Various image viewers are available under Linux, but if you want to view RAW images, the choice becomes considerably restricted. You are left with veterans like Geeqie [1] and Shotwell [2]. Both offer far more than merely displaying the many different formats – each in their own way.

PhotoQt [3] – a relatively young project – is now trying to gatecrash the veteran party. Its first release, version number 1.2, dates back to January 2015. This article is based on the current 1.5.1 version. Many repositories already contain the software, and the installation does not typically cause any problems. On the homepage, you will find instructions for many distributions. On Ubuntu version 15.04, you can do:

$ sudo apt-add-repository ppa:lumas/photoqt
$ sudo apt update
$ sudo apt install photoqt

to set up PhotoQt from the PPA for the project.

Requirements

If you use an image viewer for a larger number of images, some features are useful or even essential. These in particular include different ways to browse or sort the images. The program needs to be able to evaluate the metadata contained in the images, that is, the Exif tags. Additionally, it should ideally offer an option for including reviews or keywords in the search.

In the case of RAW images, access to embedded thumbnails is a big help, and it accelerates the display in many cases. The RAW formats also contain metadata. It is particularly useful if you also have the option of using the raw data for the preview.

As the PhotoQt developer Lukas Spies states on the homepage, his interests are strongly focused on using the graphics processing unit (GPU). Today, this is often the most powerful processing unit in a computer.

Features

PhotoQt relies on modern technology with a combination of Qt5/QML. The program supports many formats through the use of GraphicsMagick and LibRaw. However it is pretty lame at times, such as when loading the thumbnail. And, PhotoQt only supports images in 32-bit color depth.

The program has functions for rotating, mirroring, and enlarging the pictures. You can directly rename, copy, move, and delete the image files. For many actions, keyboard shortcuts are available (Table 1); you can modify these if necessary and add your own.

The wallpaper function did not work consistently in the test. A slideshow displays a series of images without the need to switch manually. If necessary, the program can add the Exif information for the displayed images.

PhotoQt allows many customizations, more than many other viewers. It caches thumbnails of the loaded images. If you need quick access to the program, you can drop it into the system tray. Some rudimentary options let you control the behavior of the software when launched.

Hands On

By default, PhotoQt launches in full-screen mode and loads the images from the current directory. Alternatively, there is a mode with window decorations, and a several options that let you set the behavior of the program. Thanks to the semitransparent interface, you still see the desktop despite displaying images. This is modern but has no practical value (Figure 1). At the center, animated bars visualize loading the images.

Figure 1: PhotoQt presents itself as simple and functional at startup time. The program's semitransparent interface offers little added value in practice.

By default, there only a few ways to toggle between images. For example, it is not possible to toggle between images with the mouse wheel. However, you can customize the controls to suit your needs. To do so, use the menu that opens up when you move the mouse pointer to the right edge of the screen (Figure 2). Below Settings is a comprehensive dialog with several tabs with which you can configure nearly all the aspects of the program.

Figure 2: The menu and other Windows remain hidden and appear only when you move the mouse pointer to the edges of the screen.

Below Shortcuts, you can map keys and mouse events to functions (Figure 3). To change the images with the mouse wheel, first select the Next image function and then perform the desired action. Then, repeat the whole thing for the Previous image function. Then, select Save changes and close to quit the dialog.

Figure 3: Below hortcuts users can customize the mappings for key and mouse events. You select additional actions in the right column.

If you were looking for a context menu that is quickly accessible with special functions in PhotoQt, you can stop now; this tends to make working with the program unnecessarily complicated. The online guide provides information about the most important settings [4] among other things. However, the document completely ignores the filters.

PhotoQt can display the metadata for images (Figure 4). Essentially the software is limited to the Exif information. The corresponding function still showed some weaknesses with most of the (raw) formats, delivering only a File format is not supported message; however, if necessary, you can display a histogram.

Figure 4: Although the program usually displays the important metadata from the EXIF tags, it exhibits some weaknesses especially with RAW formats.

PhotoQt transfers the images to Gimp for processing – this is probably not the right way to handle RAW images. A RAW converter would be the better choice. However, Gimp does let you automatically call a converter (now including Darktable).

The Viewer uses a bar at the bottom of the window for the thumbnails (Figure 5). However, it only appears when you move the mouse pointer to the bottom. You can use the mouse to browse the images in the bar and click to select the one you want.

Figure 5: The bar with the preview images appears as soon as you move the mouse pointer to the bottom of the screen.

However, the main window does not always show the image under the mouse pointer in the preview bar, but the one that is highlighted in the bar – this is somewhat confusing at first. Caching also apparently doesn't work when displaying RAW images; the result being that image changes can take a very long time.