Fixed sized graphics

by Finnian Reilly (modified: 2011 Nov 19)

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.


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 The 32 bit or 64 bit versions are downloaded depending on the value of ISE_PLATFORM.


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.

  • Colin Adams (12 years ago 21/11/2011)

    svg-vision library

    Note that another way of generating your SVG graph is to use the svg-vision library (should you be starting from a graph drawn via vision2).

    • Jocelyn-Fiat (12 years ago 22/11/2011)


      From what I read, it is parsing the SVG data (XML), and draw the SVG using Vision2 routines on EV_DRAWABLE

      • Colin Adams (12 years ago 22/11/2011)

        It generates SVG data

        I ought to know, as I wrote it.

        Basically it implements an EV_DRAWABLE as an SVG file.

        Parsing SVG files to produce Eiffel Vision graphs would be a nice complementary project, but much more difficult, I think.

        • Jocelyn-Fiat (12 years ago 22/11/2011)

          It might sound weird, but probably producing Eiffel Vision graphs might help covering more of SVG since I am not sure with Vision2 EV_PIXMAP or EV_PIXEL_BUFFER have features to rotate and so on (however I am not an vision2 drawing expert)

          In any case, this sounds a nice library to use and to extend.

    • Finnian Reilly (12 years ago 22/11/2011)

      SVG Vision

      I am very curious to see how it matches up to librsvg. Will be giving it a try.

      • Finnian Reilly (12 years ago 22/11/2011)


        Sorry I misunderstood the purpose. It doesn't render SVG but I can see how would be very useful for exporting to other applications or saving drawing actions input by the user.

  • Finnian Reilly (12 years ago 22/11/2011)

    Further enhancements

    The DLL dependencies on Windows add up to 11 mb. To reduce the memory foot print I have made the module dynamically loadable. If the PNG output is already cached there is no need load it.

    The library has now been tested on Window 7 (32 + 64 bit), Ubuntu 64 bit. Testing on XP 32 tonight.