Usinfg CImg lib with Qt for faster image resizing. Is It worth it to dig?

Question

My project is to batch resize a big number (30000) of small jpeg images (400*300) to 15 different destination sizes (which are smaller than the source).

I first created a multi-threded command line tool based on CImg and libjpg to batch process the images. And is worked as a charm on my 4*2 cores Ubuntu notepad (20 minutes).

Now the client wants a GUI for the tool, with progress bars, estimated time, previews, ... and so on! And want it to work on Windows too. He wants a sort of image manager.

I started porting the pure C++ command line to Qt and its QImage class (QtConcurrent::run for multi-threading), and here the problem : On the same machine it runs 3 times slower on Windows 7 x64 (about 50 minutes)!

So how to use Qt for reading images and CImg for resizeing them? And will it run faster?

No, my client has many websites to promote his products, and each website has its own image sizes. — Yassine Zaroui, Jul 12 '11 at 12:27

score 3 · Answer 1 · answered Jul 16 '11 at 16:19

Here is what I have done :

void convert(QStringList files, QString destDir) {
    foreach (QString file, files) {
      CImg<unsigned char> image(file.toStdString().c_str());
      QString destFile = destDir + QFileInfo(file).fileName();
      image.get_resize(200, 200, -100, -100, 1).save_jpeg(destFile.toStdString().c_str(), 70);
  }
}

instread of :

QImage img;
foreach (QString file, files) {
img.load(file);
QString destFile = destDir + QFileInfo(file).fileName();
img.scaled(200, 200, Qt::IgnoreAspectRatio, Qt::FastTransformation).save(destFile, 0, 70);
}

It runs 2 to 3 times faster

score 2 · Accepted Answer · answered Jul 12 '11 at 11:48

2

If it will run faster is difficult to tell without measuring (though I would guess it is), but it is nonetheless a good idea. Create a interface (API not GUI) for your command line tool, build the gui parts in Qt and call the API of your image resizing backend from it.

This is a good (I would even call it a fundamental principle) design for application development anyway. For such a small project it is sometimes overkill, but since you already have the "backend" part anyways your fine.

answered Jul 12 '11 at 11:48

Fabio Fracassi

3,791
1
18
17

First, As you said, it is a small project and I don't want to manage two different independent projects (API & GUI)! Seceond, the command line is linux only now, I will have to "windowsize" it. And last but not least, I am beginning Qt, I still don't know how to work with external APIs and make it cross-platform. – Yassine Zaroui Jul 12 '11 at 12:33
1

@yzaroui: just turn your current implementation into a library. Then, write a brand new GUI program, which will utilise its functions (and a set of library functions is an API by definition). – vines Jul 12 '11 at 12:42
as @vines says, you do not need to manage them independently, turn them into a library, or just include everything into one project. The separation is purely conceptual. There is nothing special in using neither internal nor external APIs and Qt doesn't change that. – Fabio Fracassi Jul 12 '11 at 12:52

Usinfg CImg lib with Qt for faster image resizing. Is It worth it to dig?

2 Answers2