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I am trying to understand how the BLAM addon works. In the Blender default scene I made an image of a cube with camera focal length 35mm. Then I created a new blend file and opened this image in the movie clip editor and did draw the lines requested for camera calibration with BLAM.

enter image description here

After BLAM camera calibration the focal length was set at 32.37. I really don't understand this (the image was made in blender with 35.0).

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Blams accuracy depends on your line drawing accuracy. Maybe if you would repeat the cube line pixel to pixel it would calculate focal length correctly, but I bet there would still be some minor inaccuracy. As for the mesh reconstruction, even in introduction video is stated, that resulting object will probably not be perfect, so you should expect that. BLAM is not perfect, but certainly changes tens of minutes of tweaking into few minutes of tweaking, so it's not bad at all.

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  • $\begingroup$ I tried again with a line thickness of 1 pixel and now BLAM tells me 30.7 focal length. I don't think it is about drawing the lines even more precise. It may be a factor but I think my lines were pretty good $\endgroup$
    – user13877
    Nov 10, 2016 at 21:46
  • $\begingroup$ I managed to get 34.89 focal length at the best and precision of line drawing was main variable impacting the outcome.Here is the blend file check it out and see if you get the same result by not changing anything and pressing Calibrate active camera. $\endgroup$
    – Mzidare
    Nov 11, 2016 at 10:52
  • $\begingroup$ Thanks. You are right. Would you mind also trying at the file that I attached at my question ? $\endgroup$
    – user13877
    Nov 11, 2016 at 16:13
  • $\begingroup$ I guess I'm getting better at it. With your file I managed to get 34.92 so almost there.Here's your file after upgrading line drawing. $\endgroup$
    – Mzidare
    Nov 11, 2016 at 19:20
  • $\begingroup$ Jesus Christ. Tomorrow I must have a look at your special talents $\endgroup$
    – user13877
    Nov 12, 2016 at 1:13

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