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Thread: Maintaining Aspect Ratio of Image in Qt

  1. #1
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    Post Maintaining Aspect Ratio of Image in Qt

    Dear All,

    Is there any way for maintaining the aspect ratio for the Image being taken from the db.
    What exactly I am doing is I am populating db with the image (the datatype for the image is BLOB in DB) & later on I am displaying that image on the reports.

    The code I have used for displaying the Image on the reports is below,

    Qt Code:
    1. QPainter *reportPainter;
    2. QByteArray myImage;
    3. p.loadFromData(myImage); //myImage = Acquired from the db
    4. p.scaled(120,60,Qt::KeepAspectRatio); //This is not required as x & y co-ordinates are fix
    5. reportPainter->drawImage(QRect(60,0,120,60),p.toImage()); //Adding image in QPainter with rectangle size kept fixed.
    To copy to clipboard, switch view to plain text mode 

    In Line of Code no. 4 : Image is scaled to fix width & height (viz. not the requirement, as it should be calculated based on the acquired image height & width)
    In Line of Code no. 5 : Similarly, the Image is drawn in the fix rectangle.

    How to get the image's height & width present in the db? or Is there any other way to do so?
    And yes I want to maintain the aspect ratio of the image in the report acquired from the database.

    Thanks in Advance.
    Regards,
    Rohit K

  2. #2
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    Default Re: Maintaining Aspect Ratio of Image in Qt

    Line 4 loads them image, it does not scale it at all.
    Line 5 is a very expensive NOOP, you are creating a scaled version of the image and then ignore it (return value not used at all).
    In Line 6 is another unneeded operation, i.e. toImage(). If you want to work with QImage, use QImage instead of QPixmap. Right now you convert from image to pixmap (internally in line 4) and then back again in line 6.

    Cheers,
    _

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    Default Re: Maintaining Aspect Ratio of Image in Qt

    Quote Originally Posted by anda_skoa View Post
    Line 4 loads them image, it does not scale it at all.
    Yeah I know this.
    Line 5 is a very expensive NOOP, you are creating a scaled version of the image and then ignore it (return value not used at all).
    What is the right way to do it?
    Does storing the returned value really helps?
    In Line 6 is another unneeded operation, i.e. toImage(). If you want to work with QImage, use QImage instead of QPixmap. Right now you convert from image to pixmap (internally in line 4) and then back again in line 6.
    I have used QByteArray to retrieve image from the database (present as BLOB in db) hence, i am not converting the image i am loading the data (present in QByteArray datatype) into the QPixMap.
    I have used QPixMap instead of the QImage because i want to maintain the aspect ratio , as it cannot be done using the QImage (I have tried it before & Image was coming distorted using QImage) & hence, again converting the QPixMap to QImage to display it on the QPainter .

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    Default Re: Maintaining Aspect Ratio of Image in Qt

    Quote Originally Posted by rohitkk View Post
    Does storing the returned value really helps?
    It helps if you want to use the scaled image. If your goal is to use CPU time then you can of course ignore the result.
    However, from your general description it sounds like you want the scaled image, not the original.

    Quote Originally Posted by rohitkk View Post
    I have used QByteArray to retrieve image from the database (present as BLOB in db) hence, i am not converting the image i am loading the data (present in QByteArray datatype) into the QPixMap.
    Loading is always done through QImage, so loading into QPixmap loads into a QImage and then converts it to a QPixmap.

    Quote Originally Posted by rohitkk View Post
    I have used QPixMap instead of the QImage because i want to maintain the aspect ratio
    What?
    Qimage::scaled() takes the same arguments.

    Cheers,
    _

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    Default Re: Maintaining Aspect Ratio of Image in Qt

    Ok .
    I have modified my Code.
    Qt Code:
    1. QImage img;
    2. if (img.loadFromData(schoolLogo)) {
    3. reportPainter->drawImage(QRect(60,0,120,60),img.scaled(img.size(),Qt::KeepAspectRatio));
    4. }
    To copy to clipboard, switch view to plain text mode 

    Please see the screenshot for ,
    screen 1 : my actual Image (.jpg)images (1).jpg
    screen 2: In my pdf that I have generated containing that picture Untitled.jpg

    Hence, in screen 1 viz the actual image , have different height & width.
    whereas in screen 2 viz the image on the pdf , where the aspect ratio is not at all maintained.

    Please help me out to maintain the aspect ratio of the image & why isn't the Qt::KeepAspectRatio not working as desired?

    Thanks in advanced.

    Regards,
    Rohit

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    Default Re: Maintaining Aspect Ratio of Image in Qt

    What do you think this part of the code does?
    Qt Code:
    1. img.scaled(img.size(),Qt::KeepAspectRatio)
    To copy to clipboard, switch view to plain text mode 
    I.e. in words, what do you expect as the return value of img.scaled()?


    Cheers,
    _

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    Default Re: Maintaining Aspect Ratio of Image in Qt

    Try this as it worked for me:
    Qt Code:
    1. //viewWidth,viewHeight is the size of the image in the aspect ratio needed.
    2. image = im.scaled(viewWidth,viewHeight,Qt::IgnoreAspectRatio,Qt::SmoothTransformation);
    To copy to clipboard, switch view to plain text mode 

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    Default Re: Maintaining Aspect Ratio of Image in Qt

    What do you think this part of the code does?
    Qt Code:
    1. img.scaled(img.size(),Qt::KeepAspectRatio)
    To copy to clipboard, switch view to plain text mode 
    What is the correct way to do it?
    As you know I have fixed size rectangle,
    Qt Code:
    1. QRect(60,0,120,60)
    To copy to clipboard, switch view to plain text mode 
    And I want to display the image in this rectangle with it's aspect ratio maintained.

    I.e. in words, what do you expect as the return value of img.scaled()?
    A scaled Image . Right?

  9. #9
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    Default Re: Maintaining Aspect Ratio of Image in Qt

    Quote Originally Posted by rohitkk View Post
    What is the correct way to do it?
    Correct depends on the desired outcome.
    Your code is correct if you want an expensive copy of the original image.
    Quote Originally Posted by rohitkk View Post
    As you know I have fixed size rectangle,
    Qt Code:
    1. QRect(60,0,120,60)
    To copy to clipboard, switch view to plain text mode 
    Which is why I was wondering why you are not using that size instead of the image's own size.

    Quote Originally Posted by rohitkk View Post
    A scaled Image . Right?
    Yes, scaled to exactly the same size.

    I have troubles understanding why you are not passing the size you want to scale to.


    Cheers,
    _

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    Default Re: Maintaining Aspect Ratio of Image in Qt

    Quote Originally Posted by kapoorsudhish View Post
    Try this as it worked for me:
    Qt Code:
    1. //viewWidth,viewHeight is the size of the image in the aspect ratio needed.
    2. image = im.scaled(viewWidth,viewHeight,Qt::IgnoreAspectRatio,Qt::SmoothTransformation);
    To copy to clipboard, switch view to plain text mode 
    What will be view width & view height?
    Is it the width & height of the fixed size rectangle in which image will be displayed? or width & height of the image?


    Added after 10 minutes:


    @anda_skoa
    Up till now u must have understand my problem ...
    plz provide me the solution/...

    Regards,
    Rohit

    Is there any way to maintain aspect ratio ? or I have to calculate the aspect ratio?
    Last edited by rohitkk; 9th May 2014 at 12:24.

  11. #11
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    Default Re: Maintaining Aspect Ratio of Image in Qt

    Quote Originally Posted by rohitkk View Post
    Up till now u must have understand my problem ...
    plz provide me the solution/...
    Come on, you are a software developer

    QImage::scaled() needs two things from you
    * a target size hint
    * an aspect hint

    You have the latter correct: Qt::KeepAspectRatio

    You have a target rectangle: QRect(60,0,120,60)

    Go see if you can find a method that results in the size of a QRect.

    Cheers,
    _

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    Default Re: Maintaining Aspect Ratio of Image in Qt

    The viewWidth/viewHeight are the width and height of the display area rectangle (fixed size rectangle) where the image needs to be displayed. By passing it to the image scaled function the input image gets scaled to the width and height of the display rectangle area.

    Also to maintain aspect ratio my suggestion is to adjust the aspect ratio of the display rectangle and not of the image.

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