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

[Kermit Mei]

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