250. If you set your speed too high, you may not capture all of the available strobe light. For continuous lighting, your shutter-speed is more flexible. Slower shutter speeds invite the risk of vibrations which can adversely affect the sharpness of your shot.
  • ISO: Low between 100-400 is good, and with a better camera sensor, you can set a higher ISO with lower noise.
  • Auto-focus? It can be ok for some subjects, but leaving the camera to choose the focus might lead to out of focus shots. Fixed focus limits the risk of this happening, and to avoid your subject slipping in and out of focus you need to ensure your focus and subject are close to centred.

There are more camera tips available here in: this tutorial.


4. Position of your Subject

The positioning of your subject is very important to minimise the need to touch the subject throughout your capture session:

  • Centre your Subject: If your subject is not centred then it will drift across the frame of your shot, and in some cases you will not capture all the available surface.
  • Test Runs: It helps to do a few test runs looking through the camera viewfinder, ensuring that the subject is captured fully in each frame.
  • Elevate your Subject: By doing this you can get lower angles without the turntable base obscuring your view.
  • Subject Stability: Vibrations and abrupt movements can shift the subject, occasionally tipping them over. Be very careful when securing delicate and expensive subjects.

5. Capture Elevations & Framing

Now the subject is set up on turntable, we turn our attention to the camera:

  • Shoot in a Formal Order: Try to have every image overlapping the one before and the one after. This will help speed up alignment. Your capture start point should be feature rich so when your elevation changes, the images between elevations match sequentially.
  • Calculating the amount of photos you need per rotation depends on occlusions, subjects surface and thickness of features.
  • Between 24 shots (15°) and 48 shots (7.5°) degrees is typically all you need – For some cases you might need even more photos (72 shots (5°) or higher), but try with less to begin with and you might be surprised how well your subject reconstructs. The more photos you take, the longer the project will take to process.
  • Make sure your elevations are evenly distributed, and there is enough vertical overlap to connect each height pass, it’s quite easy to have too little overlap vertically, so be mindful of this.

6. Image Pre-processing: Lightroom

Now we have our photographs, we transfer them onto the computer. For this example we will be using lightroom, but most things we do here can be done in other programs.

  • Firstly, we import our RAW images into lightroom, then we tweak settings to suit the subject. We can make changes to one image, then sync the changes across all images, then refine individual images later if need be.
  • Set the white balance, adjust exposure, and tweak other settings to suit. There is no one-size-fits-all setting, but the key objectives are to have clearly visible sharp features, and a uniform illumination.
  • It’s important to avoid changing the shape, size or distortion of the images – This distortion is used in Zephyr to calculate reconstruction and is found in the image metadata.
  • To help mask generation, flatten the blacks or overexpose the white so you have a “perfectly” uniform background. Be careful not to affect the subject too much when doing this, it’s more important that your subject is perfect, than your mask is perfect.
  • People typically export the adjusted images as 100% JPG or 8bit TIFF, but Zephyr can accept a wide range of image formats, such as 16bit TIFF or RAW files.

7.A Mask Automation: 3DF Masquerade

As introduced in our previous Masquerade tutorial you can use the blue and red strokes to automatically compute backgrounds. Please note that you can use the cog icon to automatically compute the next image or the full dataset from the tools menu.

However, 3DF Masquerade also offers two automation tools specifically designed for turntables: a turntable mode and color removal tool. You can find below the two videotutorials for this two specific functions.


7.B Mask Automation: Photoshop

Alternatively, the final stage before we enter 3DF Zephyr can be done with external software as well! In this example we shall cover the workflow for the automation of mask generation using Photoshop.

  • Import one of the images you processed into Photoshop.
  • Open the ‘Window’ dropdown menu.
  • Open the ‘Actions’ panel.
  • Create a new action; Give it a name like ‘maskAction_[subject]’, and press ‘Record’ – Be careful now, every click is recorded so try not to make mistakes!
  • Reset foreground/background colours by pressing ‘D’
  • Open the ‘Select’ dropdown menu.
  • Open the ‘Colour range’ window and choose your selection as shadows (or highlights for white background)
  • Tweak the fuzziness and range sliders until you have a clean silhouette of your scan, contrasting against the background.
  • Close the ‘Colour range window and add a layer to your workspace. Select this new layer and fill the marquee selection with white.
  • Add another layer and put that directly below the other one. Make this black.
  • Black is the masked area, whereas white is Visible to Zephyr. Add an invert filter on top of your layers if you need to flip the colours.
  • Save as a low quality JPG – It doesn’t need to be a massive file because it’s just a mask.
  • End the action recording

Now we have this action saved, we can automate the creation of a whole folder full of images!

  • File>Automate>Batch…
  • Pick the action you created from the dropdown menu
  • Choose the source folder which contains the images you wish to mask, and set the output destination folder to the same source.
  • File naming convention should be; documentName_masked.jpg
  • Hit OK and let the computer do the hard work. Go grab a coffee or tea. Or take more photos, up to you!

Now we have our photos and masks all set up, we are ready to enter the next phase as usual. If you haven’t yet, make sure to check out our other tutorials