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david3_user_manual:structured_light

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(click here to get to DAVID 2.x User Manual)

DAVID 3.x User Manual Pages

  1. LASER Scanning:
  2. STRUCTURED LIGHT Scanning:

Structured Light: Hardware Setup and Calibration

Introduction

Structured Light Scanning (SL) is 3D scanning with a video projector instead of the laser. DAVID will use the video projector to project a number of stripe patterns onto the object. The camera will watch that from a different point of view (above, below, left or right of the projector) and take (at least) one image of each pattern. Based on the deformations of the stripe patterns in these images, DAVID will compute a precise 3D mesh of the object surface.

Camera calibration is the same as for laser scanning. Scanning however is much faster and simpler:

  • Calibration panels can be removed after camera and projector calibration
  • No moving parts
  • One click only
  • Scan takes only a very few seconds

Mobile scanner: It is easily possible to mount camera and projector with a fixed distance to one stand (e.g. tripod). Once calibrated, this mobile scanner can be moved and scan anywhere, as long as camera and projector remain fixed and their optics remain unchanged. The DAVID SLS-2 has this kind of setup.

Step 1: Projector Setup in Microsoft Windows

Connect your projector to your PC (usually via VGA, DVI, HDMI, or DisplayPort).

We recommend to disable all filters that your projector may have for image “improvement”, like increased sharpness or contrast, frame interpolation etc. If your projector has presets, choose “default”, “neutral”, “video” or “cinema”. Presets like “dynamic” or “game” are usually bad for scanning. Also switch off any automatic or manual Keystone Correction!

Short story: Set up your projector as extended desktop (screen 2). Set both your monitor and projector to their respective native resolution. Go to Step 2.

Long story: Windows does not make a difference between a projector and a monitor: Both are “screens” or “displays” with a certain resolution. It can be set to show the same content on both (clone), or different content (extended desktop) that expands from one screen to the other. That's what we need! Icons, program windows and mouse pointer etc. can be placed on either screen, or moved from one to the other. The only thing that is “cloned” on both displays is your desktop background image.

Fast version: In Windows 7, you can press Win+P to get a menu like this. Choose “Extend”, usually then you are done:

In any Windows version, you can right-click on your desktop and choose “Properties” or “Screen Resolution” (depending on your Windows version). Choose “Extend”, and set each display to its native resolution (maximum). You can click “Identify” to see which is which. Also you can choose which is your “main display” (should be your monitor).

Move your mouse pointer around toward the display borders to see how it leaves one display and appears on the other. You can move any windows, desktop icons or even the taskbar where you want, but usually you will want them all on your monitor.

Step 2: Hardware Placement

The projector must be placed left, right, above or below the camera, with of course both looking toward the object. It is reasonable to mount both together on one mobile tripod (“The Scanner”), but also you can just place them above or next to each other on a table. The scale of the whole scanner depends on the object size. The following table contains guide values for the DAVID SLS-1 scanner (12mm lens, medium-size projector). Webcams usually have a wider viewing angle and need to be set up at shorter distances.

Scanned object size Calibration pattern scale Distance cam-projector Distance scanner-object Expected resolution (SLS-1)
<30 mm 30 mm ca. 30 mm ca. 90 mm < 0.05 mm
50 mm 60 mm ca. 40 mm ca. 120 mm 0.05 mm
70 mm 60 mm ca. 65 mm ca. 180 mm 0.08 mm
90 mm 120 mm ca. 80 mm ca. 220 mm 0.10 mm
120 mm 120 mm ca. 110 mm ca. 300 mm 0.13 mm
150 mm 120 mm ca. 125 mm ca. 350 mm 0.15 mm
200 mm 240 mm ca. 160 mm ca. 450 mm 0.2 mm
300 mm 240 mm ca. 250 mm ca. 700 mm 0.3 mm
500 mm 240 mm ca. 400 mm ca. 1200 mm 0.5 mm
in general similar to object angle 15°-25° between them object should fill camera view around 0.1% of object size

Step 3: Menu "Hardware Setup"

Start DAVID 3 and open the Hardware Setup menu at the left. Hardware Setup Menu

Projector Setup

Select “Structured Light Setup” as your Setup Type. Set the Screen ID so that the stripe pattern is shown on your projector (usually 2) - if this is not possible, go back to Step 1 and be sure to set up your projector as an extended screen.

Place the object so that the projector illuminates the object surface - not less, but also not much more. Adjust the projector focus so that the projection is sharp. Depending on your projector model, if the projector is perfectly focused, the fine lines between the projector's pixels may cause a fine noise in the scan. If that happens, we recommend to set the projection slightly out of focus. This will not reduce the scan quality.

Camera Setup

In DAVID, select your camera as video source (usually “DAVID-CAM-…”). You should see the live camera image. If necessary, adjust the camera focus and aperture.

Position the camera so that the projected pattern on the object surface (nearly) fills the camera view. If the camera view shows much more than the scanned surface, you should reduce the distance between camera and object. Fix the ball joint (where available). Adjust camera focus to get a sharp image.

:!: A very important step is the adjustment of image brightness. It depends on

  • Exposure time ← set so that the image is steady and does not flicker or pulse - usually 1/60s is fine
  • Camera settings (“gain”, “brightness”) ← should usually be kept at default (neutral)
  • Projector brightness ← should be set to maximum
  • Mechanic lens aperture

If you need a fixed Exposure Time to avoid flickering/pulsing, in most cases you should use the mechanic lens aperture size to adjust the image brightness. If your camera does not allow that (webcam), you may have to reduce Projector Brightness.

DAVID measures the image intensities along 3 horizontal and 3 vertical lines and displays them as red curves. It is very important to find good settings so that these red curves are well saturated. Regard those image regions that show the wave patterns: The red intensity curves must be sinusoidal and must not be over- or under-saturated. In other words, the red sine curve must not be cut off at the blue borders.

too dark - sine is too flat! good (but nearly too much) too bright - sine is cut off!

The following images illustrate the same sine waves, but during scanning. This image shows a perfect setup:
good sine wave profile lines(red)

On the following image the sine waves are distorted (over-saturated). This will cause waves in the scan result. Avoid this by decreasing camera exposure or brightess. If that is not possible, you can decrease the projector brightness with the slider at the top.
oversaturated

The next image is too dark. In this case increase camera exposure and/or brightness.
undersaturated

The hardware is now optimized for your object and needs to be calibrated. From now on, keep the whole scanner fixed.

Step 4: Menu "Calibration"

Choose a calibration pattern that fits your object size / desired scan volume (see table above). Its angle must be precisely 90°.

Remove the object and set up the calibration corner in the same area in front of the scanner. If you have a mobile scanner, you can move or tilt the whole unit, but keep camera and projector fixed with respect to each other. The camera does not need to see ALL markers, but at least the 6 rings and a few dots. The whole camera image should be filled with the calibration markers. The camera should not be able to see past the calibration panels.

In DAVID, open the Calibration menu. Calibration Menu

Enter the correct calibration scale length at “Calibr. Scale”. You can find it at the side of the calibration pattern.

Check the camera image again. In those areas that show the wave pattern, the red intensity curves must not be cut off at the blue borders.
If the scanned object is significantly darker than the white calibration panels, the sine waves will be distorted now. Reduce the “Projector Brightness” slider to correct this.

The camera image should look approx. like the following:

Click “Calibrate” to calibrate the whole scanner. In this step the software will first measures the position, orientation, focal length and distortion coefficients of the camera. Then it will project a pattern sequence in order to determine the same optical properties of the projector. If Texturing is not switched off, it will also perform a white balance measurement.

After a successful calibration, DAVID will project a checkerboard pattern, whose corner points should fall exactly onto the calibration markers.

Calibration Completed

Your scanner is now calibrated. This refers to the positioning of camera and projector relative to each other, and the focus and brightness settings. You can move, rotate and tilt the scanner as a whole, and you can close and restart the DAVID software without losing the calibration.

However, if you move/rotate camera or projector separately, or change the focus (e.g. in order to scan something much smaller or larger), you will have to repeat the whole calibration procedure.

You can now perform scans.

david3_user_manual/structured_light.txt · Last modified: 2014/08/15 13:05 by sven