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Coordinate System

WIP

All 2-D stimulus positions use a pixel-space coordinate system:

  • Origin at the screen centre
  • X increases to the right
  • Y increases upward
  • Units are pixels
                  +Y
                   │
                   │
    ───────────────┼───────────────  X
                   │
                   │
                  -Y

A rectangle at (x=0, y=0) is centred on screen. A rectangle at (x=200, y=0) is 200 pixels to the right of centre.

Examples

Position Meaning
x=0, y=0 Screen centre
x=500, y=0 500 px right of centre
x=0, y=-300 300 px below centre
x=-100, y=200 Upper-left quadrant

Screen size

Query the display dimensions at runtime:

info = conn.system.query_server_info()
w, h = info.display_width, info.display_height
# Top-right corner of the screen:
x_max = w / 2
y_max = h / 2

Orientation

Stimulus rotation (orientation_deg) is measured in degrees, counter-clockwise from the positive X axis. A rectangle with orientation_deg=45 is rotated 45° CCW.

Notes

  • The coordinate system matches PsychoPy's units="pix" mode.
  • The server's internal vertex shader converts pixel coordinates to Vulkan NDC at draw time. Clients never need to think about NDC.