The Measurement Tool
The measurement tool is a compelling feature on paper. You aim your RealSense Snapshot at an object, snap a photo and use the tool to determine distances between objects, measure objects and even calculate the area of something.
However, to be blunt, you should manage your expectations of what this tool can actually do.
Dell was clear in its various guides that the measurement tool really only works under certain conditions. We were advised to be within 3-15 feet of the subject, make sure to shoot straight on (not at an angle) and ensure that there’s plenty of light. But not bright, direct sunlight. Oh, and we were to also avoid reflective or see-through surfaces, such as glass.
We were further advised to be sure that there were no smudges on the camera lenses; they can cause inaccuracies.
Dell recently pushed out an update that includes a new Dynamic Calibration tool to help ensure better accuracy in depth shots. Apparently, the Snapshot camera calibration can slip over time as you use the device, or even just during shipping. According to a Dell document, the calibration is automatic and occurs while a depth shot is being processed; if there's an issue, you'll see a pop-up notification informing you of the problem (and offering the automatic solution).
When you snap a photo, you enter the edit mode and tap the measurement tool. Simply tap two points on the image to create a measurement. You can fine-tune the measurements using a (rather nifty) feature: when you tap and hold one of the corners of your lines, you get a little magnified look at what you’re doing. It’s delicate, but it works well.
We dutifully obeyed these instructions, but our results were mixed, to say the least. We shot a variety of objects in different lighting conditions -- all more or less optimal -- and found that the tool was frequently off.
For example, note the measurements on this bridge. According to the RealSense Snapshot camera, it’s 15’6” across. That’s not completely far off; it’s actually 15’1”.
However, note the three measurements of the rails, bottom to top. The camera thinks they’re 2’2”, 4’1” or 7’4”. In reality, each of those distances is actually a hair under 2’5”. Also, the 3’9” span across the two posts is actually 3’1.75”.
We showed this parade of inaccuracy to Dell, and a representative noted that there were mitigating factors to consider. “The bridge shot has areas where the background is very similar in color to the foreground, and this is likely causing the software to calculate an incorrect depth for some of those points,” he said. He also noted that in terms of the bridge's span, a 5-inch error over that distance is still 97.5-percent accurate.
That’s fair enough, we suppose. Let’s agree that the lighting and contrast between subject and background was not optimal in the bridge image.
Here’s another example:
We shot these bricks in mid-afternoon light, safely away from the sun’s direct glare, but thoroughly well-lit, from just over 3 feet away. The bricks are 8.25” across. The RealSense camera guessed some of them fairly accurately at 8”, but the one is more than an inch off at 7”, and the tool was unable to even venture a guess for one of the bricks.
Could we have shot more straight-on here? Possibly, yes. But it's extremely difficult to do so without a tripod, and in real-life situations, this is about as good as you can do.
Here’s another example:
Determining couch size has been Dell’s go-to example of the measurement tool’s usefulness. You’re in a store, and you need to know if that funky divan will fit in your living room. You whip out your Dell Venue 8 7000 tablet, snap a pic and measure away.
We tried this specific use case and found that it was slightly off, too. The span of our test image is 6’2”, and the tool had it at 5’11”. That’s close -- probably close enough to determine if you can fit it in your living room -- but not precise.
In some cases, though, the measurement tool was more accurate. Take for example this picture frame:
The sides of the frame are actually 16.25”, so the tool was accurate within a quarter-inch (it doesn’t offer measurements less than an inch, so being within an inch is as accurate as the tool can possibly be).
With the measurement tool, you can also check for the area of a plane. You simply tap for points, click to drag the points to exact spots, and poof, you get the area.
Or not. In this image, we shot a simple, rectangular box as close to straight on as possible. Again, we're holding a tablet and shooting without a tripod. The tool wasn’t able to measure all the sides and thus produced an “unknown” result. Oddly, the line measurement in the image on the left is actually correct.
Using that snapshot of the picture frame (above), we were able to get a reasonably accurate result, though. One of the sides registered an inch off, and therefore the area was slightly incorrect, but all things considered this particular area calculation was quite good.
We should note that we got more accurate results when shooting close to an object and measuring something small, or shooting farther away and measuring a larger distance. For instance, if you want to know the width of a brick, you should be within about 3 feet, not 15.