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Thread: Please help - Refreshing a QPixmap repeatedly in a for loop ( ??? )

  1. #1
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    Default Please help - Refreshing a QPixmap repeatedly in a for loop ( ??? )

    Hello Everyone,

    I am manipulating a rectangular array pixel-by-pixel with QImage and setpixel. Next, I convert the QImage to a QPixmap, and display it through a QLabel. This process works pretty well in itself, but, I would like to be able to manipulate the image's pixels over and over, and display the "updated" Qimage (or Qpixmap) file in the label.

    Right now, the QLabel remains white/blank until the for.. loop it is in, finishes. Instead, I'd like to see the changed pixmap after each 'step' within the for loop.

    Any suggestions? Is there a method to successfully output the QImage in my QLabel after each loop through the step loop? The ideal result would be a Qlabel that displays the Qpixmap which, after each time through the for (step<numbstep) loop, the color is slightly modified via adjusting the R G B values. Right now, it only outputs the "final" color. (Code Below):


    Qt Code:
    1. #include "widget.h"
    2. #include "ui_widget.h"
    3. #include <QLabel>
    4. #include <iostream>
    5. #include <QImage>
    6. #include <QPixmap>
    7.  
    8.  
    9. using namespace std;
    10.  
    11.  
    12. Widget::Widget(QWidget *parent) :
    13. QWidget(parent),
    14. ui(new Ui::Widget)
    15. {
    16. ui->setupUi(this);
    17.  
    18. mylabel = new QLabel; //
    19. mylabel->show(); //
    20.  
    21. myimage = QImage(100,100,QImage::Format_RGB32);
    22.  
    23. for(int step=0;step<250;step++)
    24. {
    25. da = 3;db=1;dc=.5;
    26.  
    27. for(int i=0;i<100;i++)
    28. {
    29. for(int j=0;j<100;j++)
    30. {
    31.  
    32. QRgb colorval;
    33. colorval = qRgb(a,b,c);
    34.  
    35. myimage.setPixel(j,i,colorval);
    36. //cout << " Setting " << i << " " << j << endl;
    37. }
    38.  
    39. }
    40. a = a + da;
    41. b = b + db;
    42. c = c + dc;
    43.  
    44. QPixmap mypixmap = QPixmap::fromImage(myimage);
    45.  
    46. mylabel->setPixmap(mypixmap);
    47. mylabel->setScaledContents(true);
    48. mylabel->update();
    49.  
    50. }
    51. }
    52.  
    53. Widget::~Widget()
    54. {
    55. delete ui;
    56. }
    To copy to clipboard, switch view to plain text mode 

  2. #2
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    Default Re: Please help - Refreshing a QPixmap repeatedly in a for loop ( ??? )

    [wiki]Keeping the GUI Responsive[/wiki]
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

    Please ask Qt related questions on the forum and not using private messages or visitor messages.


  3. #3
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    Default Re: Please help - Refreshing a QPixmap repeatedly in a for loop ( ??? )

    Okay, thanks, I took a look at your post, and implemented :
    Qt Code:
    1. QCoreApplication::processEvents();
    To copy to clipboard, switch view to plain text mode 
    after setting my pixmap from my Qimage.

    Now, I can visually notice the pixmap being updated with a new color, rapidly.

    Onto another issue, is there a simple way to delay a step by a given amount of ms or seconds? The pixmap is updated a little too rapidly at this point. It would be nice to be able to control the time the pixmap is displayed.

    Right now the only way I can "trick" the program into slowing down is to perform many more steps in my step loop (~20,000 steps) and output my pixmap every 100 steps or so.

    Thanks,
    Last edited by mkmartin06; 16th June 2011 at 21:08.

  4. #4
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    Default Re: Please help - Refreshing a QPixmap repeatedly in a for loop ( ??? )

    Read the article again. Implement the "step by step" approach, just adjust the timer.
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

    Please ask Qt related questions on the forum and not using private messages or visitor messages.


  5. #5
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    Default Re: Please help - Refreshing a QPixmap repeatedly in a for loop ( ??? )

    My example code that I provided in my first post is a greatly simplified case of what actually occurs in my for loop. When I try to implement
    Qt Code:
    1. QCoreApplication::processEvents();
    To copy to clipboard, switch view to plain text mode 
    in my program that does a lot of calculations within the loop, the program only displays the final step's pixmap in the QLabel. The intermediate pixmaps which I need displayed never show up. Any suggestions?
    Last edited by mkmartin06; 16th June 2011 at 21:58. Reason: updated contents

  6. #6
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    Default Re: Please help - Refreshing a QPixmap repeatedly in a for loop ( ??? )

    Quote Originally Posted by mkmartin06 View Post
    Any suggestions?
    Yes. Don't use processEvents() and redesign your code. Read what the article says on using processEvents(), especially the part starting with "This approach has significant drawbacks". And then read my previous post again and follow the guideline contained in it.
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

    Please ask Qt related questions on the forum and not using private messages or visitor messages.


  7. #7
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    Default Re: Please help - Refreshing a QPixmap repeatedly in a for loop ( ??? )

    Ok. That is what I did. It works now. Have a nice day.

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