## 2. Creating and Manipulating Emitters

Go to the "Emitter" page and click on the button with the "plus" sign on the lower left corner of the page. A new emitter has been added. Its coordinates can now be modified, and it is displayed as a red dot in the Scene. The coordinates can be modified using the sliders and by entering the respective values into the entry widgets at the right side of the sliders. Once the coordinates are chosen, the direction of the emitter can be chosen. Choosing 'forward + backward' will trace the particle forward and backwards in time from the emitter position.

In Python, assuming that the emitter coordinates are (0, 2000, 0), the same is achieved with

tr.emitters.append((0, 2000, 0))

Although in many cases, a single emitter (consisting of several emitter points) is sufficient, the number of emitters that can be added is not restricted. The list widget to the left of the "Emitter" page allows to select each emitter. Clicking the "minus" button at the lower left corner will delete the selected emitter.

An emitter consists in fact of a 3D cubic grid of emitter points. By default, the size of the grid is 0, which means that the grid is fully degenerated and only a single emitter point exists. The "Size" sub-page on the "Emitter" page allows to select the size in x/y/z for each of the emitters. Usually, the cube's size is made nonzero in the directions perpendicular to the flow direction, and usually these are y and z. Once this is done, 4 particle traces are extracted.

To have more emitter points, increase the number of intervals in the appropriate directions. This can be adjusted on the "Number" sub-page on the "Emitter" page. By default, these values are 1 for all directions. Setting the y-value to 2 and the z-value to 3 will create 3x4 emitter points.

When an emitter has been added by means of the GUI, the grid and centre point of the grid is displayed by default. This setting can be toggled with the "show grid" check box on the "Emitter" page.