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  4. Dump QImage raw pixel data into `std::vector<char>`

Dump QImage raw pixel data into `std::vector<char>`

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  • C Offline
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    Christian Ehrlicher
    Lifetime Qt Champion
    wrote on 6 Dec 2020, 08:09 last edited by
    #37

    Use mutexes or copy the data - I would suggest reading a little bit on how threading works. E.g. https://doc.qt.io/qt-5/threads-synchronizing.html - c++ uses the same mechanisms

    Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
    Visit the Qt Academy at https://academy.qt.io/catalog

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      abmyii
      wrote on 6 Dec 2020, 12:34 last edited by
      #38

      Whoops, I've been saying malloc (which is apparently thread-safe) all along when in fact the actual function I'm using is memcpy... Does that change things and/or can I use malloc instead?

      J 1 Reply Last reply 6 Dec 2020, 13:22
      0
      • A abmyii
        6 Dec 2020, 12:34

        Whoops, I've been saying malloc (which is apparently thread-safe) all along when in fact the actual function I'm using is memcpy... Does that change things and/or can I use malloc instead?

        J Offline
        J Offline
        JonB
        wrote on 6 Dec 2020, 13:22 last edited by
        #39

        @abmyii
        Hence what @Christian-Ehrlicher has been saying above. malloc and memcpy do totally different things, it makes no sense to ask whether one can be used in place of the other....

        A 1 Reply Last reply 6 Dec 2020, 14:49
        0
        • J JonB
          6 Dec 2020, 13:22

          @abmyii
          Hence what @Christian-Ehrlicher has been saying above. malloc and memcpy do totally different things, it makes no sense to ask whether one can be used in place of the other....

          A Offline
          A Offline
          abmyii
          wrote on 6 Dec 2020, 14:49 last edited by abmyii 12 Jun 2020, 14:50
          #40

          @JonB I understand - I got confused due to my unfamiliarity with the functions but I never asked if I could use malloc instead of memcpy - I asked if I could use push_back instead of malloc (sic).

          J 1 Reply Last reply 6 Dec 2020, 15:04
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          • A abmyii
            6 Dec 2020, 14:49

            @JonB I understand - I got confused due to my unfamiliarity with the functions but I never asked if I could use malloc instead of memcpy - I asked if I could use push_back instead of malloc (sic).

            J Offline
            J Offline
            JonB
            wrote on 6 Dec 2020, 15:04 last edited by JonB 12 Jun 2020, 15:06
            #41

            @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

            I never asked if I could use malloc instead of memcpy [sic.]

            in fact the actual function I'm using is memcpy... Does that change things and/or can I use malloc instead? [*sic.]

            If you say so! I'll leave others to untangle, who clearly understand what you are saying better than I :) Don't worry about explaining to me.

            A 1 Reply Last reply 6 Dec 2020, 15:09
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            • J JonB
              6 Dec 2020, 15:04

              @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

              I never asked if I could use malloc instead of memcpy [sic.]

              in fact the actual function I'm using is memcpy... Does that change things and/or can I use malloc instead? [*sic.]

              If you say so! I'll leave others to untangle, who clearly understand what you are saying better than I :) Don't worry about explaining to me.

              A Offline
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              abmyii
              wrote on 6 Dec 2020, 15:09 last edited by abmyii 12 Jun 2020, 15:10
              #42

              @JonB OK, you got me! Too much to keep track of...

              I tried this and it worked, but is quite a lot slower than memcpy:

              std::vector<unsigned char> vectorBuffer(charBuffer, charBuffer + length);
              fb = vectorBuffer;
              
              J C 2 Replies Last reply 6 Dec 2020, 15:20
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              • A abmyii
                6 Dec 2020, 15:09

                @JonB OK, you got me! Too much to keep track of...

                I tried this and it worked, but is quite a lot slower than memcpy:

                std::vector<unsigned char> vectorBuffer(charBuffer, charBuffer + length);
                fb = vectorBuffer;
                
                J Offline
                J Offline
                JonB
                wrote on 6 Dec 2020, 15:20 last edited by
                #43

                @abmyii
                I am lost in the complexities of your questions :) But I would suggest heeding @Christian-Ehrlicher's advice, he is usually right! He last offered:

                Use mutexes or copy the data

                So why not pursue that?

                A 1 Reply Last reply 6 Dec 2020, 15:56
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                • A abmyii
                  6 Dec 2020, 15:09

                  @JonB OK, you got me! Too much to keep track of...

                  I tried this and it worked, but is quite a lot slower than memcpy:

                  std::vector<unsigned char> vectorBuffer(charBuffer, charBuffer + length);
                  fb = vectorBuffer;
                  
                  C Offline
                  C Offline
                  Christian Ehrlicher
                  Lifetime Qt Champion
                  wrote on 6 Dec 2020, 15:29 last edited by
                  #44

                  @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

                  std::vector<unsigned char> vectorBuffer(charBuffer, charBuffer + length);
                  fb = vectorBuffer;

                  This does two memcpy for no good reason. Since I don't know what libvncserver function you use you're on your own. Read the documentation of them and also read what's needed for threadsafe access.

                  Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                  Visit the Qt Academy at https://academy.qt.io/catalog

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                  • J JonB
                    6 Dec 2020, 15:20

                    @abmyii
                    I am lost in the complexities of your questions :) But I would suggest heeding @Christian-Ehrlicher's advice, he is usually right! He last offered:

                    Use mutexes or copy the data

                    So why not pursue that?

                    A Offline
                    A Offline
                    abmyii
                    wrote on 6 Dec 2020, 15:56 last edited by
                    #45

                    @JonB Haha, you think you are the only one?!

                    I am planning to try the mutex idea - I haven't had a chance to look it up and figure out how to do it yet. A basic QMutex lock/unlock didn't fix it (though I probably applied it wrong)...

                    @Christian-Ehrlicher Ah, I see. What "libvncserver function" do you mean?

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                    • C Offline
                      C Offline
                      Christian Ehrlicher
                      Lifetime Qt Champion
                      wrote on 6 Dec 2020, 16:09 last edited by
                      #46

                      @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

                      What "libvncserver function" do you mean?

                      You are using libvncserver, not me.

                      Giving up here now.

                      Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                      Visit the Qt Academy at https://academy.qt.io/catalog

                      A 1 Reply Last reply 6 Dec 2020, 21:37
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                      • C Christian Ehrlicher
                        6 Dec 2020, 16:09

                        @abmyii said in Dump QImage raw pixel data into &#x60;std::vector<char>&#x60;:

                        What "libvncserver function" do you mean?

                        You are using libvncserver, not me.

                        Giving up here now.

                        A Offline
                        A Offline
                        abmyii
                        wrote on 6 Dec 2020, 21:37 last edited by abmyii 12 Jun 2020, 21:39
                        #47

                        Going to mark this thread as closed since it seems the issue lies with VNC, not Qt. Thank you all for your suggestions and helping me to isolate the problem.

                        @Christian-Ehrlicher Sorry for stressing you out - I thought you were referring to a specific function for some reason. I really need to slow down when reading and comprehend rather than just having a quick look and then replying which is causing confusion on all sides. Thank you again for your patience and assistance throughout.

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                        • H Offline
                          H Offline
                          heatblazer
                          wrote on 17 Jul 2024, 10:06 last edited by
                          #48

                          Hi I've done something recently I've needed so my approach was:

                          total_len = qimg.width() * qimg.height();
                           std::vector<unsigned int> data;
                           data.reserve(total_len);
                           for(int i=0; i < qimg.height(); i++)
                               for(int j=0; j < qimg.width(); j++){
                                   data.push_back(qimg.pixel(j, i));
                               }
                          

                          I needed it as an rgb pixels in order to do some image manipulations, but I believe you can also take the uchar data.

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