Locations and sizes

When defining a drawing surface you need to understand how operating system (OS), area and graphics coordinate systems work together. For simplicity, consider a full HD screen, i.e., 1920x1080 resolution, as shown in the figure 1 below. The upper left and lower right coordinates in OS coordinate system are (0,0) and (1920,1080), respectively. A green rectangle shows the area where the graphics of your application appears and for familiarity the horizonal gray rectangle at the bottom visualizes a taskbar.

To defined the graphics are you need to define window, area and graphics locations and their sizes.

1. Point A: Define the window location relative to OS coordinate origin (0,0) and window size. This is an operating system window.
2. Point B: Define the area location relative to the window lower left corner. In reality, the area is an OpenGL viewport.
3. Point C: Define the application coordinate that matches the upper left corner of the area.

Fig.1. Locations

Here's the figure 1 as it would appear in screen.xml file.

<?xml version="1.0" encoding="UTF-8" standalone="no" ?>
<multihead>
  <window>
    <area>
      <graphicslocation>0 0</graphicslocation>  /* Point C */
      <graphicssize>654 341</graphicssize>
      <location>381 252</location>  /* Point B */
      <size>654 341</size>
    </area>
    <location>313 191</location>  /* Point A */
    <size>1157 656</size>
  </window>
</multihead>

Note that the graphics location, point C, doesn't define a location on screen, but an application coordinate that matches to the upper left corner of the area. For example, if your application draws a single pixel to location (-1,-1) with C(0,0), the pixel is not visible since is just outside the visible area. Changing the graphics location to C(-1,-1) makes the pixel visible to the upper left corner of the area.

The graphics size defines a range of application coordinates that map inside the area, but the size doesn't have anything to do with the amount of physical pixels in the area. For example, if your application draws a single pixel to location (654, 341), the pixel is not visible (note that the coordinates start from zero) since it's just outside the visible area. However, if you double the graphics size to (1308,682), then the pixel appears in the center of the area. Graphics drawn to the area is stretched according to graphics size and to avoid unnatural stretching scale width and height evenly.