OSD overlay limited to 1920x1080 — cannot center or scale for resolutions above 1080p

[AlexBusko]

When running msposd on SSC338Q with video resolution higher than 1920x1080 (e.g. 3840x2160), the hardware overlay region is always capped at 1920x1080. The OSD does not scale to fill the full video frame and appears offset rather than centered.

Hardware / Software

• SoC: SSC338Q
• msposd version: 63499fe8-dirty, compiled 20260213_112114
• Video config: size: 3840x2160 in /etc/majestic.yaml or even 1920x1080
• Flight controller: ArduPilot (ArduCopter), MSP DisplayPort
• Ground station: QGroundControl (no built-in OSD position editor like Mission Planner, so OSD element positioning is done via OSD1_* parameters manually)

Steps to reproduce

msposd --master /dev/ttyS2 --baudrate 115200 --osd -r 20 --ahi 1 -v -x 0 -z 3840x2160

Log output

Loading /usr/share/fonts/font.png for 3840x2160
Glyph size: 36:54 on a 53:20 matrix. Overlay: 1800x1000
Glyph size: 36:54 on a 53:20 matrix. Overlay: 1920x1080
Create_region PixelFormat:3 Size: 1920:1080 X_Offset:960 Y_Offset:0 results: 0

Despite requesting -z 3840x2160, the actual overlay is created as 1920x1080. X_Offset is calculated as (3840-1920)/2 = 960, but the OSD still does not appear centered on screen. Y_Offset remains 0 instead of expected (2160-1080)/2 = 540.

Current workaround

Using an artificially inflated -z value to shift the overlay position:

msposd --master /dev/ttyS2 --baudrate 115200 --osd -r 20 --ahi 1 -v -x 1 -z 2760x1980

This roughly centers the OSD but does not properly scale it — elements remain rendered within 1920x1080 bounds. Since QGroundControl lacks a visual OSD grid editor (unlike Mission Planner), adjusting individual element positions to compensate is cumbersome and does not fix the AHI (Artificial Horizon) placement, which is rendered by msposd directly.

Expected behavior

Either:

1. Support overlay regions larger than 1920x1080 on SSC338Q (if RGN hardware supports it), or
2. Properly center the 1920x1080 overlay both horizontally and vertically when video resolution exceeds overlay size (correct Y_Offset calculation).

[tipoman9]

This is a limitation of the logic used by the Flight Controller to display the OSD on the video. It works by placing characters on a fixed 53×20 matrix.

There are two types of fonts to cover the two most popular resolutions
for 720p
every character has dimensions of font_width = 24 , font_height = 36
This gives:
53 * 24 = 1274
20 * 36 = 720
So this matches 1280x720

for 1080p
the character dimensions are font_width = 36 , font_height = 54 . Do the maths ....

So there is no standard way to support larger overlay resolutions, unless a custom font file with bigger characters is created and then the code is updated. Using 36x54 characters with some offset to spread them evenly across the screen may work also, but will look ugly....
Regarding centering, you can experiment with -x --matrix argument:
OSD matrix [0- 53:20 , 1- 50:18 chars, 11- Variable font size, 9-bottom align 720p mode, 8-center align 720p mode]
Value 8 may fix centering, though I've never tested it for such video size.

EDIT:
Maybe the approach I use for mixing larger and smaller fonts (matrix = 11) could be adapted, but explaining how to build the OSD layout properly for it seems difficult for general pilots.