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Dimensions of the TargetElement Image in Vuforia Database

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  • Dimensions of the TargetElement Image in Vuforia Database

    Game manual pt 2 indicates the size of the sticker on the SkyStone as 7 x 3.75" (including the width of the black bars on the right and left margins. When I copy it from the game manual and print it at 7 x 3.75", the actual useable image (i.e., excluding the black bars) measures 5 x 3.75".

    But the Skystone.xml file indicates target dimensions for this image of 4 x 3.75" (101.599998 x 95.250000 mm).

    My testing with Vuforia so far: when the camera is 12 inches away from the image, Vuforia says it is 10 inches away. Also, when the camera is panned 5 inches (going from left edge of image in center to right edge in center), Vuforia says it has panned only 4 inches.

    I'm wondering, was the image intentionally cropped to a smaller size before the Vuforia dataset was created, was there an error in entering image dimensions, or am I making some kind of mistake in my analysis?

  • #2
    The skystone stickers can be purchased from AndyMark using part number am-4018_sticker (found in field setup guide). They are $3 each, if I recall correctly. I am guessing the examples in the game manual were not intended to be actual size.


    • #3
      That's an interesting observation.

      During my testing, I discovered the exact same image panning problem (4" reported with 5" movement), but when others tested the same scenario, they did not see the discrepancy.
      At the time, I'm sure I checked the XML file to verify the dimensions. It's possible that there were two different filed floating around at the time.

      I finally put it down to the possibility that the older ZTE phone I was using had incorrect camera calibration, although I did not see this problem on the other images.

      FWIW... What type of phone are you using?

      Now that you've brought up the image size discrepancy, I'm going to double check that. I'd be glad to find an actual problem.


      • #4
        Follow up...
        Note: With the reported image distance, you need to be sure that you take into account the camera offset that is loaded into your opmode.
        The sample OpMode has several inches of forward camera offset built in (since it is unlikely that the phone will be at the center of the robot) so some constant offset is expected.


        • #5
          OK, So I just measured an actual SKYSTONE target and the image portion of the sticker matches the XML file, (101.599998 x 95.250000 mm). so it sounds like the self-printed image is the problem.

          While I track this down, I would resize your printed image so the image area is (102 x 95 mm) & retest.


          • #6
            Yeah, so it looks like the target image in the manual is incorrect. it must have been an earlier prototype image that was not updated when a change was made.
            I guess nobody expected it would be used as an actual test target (D'OH!)

            I compared the two images and the content is exactly the same, but the image in the manual is stretched horizontally. It takes up more of the sticker than it should.

            Tom is now aware of the problem and will take action to get a solution.

            But as a work-around, if you crop out the image portion and resize it to 101.599998 x 95.250000 mm, it will match a real sticker. For Vuforia, the outer black portion does not matter.


            • #7
              Philbot Thanks for the info. My test was with a Moto G4. I specify that the camera is at (0,0,0) in robot coordinates. I did notice last year that I was getting a small error (around 4 percent) in distance estimates that varied between phone models. That was correctable by tweaking the numbers in the XML file (though we ultimately didn't use Vuforia for competition).


              • #8
                Folks - FIRST should be releasing the PDF files with the Skystone and bridge target images included soon. The image of the skystone in the Game Manual Part 2 is not to scale as others have noted so I would avoid using it for development and wait for the PDFs.

                Additional info is available here.
                Hello all. I can't find printable images for the stickers for Skystones nor the images that would go on the Skybridge legs. This PDF has the 8...