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Save QImage from BYTE buffer segfaults ?

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  • S SGaist
    6 Jul 2020, 20:10

    Can you get the stack trace of the crash ?

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    R-P-H
    wrote on 6 Jul 2020, 21:26 last edited by
    #15

    @SGaist said in Save QImage from BYTE buffer segfaults ?:

    Can you get the stack trace of the crash ?

    I'm testing the application on another machine using the executable. I don't think I'd be able to get it without QT or another IDE right ?

    1 Reply Last reply
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    • S Offline
      S Offline
      SGaist
      Lifetime Qt Champion
      wrote on 7 Jul 2020, 20:48 last edited by
      #16

      Windows machine ?

      Interested in AI ? www.idiap.ch
      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

      R 1 Reply Last reply 7 Jul 2020, 21:59
      0
      • S SGaist
        7 Jul 2020, 20:48

        Windows machine ?

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        R-P-H
        wrote on 7 Jul 2020, 21:59 last edited by
        #17

        @SGaist said in Save QImage from BYTE buffer segfaults ?:

        Windows machine ?

        Yes it is. Windows 7 and also Windows 10.

        1 Reply Last reply
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        • M Offline
          M Offline
          mranger90
          wrote on 8 Jul 2020, 13:44 last edited by
          #18

          try:
          BYTE *buf = new BYTE[imWidth * imHeight];

          R 1 Reply Last reply 8 Jul 2020, 16:26
          1
          • M mranger90
            8 Jul 2020, 13:44

            try:
            BYTE *buf = new BYTE[imWidth * imHeight];

            R Offline
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            R-P-H
            wrote on 8 Jul 2020, 16:26 last edited by
            #19

            @mranger90 said in Save QImage from BYTE buffer segfaults ?:

            try:
            BYTE *buf = new BYTE[imWidth * imHeight];

            Hi, it is not crashing anymore, however the image seems to be coming out completely black. I will confirm tomorrow if the image is supposed to be completely black or not...

            1 Reply Last reply
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            • J Offline
              J Offline
              Jarek B
              wrote on 8 Jul 2020, 16:32 last edited by
              #20

              From the documentation
              https://doc.qt.io/qt-5/qimage.html#QImage-4
              data must be 32-bit aligned, and each scanline of data in the image must also be 32-bit aligned.

              So (as far as I understand) that is not enough to have memory buffer aligned to 32 bit address, also each line have to be aligned to 32 bit address. So if you have image that is 260x300 pixels, a whole line will take 288 bytes. So for whole image you should have 288*300 bytes which is more than you have provided.

              R 1 Reply Last reply 8 Jul 2020, 16:48
              2
              • J Jarek B
                8 Jul 2020, 16:32

                From the documentation
                https://doc.qt.io/qt-5/qimage.html#QImage-4
                data must be 32-bit aligned, and each scanline of data in the image must also be 32-bit aligned.

                So (as far as I understand) that is not enough to have memory buffer aligned to 32 bit address, also each line have to be aligned to 32 bit address. So if you have image that is 260x300 pixels, a whole line will take 288 bytes. So for whole image you should have 288*300 bytes which is more than you have provided.

                R Offline
                R Offline
                R-P-H
                wrote on 8 Jul 2020, 16:48 last edited by
                #21

                @Jarek-B said in Save QImage from BYTE buffer segfaults ?:

                From the documentation
                https://doc.qt.io/qt-5/qimage.html#QImage-4
                data must be 32-bit aligned, and each scanline of data in the image must also be 32-bit aligned.

                So (as far as I understand) that is not enough to have memory buffer aligned to 32 bit address, also each line have to be aligned to 32 bit address. So if you have image that is 260x300 pixels, a whole line will take 288 bytes. So for whole image you should have 288*300 bytes which is more than you have provided.

                Hi, interesting...I have not seen any examples like that. How are you arriving at 288 bytes ? Are you suggesting I make the buffer 288x300 bytes instead ?

                M J 2 Replies Last reply 8 Jul 2020, 17:00
                0
                • R R-P-H
                  8 Jul 2020, 16:48

                  @Jarek-B said in Save QImage from BYTE buffer segfaults ?:

                  From the documentation
                  https://doc.qt.io/qt-5/qimage.html#QImage-4
                  data must be 32-bit aligned, and each scanline of data in the image must also be 32-bit aligned.

                  So (as far as I understand) that is not enough to have memory buffer aligned to 32 bit address, also each line have to be aligned to 32 bit address. So if you have image that is 260x300 pixels, a whole line will take 288 bytes. So for whole image you should have 288*300 bytes which is more than you have provided.

                  Hi, interesting...I have not seen any examples like that. How are you arriving at 288 bytes ? Are you suggesting I make the buffer 288x300 bytes instead ?

                  M Offline
                  M Offline
                  mranger90
                  wrote on 8 Jul 2020, 17:00 last edited by mranger90 7 Aug 2020, 17:01
                  #22

                  @R-P-H Of course it's black, the data never gets initialized.
                  Also, the reason why it doesn't crash is because:
                  BYTE *buf = new BYTE(x * y); // allocates a single byte, initialized to the value of (x * y)
                  BYTE *buf = new BYTE[x * y]; // allocates an array of bytes whose size is x * y

                  R 2 Replies Last reply 8 Jul 2020, 17:08
                  0
                  • M mranger90
                    8 Jul 2020, 17:00

                    @R-P-H Of course it's black, the data never gets initialized.
                    Also, the reason why it doesn't crash is because:
                    BYTE *buf = new BYTE(x * y); // allocates a single byte, initialized to the value of (x * y)
                    BYTE *buf = new BYTE[x * y]; // allocates an array of bytes whose size is x * y

                    R Offline
                    R Offline
                    R-P-H
                    wrote on 8 Jul 2020, 17:08 last edited by
                    #23

                    @mranger90 said in Save QImage from BYTE buffer segfaults ?:

                    @R-P-H Of course it's black, the data never gets initialized.
                    Also, the reason why it doesn't crash is because:
                    BYTE *buf = new BYTE(x * y); // allocates a single byte, initialized to the value of (x * y)
                    BYTE *buf = new BYTE[x * y]; // allocates an array of bytes whose size is x * y

                    I don't quite understand what you're saying to be honest. Where are you saying the issue is ?

                    M 1 Reply Last reply 8 Jul 2020, 17:27
                    0
                    • R R-P-H
                      8 Jul 2020, 17:08

                      @mranger90 said in Save QImage from BYTE buffer segfaults ?:

                      @R-P-H Of course it's black, the data never gets initialized.
                      Also, the reason why it doesn't crash is because:
                      BYTE *buf = new BYTE(x * y); // allocates a single byte, initialized to the value of (x * y)
                      BYTE *buf = new BYTE[x * y]; // allocates an array of bytes whose size is x * y

                      I don't quite understand what you're saying to be honest. Where are you saying the issue is ?

                      M Offline
                      M Offline
                      mranger90
                      wrote on 8 Jul 2020, 17:27 last edited by
                      #24

                      @R-P-H here is the amended code

                      // allocates a buffer of imWidth * imHeight bytes, which  depending on the compiler 
                      // settings will either be initialized to zero, or may contain garbage
                      BYTE *buf = new BYTE[imWidth * imHeight];
                      
                      // creates a QImage based on the data in the buffer, 
                      QImage img(buf, imWidth, imHeight, QImage::Format_Grayscale8);
                      
                      // saves the data
                      img.save("image.bmp", "BMP");
                      delete [] buf;
                      
                      

                      The point is that you never put actual image data into the buffer.

                      R 1 Reply Last reply 8 Jul 2020, 17:56
                      0
                      • M mranger90
                        8 Jul 2020, 17:27

                        @R-P-H here is the amended code

                        // allocates a buffer of imWidth * imHeight bytes, which  depending on the compiler 
                        // settings will either be initialized to zero, or may contain garbage
                        BYTE *buf = new BYTE[imWidth * imHeight];
                        
                        // creates a QImage based on the data in the buffer, 
                        QImage img(buf, imWidth, imHeight, QImage::Format_Grayscale8);
                        
                        // saves the data
                        img.save("image.bmp", "BMP");
                        delete [] buf;
                        
                        

                        The point is that you never put actual image data into the buffer.

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                        R-P-H
                        wrote on 8 Jul 2020, 17:56 last edited by R-P-H 7 Aug 2020, 17:59
                        #25

                        @mranger90 said in Save QImage from BYTE buffer segfaults ?:

                        @R-P-H here is the amended code

                        // allocates a buffer of imWidth * imHeight bytes, which  depending on the compiler 
                        // settings will either be initialized to zero, or may contain garbage
                        BYTE *buf = new BYTE[imWidth * imHeight];
                        
                        // creates a QImage based on the data in the buffer, 
                        QImage img(buf, imWidth, imHeight, QImage::Format_Grayscale8);
                        
                        // saves the data
                        img.save("image.bmp", "BMP");
                        delete [] buf;
                        
                        

                        The point is that you never put actual image data into the buffer.

                        This is already what I did according to your first post. The buffer IS populated, I just didn't show that API call. I will modify my original post to make this clear...

                        So the result is a completely black image. This may be normal but I will have to check with the API provider.

                        EDIT: I have edited my original post.

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                        0
                        • S Offline
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                          SGaist
                          Lifetime Qt Champion
                          wrote on 8 Jul 2020, 20:48 last edited by
                          #26

                          Is that some private library you can't share ? Or would it be possible to have more information about that API ?

                          Interested in AI ? www.idiap.ch
                          Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                          R 1 Reply Last reply 9 Jul 2020, 09:46
                          0
                          • S SGaist
                            8 Jul 2020, 20:48

                            Is that some private library you can't share ? Or would it be possible to have more information about that API ?

                            R Offline
                            R Offline
                            R-P-H
                            wrote on 9 Jul 2020, 09:46 last edited by
                            #27

                            @SGaist said in Save QImage from BYTE buffer segfaults ?:

                            Is that some private library you can't share ? Or would it be possible to have more information about that API ?

                            Your assumption is correct. There really aren't any additional details on using this API function. It simply takes as input a reference to the buffer and a unique id of the capturing device. It then populates the buffer in place. The example in the documentation initializes the buffer the same way as shown in my original post.

                            If we are sure the error is on the API side, then they will need to fix that.

                            I made the assumption that the error was in saving the image to disk.

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                            • S Offline
                              S Offline
                              SGaist
                              Lifetime Qt Champion
                              wrote on 9 Jul 2020, 11:15 last edited by
                              #28

                              Then are you sure their BYTE type is not something custom ? Do they also show how to access the data ?

                              Is it the Windows BYTE type ?

                              Interested in AI ? www.idiap.ch
                              Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                              R 1 Reply Last reply 9 Jul 2020, 15:40
                              0
                              • R R-P-H
                                8 Jul 2020, 16:48

                                @Jarek-B said in Save QImage from BYTE buffer segfaults ?:

                                From the documentation
                                https://doc.qt.io/qt-5/qimage.html#QImage-4
                                data must be 32-bit aligned, and each scanline of data in the image must also be 32-bit aligned.

                                So (as far as I understand) that is not enough to have memory buffer aligned to 32 bit address, also each line have to be aligned to 32 bit address. So if you have image that is 260x300 pixels, a whole line will take 288 bytes. So for whole image you should have 288*300 bytes which is more than you have provided.

                                Hi, interesting...I have not seen any examples like that. How are you arriving at 288 bytes ? Are you suggesting I make the buffer 288x300 bytes instead ?

                                J Offline
                                J Offline
                                Jarek B
                                wrote on 9 Jul 2020, 11:51 last edited by Jarek B 7 Sept 2020, 11:52
                                #29

                                @R-P-H

                                Just round up space that you need for one line

                                280 / 32 = 8.75
                                9 * 32 = 288

                                R 1 Reply Last reply 9 Jul 2020, 15:37
                                0
                                • J Jarek B
                                  9 Jul 2020, 11:51

                                  @R-P-H

                                  Just round up space that you need for one line

                                  280 / 32 = 8.75
                                  9 * 32 = 288

                                  R Offline
                                  R Offline
                                  R-P-H
                                  wrote on 9 Jul 2020, 15:37 last edited by
                                  #30

                                  @Jarek-B said in Save QImage from BYTE buffer segfaults ?:

                                  @R-P-H

                                  Just round up space that you need for one line

                                  280 / 32 = 8.75
                                  9 * 32 = 288

                                  I tried: BYTE *buf = new BYTE(288 * 300); Result is exactly the same. Still crashes on occasion.

                                  1 Reply Last reply
                                  0
                                  • S SGaist
                                    9 Jul 2020, 11:15

                                    Then are you sure their BYTE type is not something custom ? Do they also show how to access the data ?

                                    Is it the Windows BYTE type ?

                                    R Offline
                                    R Offline
                                    R-P-H
                                    wrote on 9 Jul 2020, 15:40 last edited by
                                    #31

                                    @SGaist said in Save QImage from BYTE buffer segfaults ?:

                                    Then are you sure their BYTE type is not something custom ? Do they also show how to access the data ?

                                    Is it the Windows BYTE type ?

                                    They do not show how to access the data afterwards.

                                    It's defined as typedef unsigned char BYTE; from windows.h.

                                    1 Reply Last reply
                                    0
                                    • M mranger90
                                      8 Jul 2020, 17:00

                                      @R-P-H Of course it's black, the data never gets initialized.
                                      Also, the reason why it doesn't crash is because:
                                      BYTE *buf = new BYTE(x * y); // allocates a single byte, initialized to the value of (x * y)
                                      BYTE *buf = new BYTE[x * y]; // allocates an array of bytes whose size is x * y

                                      R Offline
                                      R Offline
                                      R-P-H
                                      wrote on 9 Jul 2020, 15:41 last edited by
                                      #32

                                      @mranger90 said in Save QImage from BYTE buffer segfaults ?:

                                      @R-P-H Of course it's black, the data never gets initialized.
                                      Also, the reason why it doesn't crash is because:
                                      BYTE *buf = new BYTE(x * y); // allocates a single byte, initialized to the value of (x * y)
                                      BYTE *buf = new BYTE[x * y]; // allocates an array of bytes whose size is x * y

                                      The image is always black and it's not supposed to be, so this doesn't work.

                                      1 Reply Last reply
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                                      • hskoglundH Offline
                                        hskoglundH Offline
                                        hskoglund
                                        wrote on 9 Jul 2020, 15:44 last edited by hskoglund 7 Sept 2020, 15:51
                                        #33

                                        Hi, you could try explicitly specify the number of bytes per line:
                                        Edit: also, it could be that your deleting the buffer before the QImage is deleted:

                                        BYTE *buf = new BYTE(imWidth * imHeight);
                                        //Populate buf with data using API call here (not shown)
                                        {
                                            QImage img(buf, imWidth, imHeight, imWidth, QImage::Format_Grayscale8);
                                            img.save("image.bmp", "BMP");
                                        }
                                        delete [] buf;
                                        
                                        R 1 Reply Last reply 9 Jul 2020, 16:20
                                        0
                                        • hskoglundH hskoglund
                                          9 Jul 2020, 15:44

                                          Hi, you could try explicitly specify the number of bytes per line:
                                          Edit: also, it could be that your deleting the buffer before the QImage is deleted:

                                          BYTE *buf = new BYTE(imWidth * imHeight);
                                          //Populate buf with data using API call here (not shown)
                                          {
                                              QImage img(buf, imWidth, imHeight, imWidth, QImage::Format_Grayscale8);
                                              img.save("image.bmp", "BMP");
                                          }
                                          delete [] buf;
                                          
                                          R Offline
                                          R Offline
                                          R-P-H
                                          wrote on 9 Jul 2020, 16:20 last edited by
                                          #34

                                          @hskoglund said in Save QImage from BYTE buffer segfaults ?:

                                          Hi, you could try explicitly specify the number of bytes per line:
                                          Edit: also, it could be that your deleting the buffer before the QImage is deleted:

                                          BYTE *buf = new BYTE(imWidth * imHeight);
                                          //Populate buf with data using API call here (not shown)
                                          {
                                              QImage img(buf, imWidth, imHeight, imWidth, QImage::Format_Grayscale8);
                                              img.save("image.bmp", "BMP");
                                          }
                                          delete [] buf;
                                          

                                          Hi, thanks for the suggestion. I tried it and result is the same...still crashes on occasion.

                                          I also tried with removing the delete [] buf line and still the same.

                                          B 1 Reply Last reply 9 Jul 2020, 16:35
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