Visual angle of Text

SIZE OF TEXT

When you create a Text visual object, you can make one of the two following parameters active to define the size of the characters of this Text object (See the definitions of these two parameters here):

• fontsize_in_postscript_points
  
• visual_angle_of_centered_x_height_deg

The argument (20 and 0) passed by default to these parameters are shown below:

my_text = PTVR.Stimuli.Objects.Text ( ..., fontsize_in_postscript_points = 20,   visual_angle_of_centered_x_height_deg = 0, ... )  

If you pass an argument (different from zero) to the parameter visual_angle_of_centered_x_height_deg, the parameter fontsize_in_postscript_points will be ignored.

ANGULAR SIZE OF TEXT : conditions of validity

Defining the parameter visual_angle_of_centered_x_height_deg is very convenient to define an angular x-height for your text. However, this nominal angular size will only correspond to the actual value if you respect some conditions that are explained in details here.

To say it quicky: your text should be positioned on a Tangent Screen and at the center of this tangent screen (i.e. at the point called Screen Origin - SO in the figures here).

Then, and only then, the angle (in degrees) subtended by the height of an 'x' (for a given font) at the origin of the current Coordinate System will correspond to the value of visual_angle_of_centered_x_height_deg .

DEFINING ANGULAR SIZE OF TEXT EVEN WHEN ACTUAL VALUE IS A GROSS APPROXIMATION

It is often convenient to use the visual_angle_of_centered_x_height_deg parameter, even when you do not want to control precisely the actual angular value of the text.

The simplest example is when you want to display labels for objects placed at short and long viewing distances: you want these labels to be visible independently of their viewing distance and using the visual_angle_of_centered_x_height_deg parameter is THE solution.

A good example is shown in the 1_global_cartesian_coordinate_system.py demo where the axes tick labels are all visible although they are placed at very different viewing distances (path of this demo: ...\PTVR_Researchers\Python_Scripts\Demos\Coordinate_Systems).

😎 Tip: You can get more information on all the Text Object parameters when you are in your Python interface development environment (IDE). For instance, in Spyder, if you see a code similar to that shown below, you can either hover with your mouse on the word Text (after "world.") or click on it and press Ctrl-i:

my_text = PTVR.Stimuli.Objects.Text ( text = 'Hello World !', 
                                    position_in_current_CS = np.array ( [ 0, 1, 1 ] ),
                                    fontsize_in_postscript_points = 60
                                )

Demos

The path of demos is: ...\PTVR_Researchers\Python_Scripts\Demos\Text

Python file	Description
10_hello_world.py	Simplest way of displaying text (character size is defined with fontsize_in_postscript_points, i.e. this is NOT an angular value)
text_height_at_different_radial_distances.py	Visually check that texts of same angular x-height at different viewing distances are visually overlapping.
Stroop_effect_with_tangent_screens.py	A simple demo of the Stroop effect with simultaneously displayed tangent screens. (character size is defined with fontsize_in_postscript_points, i.e. this is NOT an angular value - why is it here then? just for the pleasure of seeing colours)