Fixed sized graphics

Fixed size graphics

The irony of using fixed size graphics is that they appear as a different size depending on the users screen resolution. What I would like to be able to do is specify in my code the physical size of how graphics will appear. To this end I created an Eiffel library which will enable distribution of graphics in SVG format and render them as PNG to an exact size regardless of the screen resolution. The images are nicely anti-aliased and it's possible to specify the background. Rendering a dozen or so images can take a noticeable amount of time so the library caches the images for the next application session in a user data area.

Creating SVG

I use the InkScape vector graphics editor to create SVG graphics. You can of course build them from scratch but a handy shortcut is to take a large sized PNG or other fixed size format and covert it to SVG using the "Trace bitmap" facility. Ideally you should use a PNG with a transparent background and specify the background color when rendering the SVG in your app.

librsvg

The library makes use of the C component librsvg which does a pretty good job and is better than the Java batik-rasterizer which sometimes has a problem of introducing color streaks not to mention the time waiting for the JVM to load.

Monitor size

A technical hurdle to scaling the graphics correctly is being able to query the physical dimensions of the monitor to calculate the size in pixels to render the PNG. This is not available in Vision2. In Linux this was relatively easy using the xrandr API. Windows was a problem as surprisingly the Windows API does not provide any means to query the physical size of the monitor. The best solution I could find is to query the registry for the EDID data associated with the current display monitor driver. EDID is an industry standard describing display properties. It's only accurate to the nearest centimetre but good enough.

Scons build

The library also includes a scons based build process. Under Linux the process builds both librsvg and the Eiffel bridging code. Under Windows with the MSVC compiler only the bridging code and import library for librsvg is built. All other dependencies are automatically downloaded from ftp.gnome.org/pub/GNOME/binaries. The 32 bit or 64 bit versions are downloaded depending on the value of ISE_PLATFORM.

Class EL_SVG_PIXMAP

The interface for the top level class in the library, EL_SVG_PIXMAP, is shown below. The location for the SVG is specified as relative to a standardized application images location using platform neutral path steps. The image is scaled either width wise or height wise keeping the aspect ratio of the original. Procedure 'update_pixmap' caches a new PNG if either the background color or dimension changes.

class EL_SVG_PIXMAP inherits EV_PIXMAP create make_with_width, make_with_height, make_from_other feature {NONE} -- Initialization make_from_other (other: like Current) -- do end make_with_width (relative_path_steps: ARRAY [STRING]; a_width: REAL; a_background_color: EV_COLOR) -- do end make_with_height (relative_path_steps: ARRAY [STRING]; a_height: REAL; a_background_color: EV_COLOR) -- do end feature -- Access svg_file_path: FILE_NAME dimension: INTEGER -- pixel dimension width or height depending on is_width_scaled feature -- Status report is_background_color_set: BOOLEAN is_width_scaled: BOOLEAN feature -- Element change set_dimension (a_dimension: REAL) -- do end set_path_from_steps (relative_path_steps: ARRAY [STRING]) do end set_background_color (a_color: like background_color) -- do end feature -- Basic operations update_pixmap -- do end draw_centered_text (a_text: STRING; a_font: EV_FONT; a_color: EV_COLOR) -- do end end

This library will be available with the release of the Eiffel Loop libraries.