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Qt 3D Camera projection matrix

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  • B Offline
    B Offline
    Bob64
    wrote on 18 Nov 2020, 11:49 last edited by
    #1

    How is the projection matrix property of Qt 3D (Q)Camera defined? I am a complete newbie and have been trying to do some background reading as the Qt 3D docs are pretty sparse, but the only references to camera projection matrices that I have found have defined them as 3x4 matrices. This makes sense, as the mapping is from 3D homogeneous coordinates to a projection on the image plane in 2D homogeneous coordinates. However, the Qt property is a 4x4 matrix.

    P 1 Reply Last reply 18 Nov 2020, 16:02
    0
    • B Bob64
      18 Nov 2020, 18:50

      @Pl45m4 thanks for the suggestion. A quaternion is effectively a 4D vector, so if there is any connection with the 4X4 projection matrix I suppose the columns might be quaternions. As I say, I am quite new to this stuff, but my impression is that quaternions tend to be used to represent rotations, and I am struggling to see how that might relate to a projection.

      To rephrase my original question, I guess I am looking for some description of what the matrix in Qt means geometrically, and when one might need to modify its value, etc.

      P Offline
      P Offline
      Pl45m4
      wrote on 18 Nov 2020, 22:49 last edited by
      #4

      @Bob64

      Got confused by Vector4D and Matrix4x4. Sure, a quaternion is a+ib+jc+kd

      Here is probably one of the best explanations you will find out there :)
      It's Unity, but the 3D stuff is the same, but better check what your values look like.

      https://answers.unity.com/questions/1359718/what-do-the-values-in-the-matrix4x4-for-cameraproj.html


      If debugging is the process of removing software bugs, then programming must be the process of putting them in.

      ~E. W. Dijkstra

      B 1 Reply Last reply 19 Nov 2020, 10:13
      1
      • B Bob64
        18 Nov 2020, 11:49

        How is the projection matrix property of Qt 3D (Q)Camera defined? I am a complete newbie and have been trying to do some background reading as the Qt 3D docs are pretty sparse, but the only references to camera projection matrices that I have found have defined them as 3x4 matrices. This makes sense, as the mapping is from 3D homogeneous coordinates to a projection on the image plane in 2D homogeneous coordinates. However, the Qt property is a 4x4 matrix.

        P Offline
        P Offline
        Pl45m4
        wrote on 18 Nov 2020, 16:02 last edited by
        #2

        @Bob64

        I think it's a quaternion in its matrix form (X,Y,Z + Angle).
        https://doc.qt.io/qt-5/qquaternion.html

        https://en.wikipedia.org/wiki/Quaternion


        If debugging is the process of removing software bugs, then programming must be the process of putting them in.

        ~E. W. Dijkstra

        B 1 Reply Last reply 18 Nov 2020, 18:50
        0
        • P Pl45m4
          18 Nov 2020, 16:02

          @Bob64

          I think it's a quaternion in its matrix form (X,Y,Z + Angle).
          https://doc.qt.io/qt-5/qquaternion.html

          https://en.wikipedia.org/wiki/Quaternion

          B Offline
          B Offline
          Bob64
          wrote on 18 Nov 2020, 18:50 last edited by
          #3

          @Pl45m4 thanks for the suggestion. A quaternion is effectively a 4D vector, so if there is any connection with the 4X4 projection matrix I suppose the columns might be quaternions. As I say, I am quite new to this stuff, but my impression is that quaternions tend to be used to represent rotations, and I am struggling to see how that might relate to a projection.

          To rephrase my original question, I guess I am looking for some description of what the matrix in Qt means geometrically, and when one might need to modify its value, etc.

          P 1 Reply Last reply 18 Nov 2020, 22:49
          0
          • B Bob64
            18 Nov 2020, 18:50

            @Pl45m4 thanks for the suggestion. A quaternion is effectively a 4D vector, so if there is any connection with the 4X4 projection matrix I suppose the columns might be quaternions. As I say, I am quite new to this stuff, but my impression is that quaternions tend to be used to represent rotations, and I am struggling to see how that might relate to a projection.

            To rephrase my original question, I guess I am looking for some description of what the matrix in Qt means geometrically, and when one might need to modify its value, etc.

            P Offline
            P Offline
            Pl45m4
            wrote on 18 Nov 2020, 22:49 last edited by
            #4

            @Bob64

            Got confused by Vector4D and Matrix4x4. Sure, a quaternion is a+ib+jc+kd

            Here is probably one of the best explanations you will find out there :)
            It's Unity, but the 3D stuff is the same, but better check what your values look like.

            https://answers.unity.com/questions/1359718/what-do-the-values-in-the-matrix4x4-for-cameraproj.html


            If debugging is the process of removing software bugs, then programming must be the process of putting them in.

            ~E. W. Dijkstra

            B 1 Reply Last reply 19 Nov 2020, 10:13
            1
            • P Pl45m4
              18 Nov 2020, 22:49

              @Bob64

              Got confused by Vector4D and Matrix4x4. Sure, a quaternion is a+ib+jc+kd

              Here is probably one of the best explanations you will find out there :)
              It's Unity, but the 3D stuff is the same, but better check what your values look like.

              https://answers.unity.com/questions/1359718/what-do-the-values-in-the-matrix4x4-for-cameraproj.html

              B Offline
              B Offline
              Bob64
              wrote on 19 Nov 2020, 10:13 last edited by
              #5

              @Pl45m4 thank you very much. I will need to take some time to digest it properly but based on a quick skim that looks like it is very good information.

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              18 Nov 2020, 11:49

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