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Eliminate the need to use monospaced fonts to prevent ghosting #1070

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Merged
mathoudebine merged 1 commit into from
Aug 27, 2026
Merged

Eliminate the need to use monospaced fonts to prevent ghosting #1070

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19 changes: 19 additions & 0 deletions library/lcd/lcd_comm.py
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Original file line number Diff line number Diff line change
Expand Up @@ -83,6 +83,14 @@ def __init__(self, com_port: str = "AUTO", display_width: int = 320, display_hei
ImageFont.FreeTypeFont # value= a loaded freetype font
] = {}

# Remember the bounding box drawn by the last DisplayText() call at a given (x, y) origin, so the
# next call can redraw the union of the old and new areas and fully erase any previous text that is
# narrower/shorter than what replaces it, regardless of font metrics ("ghosting" prevention).
self.text_bbox_cache: Dict[
Tuple[int, int], # key=(x, y) origin as passed to DisplayText()
Tuple[int, int, int, int] # value=(left, top, right, bottom) actually drawn
] = {}

def get_width(self) -> int:
if self.orientation == Orientation.PORTRAIT or self.orientation == Orientation.REVERSE_PORTRAIT:
return self.display_width
Expand Down Expand Up @@ -304,6 +312,17 @@ def DisplayText(
# Let's extend the bounding box to the next whole pixel in all directions
left, top = math.floor(left), math.floor(top)
right, bottom = math.ceil(right), math.ceil(bottom)

# Union with the area drawn last time at this same origin, so a new value that is narrower/shorter
# than the previous one still gets its old pixels repainted with the background (prevents ghosting
# of stale glyph fragments, independent of font metrics).
bbox_key = (x, y)
prev_bbox = self.text_bbox_cache.get(bbox_key)
self.text_bbox_cache[bbox_key] = (left, top, right, bottom)
if prev_bbox is not None:
prev_left, prev_top, prev_right, prev_bottom = prev_bbox
left, top = min(left, prev_left), min(top, prev_top)
right, bottom = max(right, prev_right), max(bottom, prev_bottom)
else:
left, top, right, bottom = x, y, x + width, y + height

Expand Down
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