It is possible to use several different methods to display images. The engines can use OpenGL, MIT-SHM, Qt, gdk-pixbuf with MMX optimization. Their main difference is speed. For example, the display of images by means of Qt will be very slow (even in the case of its own MMX optimization), while gdk-pixbuf for GTK applications will work acceptably for the average image size.
Using OpenGL as a backend to view images has its advantages and disadvantages. An advantage is high speed image manipulation (zoom, rotate, shift ...), which is inaccessible to other display methods. In contrast, as a drawback, it has a very low speed of image manipulation, which may not completely fit in the video memory of a video card. For example, there's a high probability a 9600x9600 image will not fit in the video memory of a middle class video card, like a GF MX440, and zooming in the viewport will run extremely slowly. In this way, KSquirrel is great for looking at « home » pictures - images of standard sizes, scanned A4 sheets, photos from a digital camera, etc., but will run into difficulties with huge images.
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