## This time, I used a QPainter object in a very ordinary way, as would be done in genuine GUI-building.

One project which I have been posting about has been, my ongoing effort to teach myself how to use the Qt5 GUI Library, which professional programmers do use in order to design Graphical User Interfaces – GUIs – for applications / programs. In fact, in a preceding posting, I wrote that I had discovered a deliberately unusual way to use one of the object classes in Qt 5.7.1, so that a ‘QImage’ object could be rendered to the screen of a computer, without first having to be transformed into a ‘QPixmap’ object. The ordinary thing to do would be, to transform it thus, first.

That trick can increase performance if two situations are met:

• The graphical information could have started out as a bitmapped image, And
• That image could have been unusually large, so that to have to buffer the entire image in its uncompressed form several times, could pose a considerable drain on the amount of memory that even modern computers should part with.

But, when designing GUIs, the starting point of the graphical information can be, simulated brush-strokes, with a simulated ‘QBrush’ and ‘QPen’, that are orchestrated by a programmer to give logical meaning to his GUI (beyond just adding the predefined widgets to it). In that case, the trick which I just wrote about will not gain anything over what I am about to describe.

When the ‘QPainter’ object and session are initialized, their target can be a ‘QPixmap’ object, and, after a sequence of painting instructions has finished, it can be assigned to a ‘QLabel’ widget once, by way of the ‘.setPixmap()’ function, thus resulting in a custom widget. Much buffering takes place, and, if the code has been written correctly, the graphic appears on the screen. The following is the graphic which my latest exercise generated:

Admittedly, this does not correspond to the GUI of any real software. But hypothetically, it just as well might. In fact, ‘A Chess-Board’ is just as easy to construct, using the same methodology. Only, actually programming the computer to play Chess is a little harder.

The fact that I was able to post this graphic was partially due, to the added ability the Qt5 Libraries have, to Save a ‘QPixmap’ object, or a ‘QImage’ object, directly to the hard drive, using a simple, straightforward API, and to the format of a PNG File. That task is also achieved in the code sample I am about to link to.

The real caveat here is, Not to try doing funny stuff with the internal ‘.pixmap()’ pointer that belongs to the ‘QLabel’ object, because even though it can be done, writing to that internal property does not offer much in the way of performance improvements, when the goal is to design most GUIs.

(This is a link to the previous exercise.)

(Updated 8/21/2020, 21h25… )

## Basic Qt5 Slots, Signals and Menus Example

In case the reader is an aspiring programmer, but yet a hobbyist, one of the facts which he or she will already know is that, in order to design applications which have a Graphical User Interface, some predefined GUI Library is commonly used, and one example of a GUI Library which can be used is Qt 5. Therefore, in order to become proficient at using this library, like me, the reader might want a working example of:

• Signals and Slots – The way in which Qt connects user-actions on the GUI, to actions which should be triggered from the actual program,
• The use of the ‘QLabel’ class to display an image instead of more-common text,
• The design of a very basic command-menu and a keyboard shortcut,
• A QRC Resource Script.

Even though this example was created with ‘Qt Creator’ and Qt version 5.7, one of the main features of Qt Creator, its GUI Layout Designer, has been cut from the project, so that the means by which such mechanisms can be set up entirely via code, can be explored better. Also, while Qt5 maintains backwards-compatibility with Qt4 -style Signals and Slots, that are based on macros, this project makes use of the newer Qt5 semantics, that are based on function pointers, for the sake of favouring new features over old.

I can say that on my Debian 9 / Stretch computer, the example works. However, the Qt Library is designed to be cross-platform, and so the example should also work under Windows. What some people have suggested is that, in order to get such code to work under OS/X, ‘ccmake’ should be used with the ‘Unix Makefiles’ generator. This will assume that ‘XCode’ is already installed. (:1)

The Link where the compressed files, containing only source code, can be found (along some other compressed files that also contain precompiled binaries, belonging to other projects) is here:

https://dirkmittler.homeip.net/binaries/

In that Gopher-Hole, the files of interest would be ‘Creator_Test2.tar.gz‘ or ‘Creator_Test2.zip‘.