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QImage, parsing raw char from image to get alpha?

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  • M mranger90
    11 May 2019, 19:34

    What is temp[] ? is it a byte array or a pixel array ?

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    Dariusz
    wrote on 12 May 2019, 01:03 last edited by
    #3

    @mranger90 Oh sorry, missed that!

    uchar *temp = (uchar *) img.constBits();

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      Christian Ehrlicher
      Lifetime Qt Champion
      wrote on 12 May 2019, 07:58 last edited by
      #4

      @Dariusz said in QImage, parsing raw char from image to get alpha?:

      run but sadly no luck...

      What does this mean?

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        Dariusz
        wrote on 12 May 2019, 10:15 last edited by
        #5

        That this

        uchar *temp = (uchar *) img.constBits();
        
        ...
                    pixelOffset = imageWidth * he + (wi * 4);
                    alpha = temp[pixelOffset];//qAlpha(img.pixel(wi, he)); /// img.pixel work but slow due to different access function
        

        does not return alpha int. as far as I understand img.constBits() return 1d array, 4 values equal to a color, esentially A-R-G-B, so every 4th item is the alpha value and pixelOffset uses/calculates indexOffset in to the 1d array from 2d array of height*width of image.

        The loop function in 1st post shows it.

        I don't understand why the value I get from the loop above is not alpha.

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          Christian Ehrlicher
          Lifetime Qt Champion
          wrote on 12 May 2019, 10:48 last edited by
          #6

          Your offset calculation is wrong, every pixel has 4 bytes.

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          D 1 Reply Last reply 12 May 2019, 11:33
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          • C Christian Ehrlicher
            12 May 2019, 10:48

            Your offset calculation is wrong, every pixel has 4 bytes.

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            Dariusz
            wrote on 12 May 2019, 11:33 last edited by
            #7

            @Christian-Ehrlicher said in QImage, parsing raw char from image to get alpha?:

            Your offset calculation is wrong, every pixel has 4 bytes.

            Hmm did I not address that in (wi*4) ? You say bytes but isn't uchar *temp an array of ints? I'm lost o.o

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              Christian Ehrlicher
              Lifetime Qt Champion
              wrote on 12 May 2019, 12:03 last edited by
              #8

              @Dariusz said in QImage, parsing raw char from image to get alpha?:

              uchar *

              I don't see int here... only (unsigned) char

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              D 1 Reply Last reply 12 May 2019, 12:24
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              • C Christian Ehrlicher
                12 May 2019, 12:03

                @Dariusz said in QImage, parsing raw char from image to get alpha?:

                uchar *

                I don't see int here... only (unsigned) char

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                Dariusz
                wrote on 12 May 2019, 12:24 last edited by Dariusz 5 Dec 2019, 12:25
                #9

                @Christian-Ehrlicher Yeah, sorry just used to ints.

                So the:
                uchar *temp = (uchar *) img.constBits();
                Should be something like
                temp = [alpha, red, green, blue, alpha, red, green, blue, alpha, red, green, blue, alpha, red, green, blue, alpha, red, green, blue ]
                Each value being char. 0-255 - not quite sure how to deal with 32 bit+ arrays tho... are they then int *temp ? constBits return only chars... I'm lost I never dig in to 16/32+ image formats...

                so if I were to do temp[0] / temp[4],temp[8] then they all should be alphas?

                So what am I missing in
                imageWidth * imageHeightIndex + (imageWidthIndex *4)
                Which if gives correct numbers would result for 300x400 image (w x h)

                300 * 0 + (0*4) = 0
                300 * 0 + (1*4) = 4
                300 * 0 + (2*4) = 8
                

                etc etc so I'm getting correct index to retrieve alpha ?

                What am I missing ? o.O

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                  Christian Ehrlicher
                  Lifetime Qt Champion
                  wrote on 12 May 2019, 12:26 last edited by
                  #10

                  @Dariusz said in QImage, parsing raw char from image to get alpha?:

                  What am I missing ? o.O

                  Simple math I would say:
                  4 bytes per pixel, row is 300 pixels = 1200 bytes per row

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                  D 1 Reply Last reply 12 May 2019, 12:29
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                  • C Christian Ehrlicher
                    12 May 2019, 12:26

                    @Dariusz said in QImage, parsing raw char from image to get alpha?:

                    What am I missing ? o.O

                    Simple math I would say:
                    4 bytes per pixel, row is 300 pixels = 1200 bytes per row

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                    Dariusz
                    wrote on 12 May 2019, 12:29 last edited by
                    #11

                    @Christian-Ehrlicher Hmm char is 2 byte big... so do I offset by 8 instead and add up 2 items together?

                    int alpha = temp[0]<<temp[1] ?

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                      Christian Ehrlicher
                      Lifetime Qt Champion
                      wrote on 12 May 2019, 12:32 last edited by
                      #12

                      @Dariusz said in QImage, parsing raw char from image to get alpha?:

                      char is 2 byte big

                      Ok, now I'm giving up...

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                      D 1 Reply Last reply 12 May 2019, 12:33
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                      • C Christian Ehrlicher
                        12 May 2019, 12:32

                        @Dariusz said in QImage, parsing raw char from image to get alpha?:

                        char is 2 byte big

                        Ok, now I'm giving up...

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                        Dariusz
                        wrote on 12 May 2019, 12:33 last edited by
                        #13

                        @Christian-Ehrlicher said in QImage, parsing raw char from image to get alpha?:

                        @Dariusz said in QImage, parsing raw char from image to get alpha?:

                        char is 2 byte big

                        Ok, now I'm giving up...

                        Eh crap its short thats 2 byte big... char is 1 byte... ehhhhhhhh I forgot about shorts :/. Ok so esentially I have to grab 4 chars and add them up to int?

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                          SGaist
                          Lifetime Qt Champion
                          wrote on 12 May 2019, 18:58 last edited by
                          #14

                          Hi,

                          Why do you want to combine them back into and int ?

                          If you have a ARGB image, basically you have:

                          uchar *start = (uchar *) img.constBits();
                          uchar *alpha = start + 0;
                          uchar *red = start + 1;
                          uchar *green = start + 2;
                          uchar *blue = start + 3;
                          
                          // First alpha value:
                          uint8 alphaValue = *alpha;
                          // Second value of alpha
                          alphaValue = *(alpha + 4)
                          

                          if you want to sum them, then make alphaValue an int and that's all.

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