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Focal lenght

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Dimouzoi

Hello, i would like to extract the focal lenght of the camera for 3d reconstruction purposes with neural methods. I have the instricts of the camera:

    "color_format" : "RGB8",
    "depth_format" : "Z16",
    "depth_scale" : 999.99993896484375,
    "device_name" : "Intel RealSense D455",
    "fps" : 5.0,
    "height" : 720,
    "intrinsic_matrix" :
    [
        643.114013671875,
        0.0,
        0.0,
        0.0,
        642.42431640625,
        0.0,
        644.1124267578125,
        372.05221557617188,
        1.0
    ],
    "serial_number" : "146222253035",
    "stream_length_usec" : 274427956,
    "width" : 1280
 
The right value is 641.3663330078125?
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8 comments

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  • MartyG

    Hi Dimouzoi  The focal length of the image changes depending upon the resolution that is currently set.  You can find your camera's image focal length for a particular resolution using the RealSense SDK's 'rs-enumerate-devices' tool to list its intrinsics by launching it in calibration information mode with the following command.

    rs-enumerate-devices -c

     

    The focal length is described by fx and fy, which are the focal length of the image as a multiple of pixel width and height  The fx and fy values are allowed to be different but are commonly close to each other.

     

     

    Using rs-enumerate-devices will be the easiest way to be sure whether the '641' value that you listed represents fx / fy or ppx / ppy.

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  • Dimouzoi

    I did not get how exactly i get these information, can you be more detailed?

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  • MartyG

    If you are using Windows then instructions for launching in the Command Prompt interface can be found at the link below.

    https://github.com/IntelRealSense/librealsense/issues/3569#issuecomment-615365409

     

    If you are using Linux then if you have installed the RealSense SDK and included the tools and examples in the installation, input rs-enumerate-devices -c into the terminal window to launch it.

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  • Dimouzoi

    Also I have done camera calibration after the scanning, frames resolution are in 1280*720, can i find the focal length differently?

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  • Dimouzoi

    i got these values

     

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  • MartyG

    If you want to calibrate the focal length then there is a calibration tool for doing so.

    https://dev.intelrealsense.com/docs/self-calibration-for-depth-cameras#addendum-b-march-2022-focal-length-calibration

     

    In regard to alternative methods of obtaining the fx and fy focal length values, the link below discusses Python program scripting for doing so.

    https://github.com/IntelRealSense/librealsense/issues/2930#issuecomment-448603584

     

     

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  • Dimouzoi

    i have scanned a room and align the pdc according its instricts, i have calibrate the camera after that but i want the focal length of that scanning with resolution 1280*720 and fps 5. Can i get any of that information from json file?

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  • MartyG

    If you repeat the rs-enumerate-devices check for the fx and fy intrinsics at 1280x720 resolution now that you have calibrated, that will give you the values to use.  Those values should remain the same until the next time that you calibrate the camera.  The FPS will not affect the focal length.

     

    A json file does not contain the focal length information.

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