[Qt-interest] How can I read a large image fastly?

[Thomas Dähling]

Did you take a look at the overload for read that takes a QImage
pointer as parameter?

2010/2/4 Kermit Mei <kermit.mei at gmail.com>:
> Lorenzo Masini wrote:
>> Il giorno gio, 04/02/2010 alle 20.47 +0800, Kermit Mei ha scritto:
>>
>>> Hello community!
>>>
>>>      I run Qt on a slow embedded device, now we want to show some large
>>> image on it, but the speed of loading is too slowly. For example, if we
>>> decode a 6M jpg picture, we always need 100s to show it.
>>>
>>>     Does Qt4 can support that  read  parts of  the  image to make it
>>> shows quickly. For example, the picture is 4000x4000,  in fact we just
>>> need to show 320x240. Can I just show a part of the picture to make it
>>> faster?
>>>
>>> I use QImageReader like this:
>>>
>>> void Browser::paintEvent(QPaintEvent *event) {
>>>     imgReader->setFileName("./test.jpg");
>>>     imgReader->setClipRect(QRect(0,0,320,240));
>>>     imgReader->setScaledSize(QSize(3200,2400));
>>>     qDebug("canRead: %d",imgReader->canRead());
>>>     QPainter p(this);
>>>     p.drawImage(QRect(0,0,320,240),imgReader->read());
>>> }*
>>> **
>>> *But it shows all the picture which is reduced to 320x240, so the speed
>>> is also slow.
>>>
>>> Is there something wrong of the code?
>>>
>>
>> The paintEvent isn't the right place to load an image.
>> It is called every time is performed an update. For example on
>> resizeEvent.
>>
>> I think you have to load the image into another part of your code, cache
>> the resulting image, and then display it into the paintEvent.
>>
>> I don't understand if you need to show a part of the image or the image
>> resized.
>>
>> cheers
>> Lorenzo
>>
> Thanks, Cheers. But in fact, what I want to know is not how to paint it
> fast, I want to know how can I load it more fast.
>
> This paintEvent() is just a test routine, and it just load only one
> time.  In another words, I want to know how to improve the speed of
> "imgReader->read()". And why I can't load just part of it?
>
>
> B.R
> Kermit
>
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