What is edge blending and why do Indian college auditoriums need it?
Short answer: Edge blending is the technique of running two (or more) projectors side-by-side with overlapping images, and softening the overlap zone with a gamma ramp (a brightness fade curve) so the audience sees one continuous, seamless widescreen image instead of two separate projections with a bright band at the join. Indian college auditoriums with screens wider than 6–8 metres need edge blending because a single projector capable of filling that screen at useful brightness costs ₹8,00,000–₹20,00,000. Two mid-range 5,000-lumen projectors with an edge-blending processor cost a fraction of that.
The hardware you need
Two identical projectors
Identical make and model is non-negotiable for beginner edge blending. Different models produce different colour temperatures (the "warmth" or "coolness" of the white point), different gamma curves, and different native resolutions. Even matching these digitally in a processor leaves visible artefacts. Buy two units from the same production batch if possible — early batches often have slightly different optical coatings than later batches on the same model. Replace both lamps at the same time, every time, to keep lumen output matched. See the lumen and throw guide for how to calculate the right lumen class for your screen width at the overlapped resolution.
A video processor with edge-blending capability
The video processor splits a single widescreen video signal into two overlapping output signals, each covering roughly 60–65% of the total image width with a 15–25% overlap in the middle. It then applies a black level lift correction (raising the black level of the overlap zone to compensate for two projectors illuminating the same screen area) and a gamma fade curve that blends the two images smoothly. Entry-level single-channel edge-blending processors like the Datavideo VP-597 or Kramer VP-730 cost ₹25,000–₹60,000. Professional processors (Analog Way, Barco Event Master) cost ₹1,50,000–₹5,00,000.
Precision ceiling mounts at matched positions
Both projectors must be mounted at exactly the same height, roll angle (rotation around the lens axis), and vertical angle. Even 1° of tilt difference produces a vertical keystone mismatch in the overlap zone that the software cannot fully correct. Use the same mount model for both units, with the same drop rod length. Verify alignment with a laser level before commissioning.
Calibration and the India challenge
Initial calibration steps
After physical alignment, calibrate in this order: (1) geometric alignment — ensure both images have the same keystone correction and are at the same vertical position. (2) Colour matching — adjust colour temperature and white balance on both projectors to match a reference white card. (3) Overlap zone setup — set the overlap width in the processor and adjust the gamma ramp shape until the seam is invisible. (4) Black level correction — the overlap zone receives double the light of the non-overlapping zones. The processor applies a black level lift to the single-projector zones to equalise perceived black. This step is often skipped in India and results in a bright band at the seam on dark scenes.
Maintenance: the lamp ageing problem in India
In Indian auditoriums with monsoon season humidity and year-round dust, lamps degrade faster than in controlled environments. A 1,000-hour difference in lamp age between two edge-blended projectors is often enough to produce a visible brightness difference across the seam. Replace both lamps together — even if only one has technically reached end-of-life — to keep lumen output matched. See the auditorium quarterly checklist and the PRW AMC plan for how to build lamp replacement cycles into an annual service contract. Also see our detailed edge blending setup reference for advanced configuration.
A note from the PRW Engineer Team
Across 5k+ projector repairs since 2007, the most expensive edge-blending mistake we see is a college auditorium replacing one lamp in a blended pair without replacing the other. Three months later the seam is visible, the IT department blames the processor, the processor gets replaced, and the seam is still visible because the actual problem was a 800-lumen brightness differential between two projectors with 500 and 1,500 lamp hours respectively. Replace both lamps. Every time. No exceptions.