10.10 Using images and cursors

Many today graphical user interfaces extensively use (iconic) images. There are many image formats, some for specific machines, some with specific goals in mind, such as optimal compression at the loss of accuracy or provide alternatives for different screen properties.

One of XPCE's aim is to provide portability between the supported platform. Therefore, we have chosen to support a few formats across all platforms, in addition to the most popular formats for each individual platform.

10.10.1 Colour handling

Colour handling is a hard task for todays computerprogrammer. There is a large variety in techniques, each providing their own advantages and disadvantages. XPCE doesn't aim for programmers that need to get the best performance and best results rendering colours, but for the programmer who wants a reasonable result at little effort.

As long as you are not using many colours, which is normally the case as long as you do not handle full-colour images, there is no problem. This is why this general topic is handled in the section on images.

Displays differ in the number of colours they can display simultaneously and whether this set can be changed or not. X11 defines 6 types of visuals. Luckily, these days only three models are popular.

• 8-bit colour-mapped
  This is that hard one. It allows displaying 256 colours at the same time. Applications have to negotiate with each others and the windowing systems which colours are used at any given moment.

  It is hard to do this without some advice from the user. On the other hand, this format is popular because it leads to good graphical performance.

• 16-bit `high-colour'
  This schema is a low-colour-resolution version of true-colour, normally using 5-bit on the red and blue channels and 6 on the green channel. It is not very good showing perfect colours, nor showing colour gradients.

  It is as easy for the programmer as true-colour and still fairly efficient in terms of memory.

• 24/32 bit true-colour
  This uses the full 8-bit resolution supported by most hardware on all three channels. The 32-bit version wastes one byte for each pixel, achieving comfortable alignment. Depending on the hardware, 32 bit colour is sometimes much faster.

We will further discuss 8-bit colour below. As handling this is totally different in X11 and MS-Windows we do this in two separate sections.

10.10.1.1 Colour-mapped displays on MS-Windows

In MS-Windows one has the choice to stick with the reserved 20 colours of the system palette or use a colourmap (palette, called by Microsoft).

If an application chooses to use a colourmap switching to this application causes the entire screen to be repainted using the application's colourmap. The idea is that the active application looks perfect and the other applications look a little distorted as they have to do their job using an imperfect colourmap.

By default, XPCE makes a colour_map that holds a copy of the reserved colours. As colours are required they are added to this map. This schema is suitable for applications using (small) icons and solid colours for graphics. When loading large colourful images the colourmap will get very big and optimising its mapping to the display slow and poor. In this case it is a good idea to use a fixed colourmap. See class colour_map for details.

When using XPCE with many full-colour images it is advised to use high-colour or true-colour modes.

10.10.1.2 Colour-mapped displays on X11/Unix

X11 provides colourmap sharing between applications. This avoids the flickering when changing applications, but limits the number of available colours. Even worse, depending on the other applications there can be a large difference in available colours. The alternative is to use a private colourmap, but unlike MS-Windows the other applications appear in totally random colours. XPCE does not support the use of private colourmaps therefore.

In practice, it is strongly advised to run X11 in 16, 24 or 32 bit mode when running multiple applications presenting colourful images. For example Netscape insists creating its own colourmap and starting Netscape after another application has consumed too many colours will simply fail.

10.10.2 Supported Image Formats

The table below illustrates the image format capabilities of each of the platforms. Shape support means that the format can indicate transparent areas. If such an image file is loaded, the resulting image object will have an `image<-mask' associated: a monochrome image of the same side that indicates where paint is to be applied. This is required for defining cursors (see `cursor ->initialise') from a single image file. Hotspot means the format can specify a location. If a Hotspot is found, the `image<-hot_spot' attribute is filled with it. A Hotspot is necessary for cursors, but can also be useful for other images.

The XPM format (X PixMap) is the preferred format for platform-independent storage of images that are used by the application for cursors, icons and other nice pictures. The XPM format and supporting libraries are actively developed as a contributed package to X11.

10.10.2.1 Creating XPM files

Unix There are two basic ways to create XPM files. One is to convert from another format. On Unix, there are two popular conversion tools. The xv program is a good interactive tool for format conversion and applying graphical operations to images.

ImageMagic can be found at http://www.simplesystems.org/ImageMagick/ and provides a comprehensive toolkit for converting images.

The pixmap program is a comprehensive icon editor, supporting all of XPM's features. The image tools mentioned here, as well as the XPM library sources and a FAQ dealing with XPM related issues can be found at ftp://swi.psy.uva.nl/xpce/util/images/

Windows

XPCE supports the Windows native .ICO, .CUR and .BMP formats. Any editor, such as the resource editors that comes with most C(++) development environments can be used. When portability of the application becomes an issue, simply load the icons into XPCE, and write them in the XPM format using the `image ->save' method. See the skeleton below:

to_xpm(In, Out) :-
        new(I, image(In)),
        send(I, save, Out, xpm),
        free(I).

Note that the above mentioned ImageMagick toolkit is also available for MS-Windows.

10.10.2.2 Using Images

Images in any of the formats are recognised by many of XPCE's GUI classes. Table table 8 provides a brief list: