Walls are rarely optically flat
Ultra-short-throw light hits at a steep angle, so small waves, plaster texture and uneven paint can cast visible shading and distort straight edges. A wall that looks flat in normal room light may not behave like a projection surface.
Walls do not reject ambient light directionally
White paint reflects projector light and room light alike. In a dark room that can be acceptable; in a living room it usually reduces perceived contrast. A UST ALR surface is engineered to discriminate between light directions.
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Check current price ↗A wall can still be useful during planning
Before buying a screen, project onto the wall to verify approximate diagonal, furniture placement and seating comfort. Treat that as a geometry test rather than a final image-quality benchmark.
When a real screen earns its space
Once you care about daytime contrast, straight geometry, repeatable framing or a disappearing screen, a dedicated surface becomes part of the system. A motorized floor-rising option is one way to add that surface without permanently occupying the wall.