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Different devices same Focal Length

September 2, 2013 - 12:00pm #1


It seems like  getFocalLength returns the same value for different devices, even though it is clear from the video that one has wider view angle than the other.
For example, both Galaxy S4 and Galaxy Note II provide the same value:
Focal Length = 629.504028(horizontal) x 629.504028(vertical) for video resolution 640x480
Could it be that the above values are the default values, and Vuforia sets to the real values behind the scene?

Different devices same Focal Length

September 9, 2013 - 1:59am #4

Thanks for the additional clarifications, very helpful. What you say makes sense... it would indeed make sense to expect slightly different calibration data for the 2 devices, in force of the different field of view.

Actually, enhancing the camera calibration management is something we are taking into consideration for future releases of the SDK.

Different devices same Focal Length

September 9, 2013 - 1:27am #3


Thank you Alessandro for you answer.

It makes a lot of sense that different devices with different screen aspect ration will display different crops of the original 640x480 image in order to fit into the screen resolution.

By "stretched" I mean that the texture doesn't fit to (/overlay) the image target very well, at least not as good as it looks on S4.
That's the reason I thought the field of view is not correct, in such a way that the texture is a bit stretched.

To investigate this issue, I saved to file both images (raw data from S4 and from Note II), and it looked like Note II has a wider angle, on the other hand I looked into the CameraCalibration params and noticed that both devices have the same FOV.
I expected to get different values (Focal length) from these two device, but if I'm the only one experiencing this behaviour - I guess I'm doing something wrong.

Different devices same Focal Length

September 9, 2013 - 1:08am #2

Hi, the focal length may return the same value for different devices, as you indeed noticed with S4 and Note II. 

however, the fact that the texture appears stretched on the Note II may rather depend on the different screen resolution (w.r.t the S4), which may result in a different "adjustment" of the textured video background with respect to the screen; this is primarily due to the need of taking into account the difference between the screen aspect ratio and the camera image aspect ratio, where the aspect ratio is defined as the ration width / height.


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