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How to make a great seamless edge blending

  • BenQ
  • 2019-10-08
Basic Requirements for a good multi-projection setup:

To maximize exhibit effects, large exhibit spaces, such as museums, playgrounds or convention halls, may turn to image stitching through multiple projectors to achieve a wide-screen special effect. This attracts the viewers' attention with large size, high luminance and huge pixel-count. Multi-projection is therefore a common and important topic in projector applications.

Multi-projection is made possible through two or more projectors, to achieve a display larger in size and resolution. The projection is considered good when viewers are unaware of the fact that the projection is achieved through multiple projectors. Instead, to them, it appears to come from a single projector; this is the most successful seamless edge blending.

When choosing a multi-projection setup, it would be nice if the following criteria are met:

(1) Same model with the same resolution

(2) Same throw ratio and lens with the same focal length range

(3) Similar brightness, machines with near the same hours of use as projection brightness will deteriorate as hours of use increases

(4) Similar color parameters, pick specific configurations including color temperature and chromaticity coordinates. The best way to achieve parameter consistency such as brightness and color etc., is to choose machines from the same batch of production, with similar parameter configurations. This can be considered a priori criteria necessary for a successful multi-projection setup. With them, image blending would be much easier; conversely, it would be impossible to overcome certain issues if some of the criteria are not met, such as resolution or lens differences.

Aside from calculation for spacing between installations and projection size for stitching, there are two main factors for a successful multi-projection, the first is seamlessness (i.e. precise geometry adjustment), the second is image consistency (i.e. brightness and color consistency).

Back to the two main factors for successful edge blending:

1. Geometry adjustments for seamless edge blending

(1-1) Optical Alignment, adjustment of the Lens Shift and Zoom of lens increases the precision and flexibility of projection locations. There are two ways to adjust them, the first one is manual adjustment, which is a knob for Lens Shift, and a ring for Zoom, the second one is automatic adjustment. By comparison, automatic adjustment offers not only higher precision and reliability, but also speedy geometry adjustment, which is convenient and saves time.

(1-2) In terms of geometry positioning, there are two methods in general, external software or built-in image warping/edge blending processor. Aside from basic adjustment of the four corners, it also need to be able to freely adjust any regions within the projection, that way, it would be able to support edge blending of not only regular rectangle outputs, but also irregular shaped outputs such as arcs, columns etc. The higher the warping resolution (more grid blocks), the higher the amount of precision one can achieve, for a better seamless effect.

2. Image consistency (i.e. consistency of brightness and color)

(2.1) Prior to image blending apply basic configurations to each projector, including setting the same picture mode, color temperature and brightness, and measures color parameters to lower the optical parameter differences between projectors.

(2.2) After image blending make brightness and color adjustments, including increasing the white level of the overlapping region, decreasing the black level at the non-overlapping region as well as adjusting RGB color parameters, so that the overall projection has both even brightness and color.

Accurate geometry positioning and consistent imagery would be able to satisfy the performance requirements of multi-projection.

Compared to other (LCD) projection technologies, DLP projection can produce dark (black) levels with much lower brightness. Other technologies cannot output relatively low brightness for black images, which is already visible in a single projection setting; In a multi-projection setting, projection overlap would amplify the brightness of black images, which cause them to be visually not black enough. Since DLP projections have low brightness for black images, they can offer higher contrast in multi-projection settings.