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QWidget Flickering for frame by frame streaming of video

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qwidgetqpainteventflickeringvideopixmap
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  • SGaistS Offline
    SGaistS Offline
    SGaist
    Lifetime Qt Champion
    wrote on last edited by
    #14

    QImage::bits.

    Make the QImage the correct format and size.

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    Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

    S 1 Reply Last reply
    1
    • SGaistS SGaist

      QImage::bits.

      Make the QImage the correct format and size.

      S Offline
      S Offline
      SilverSurfer
      wrote on last edited by
      #15

      @SGaist
      Tried using
      QImage::QImage(uchar * data, int width, int height, Format format, QImageCleanupFunction cleanupFunction = 0, void * cleanupInfo = 0)

      data = pixel data , updated by fetchBitmap(..) function above.
      format = RGB888
      where as per documentation it does not do a deep copy. So this helps.

      But my image has R and B planes swapped.
      Can't seem to find the appropriate format .

      P.S: I am on a Windows machine.

      Thanks,
      Kshitij Mayekar

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      • SGaistS Offline
        SGaistS Offline
        SGaist
        Lifetime Qt Champion
        wrote on last edited by
        #16

        See QImage::rgbSwapped

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        Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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        • SGaistS SGaist

          See QImage::rgbSwapped

          S Offline
          S Offline
          SilverSurfer
          wrote on last edited by
          #17

          @SGaist
          QImage::rgbSwapped results in creating new QImage with is equivalent to deep copy. So It does not help. Is there any other way to dislaying the RGB planes correctly ?

          Note: Whenever I write the contents of the buffer to a file with appropriate headers I could see the Image correctly when I open the Image through windows photo viewer.

          Original Problem Statement : Display Byte Buffer(uchar*) to a QWidget without the need to deep Copy .

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          • SGaistS Offline
            SGaistS Offline
            SGaist
            Lifetime Qt Champion
            wrote on last edited by
            #18

            Where are these frames coming from ?

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            • SGaistS SGaist

              Where are these frames coming from ?

              S Offline
              S Offline
              SilverSurfer
              wrote on last edited by
              #19

              @SGaist

              These frames are received from a server.

              //Api that talks to server
              fetchBitmap(&vector[0]);
              

              I do not have access to server code.

              P.S: Server Machine is Linux and Client is Windows . Could this be the reason for RGB /BGR mismatch ?

              mrjjM 1 Reply Last reply
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              • W Offline
                W Offline
                wrosecrans
                wrote on last edited by
                #20

                Is there any chance of the paint event being triggered by the system at a moment when the pixmap isn't fully populated? If that is possible, you would get flickering regardless of whether the load function is faster or slower than real time.

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                • S SilverSurfer

                  @SGaist

                  These frames are received from a server.

                  //Api that talks to server
                  fetchBitmap(&vector[0]);
                  

                  I do not have access to server code.

                  P.S: Server Machine is Linux and Client is Windows . Could this be the reason for RGB /BGR mismatch ?

                  mrjjM Offline
                  mrjjM Offline
                  mrjj
                  Lifetime Qt Champion
                  wrote on last edited by
                  #21

                  @SilverSurfer said in QWidget Flickering for frame by frame streaming of video:

                  Could this be the reason for RGB /BGR mismatch ?

                  Well there is always Endianness but if using same sort of CPU in both ends, its likely not related to that.

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                  • mrjjM mrjj

                    @SilverSurfer said in QWidget Flickering for frame by frame streaming of video:

                    Could this be the reason for RGB /BGR mismatch ?

                    Well there is always Endianness but if using same sort of CPU in both ends, its likely not related to that.

                    S Offline
                    S Offline
                    SilverSurfer
                    wrote on last edited by
                    #22

                    @mrjj
                    Both machines are little endian.

                    mrjjM 1 Reply Last reply
                    0
                    • S SilverSurfer

                      @mrjj
                      Both machines are little endian.

                      mrjjM Offline
                      mrjjM Offline
                      mrjj
                      Lifetime Qt Champion
                      wrote on last edited by
                      #23

                      @SilverSurfer
                      Ok. so most likely they
                      are just using openCV and send as BGR888

                      So i dont see any way to convert to supported format without copy.

                      W 1 Reply Last reply
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                      • mrjjM mrjj

                        @SilverSurfer
                        Ok. so most likely they
                        are just using openCV and send as BGR888

                        So i dont see any way to convert to supported format without copy.

                        W Offline
                        W Offline
                        wrosecrans
                        wrote on last edited by
                        #24

                        @mrjj If you use a shader in a QOpenGLWidget, you can swizzle the r/g/b around arbitrarily when you do the drawing into the frame buffer, which would avoid an extra copy step to normalize the channel order.

                        S 1 Reply Last reply
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                        • W wrosecrans

                          @mrjj If you use a shader in a QOpenGLWidget, you can swizzle the r/g/b around arbitrarily when you do the drawing into the frame buffer, which would avoid an extra copy step to normalize the channel order.

                          S Offline
                          S Offline
                          SilverSurfer
                          wrote on last edited by SilverSurfer
                          #25

                          @wrosecrans said in QWidget Flickering for frame by frame streaming of video:

                          you can swizzle the r/g/b around arbitrarily when you do the drawing into the frame buffer

                          Could you provide sample code for the same ?

                          W 1 Reply Last reply
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                          • S SilverSurfer

                            @wrosecrans said in QWidget Flickering for frame by frame streaming of video:

                            you can swizzle the r/g/b around arbitrarily when you do the drawing into the frame buffer

                            Could you provide sample code for the same ?

                            W Offline
                            W Offline
                            wrosecrans
                            wrote on last edited by
                            #26

                            @SilverSurfer I don't have the time handy to do a full working example, but this is the technique in glsl:
                            https://www.khronos.org/opengl/wiki/Data_Type_(GLSL)#Swizzling

                            (The vec4 can hold "rgba" values if that's what you put in it - the example there just calls it "xyzw" because the data type generically handles any 4 floats.)

                            Basically, start from an example that draws an image with a simple glsl shader as a texture. Then hack on the shader until it flips around the colors like you want when it draws the texture. Then use your code that gets the image over the network to upload the image as the OpenGL texture every frame.

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