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

[Thomas Dähling]

To understand you correctly:

You have a picture that is 4k by 4k in it's resolution, and you only
want to display the first 320x240 pixels of it without reading in the
4000x4000 pixel image first?

In that scenario I guess the problem here is that QImageReader cannot
know when the desired clipRect area is read from the device it uses -
simply because that depends a lot upon the format the image is stored
in and thus has to read in the whole image first. You might have to
write your own reader class if you want to improve this.

/Thomas

2010/2/4 Kermit Mei <kermit.mei at gmail.com>:
> Thomas Dähling wrote:
>>
>> Did you take a look at the overload for read that takes a QImage
>> pointer as parameter?
>>
>
> Sorry, I can't understand you. Do you mean make the QImage as a buffer?
> I fact, I want it load more faster at the first time:)
>
> Thanks
> B.R
> Kermit
>
>> 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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>>>
>>
>>
>
>