QImageWriter and Writing JPEGs

If you are using the QImageWriter class in Qt to write JPEG files, you may have noticed that it creates large files.

For example, saving out a capture of a window in my app resulted in a 332 kB file. If I run that through ImageOptim [mentioned previously], this is reduced to 266 kB. Huh? What’s happening here?

Digging into the code, I looked at what jpegtran [which is what ImageOptim is running] did to reduce file size. It turns out there are two options you can turn on in libjpeg to reduce file size when writing out JPEGs: optimize and progressive scan. These are both lossless operations, so by turning both of these options on we can reduce the size of the image files without sacrificing any quality.

If I turn on just the optimize option, the file size reduces to 287 kB. Turning both optimize and progressive on reduces the files size to… 266 kB – the same as ImageOptim.

I should also note that writing PNGs using QImageWriter is almost useless given the size of the files it produces [at 100% quality]. The same window capture I mention above results in a 6.6 MB PNG! This can be reduced to a much more manageable 242 kB if you run it through ImageOptim. The problem is, unlike JPEG, the time to optimize a PNG is not trivial, so even if there are switches to do something similar, the time it takes to optimize would result in long delays writing out the PNG file. I haven’t fully investigated this, so there may yet be a solution.

To handle the JPEG issue, I filed a Qt report and patch: Support additional JPEG write options: ‘optimize’ and ‘progressive’ [QTBUG-20075].

The patch modifies the following files:

  • src/gui/image/qimageiohandler.cpp
  • src/gui/image/qimageiohandler.h
  • src/gui/image/qimagewriter.cpp
  • src/gui/image/qimagewriter.h
  • src/gui/image/qjpeghandler.cpp

It adds two options to the QImageWriter class: ImageOption::Optimize and ImageOption::Progressive. It is used like this:

Magic! Smaller JPEG files for free. I hope this patch will eventually make its way into the release version of Qt.

I can’t see any reason why the optimize option within libjpeg defaults to false in the first place – any ideas?

(19 Jan 2015): I submitted a patch for this which has been merged into the Qt codebase. This fix will be part of Qt 5.5.

ImageOptim

ImageOptim is a cool tool by Kornel Lesiński for Mac OS X used to optimize images. Why would I want to do that you ask? It reduces the size of images so they take up less disk space. This means if you are working on a website, it will load a little faster for your users. If you are working on games, it can reduce the amount of data being sent to the GPU which can speed up the loading of textures [make sure you are using lossless compression of course].

ImageOptim is a front-end to several image optimizing tools including AdvPNG, OptiPNG, Pngcrush, JpegOptim, jpegtran, and Gifsicle. It works on PNG, JPG, and GIF images.

ImageOptim Example

ImageOptim Example

Photoshop is especially bad at creating small files when it saves them. It’s incredible the number of images on the web that can be optimized to save everyone time and money. Anyone running a website should consider using ImageOptim or a tool like it on their images.

QSignalMapper Example

Qt is a great development framework. I’ve been using it for years and I’m still discovering new & improved ways to do things. For example, I had a rather clunky bit of code to put menu items in the help menu which would open up various pages on my website.

myMainWindow.h

myMainWindow.cpp

Wow. That’s chunky code. Any additional menu items required a declaration of another one line function which of course meant modifying the header and the cpp file. Messy.

Then I discovered QSignalMapper which let me tie a QString to a given QAction so I would only need one slot to handle any URL opening.

Continue reading

Hrm.

Well…  Look at that.  Yet another vanity site.

Nothing to see here.  Please move on.

Actually, this site is for me to dump little code snippets, cool tools relating to software development, or anything else I find interesting for future reference or for the edification of the masses if they choose to stop by for a virtual pint.