You can use the projector's Edge Blending feature to create a seamless image from multiple projectors. Press the Menu button. Select the Extended menu and press Enter. Select the Multi-Projection setting and press Enter. Select the Edge Blending setting and press Enter. Apr 20, 2018  And edge blending is typically only needed when the image is displayed on a surface that can be best described as environmentally neutral. In most cases, edge blending isn’t necessary when putting together environmental projection imagery. Still, edge blending is a valuable feature to look for in a projector, and a valuable skill to nail down.

Written by Paul Bourke
Assistance by Roberto Calati, and panoramic images by Peter Murphy
March 2004

Chinese version: InfoAV China, November 2004.

Implementation for Quartz Composer contributed by Matthias Oostrik:seb.zip.
Includes a Core Image Filter and an example composition for a three projectordisplay using a Matrox Triplehead2Go.

Introduction

There are a number of applications which benefit from high resolution displays,for example, immersive virtual reality and scientific visualisation. Unfortunatelyprojector technology is currently limited to around the XGA (1024x768) or SXGA(1280x1024) pixel resolution and at the time of writing the later are still ratherexpensive. The approach that can be taken then is to use low cast commodityprojectors and tile a number of them together, each driven by either a different computer or a different graphics pipe on the same computer.

The first thing one discovers with commodity projectors is that, unlike the moreexpensive models, they don't have inbuilt support for tiled displays. In additiontheir optics is such that it isn't possible to reliably align multiple projectorsin a pixel perfect way (or even close). With the tiling not being pixel perfect one ends up witheither a gap between the images or a double bright seam, both are quite objectionableespecially so for content where the camera is panning or objects are moving acrossthe seam.The solution is to overlap the two images by a significant amount and modify thepixels in the overlap region so as to make the overlap as invisible as possible.The reason why this works is that now any slight projector misalignment or lensaberration only reveals itself as a slight blurring of the image and not as a sharpseam or gap.

Overlap

The first part in the process then is to overlap the images, in this discussiona panoramic image of the interior of a Sydney landmark building will be used.The pixels in the overlap region will be blended across the overlap region, thatis, the images will be faded to black.The general approach works equally well for any number of images and also forimages that may not be aligned in a rectangular fashion. The projectors beingused here are XGA (1024x768) and the overlap region is 256 pixels. The overlapneed not strictly be a power of 2 but since the blending is performed usingtextures and OpenGL a power of 2 is the most efficient. The degree of overlapis dictated by the amount of gamma correction required (see later) and the dynamic range of the blend function. For this exercise 128 pixels couldn'tachieve as good as results as the 256 pixel overlap. The resulting imagewidth is then 2 x 1024 - 256 = 1792 pixels, the height is 768 pixels.

The image below shows the two projected images, as expected the overlap regionis brighter than it should be because it is being contributed to by twoprojectors. The application being used to present this panoramic is a locallydeveloped previewer for stereoscopic panoramic images written for OpenGLbased graphics. Creating the two overlapped images using OpenGL is particularlysimple as it only requires the appropriate change in the view frustum.Please note that the images presented here are digital photographs taken of theimages in a darkened room, there are some challenges involved with that whichmeans the colours seem a bit washed out.

Blending

Blending involves multiplying each pixel in the overlap region of one image by anamount such that when it is added to its corresponding pixel in theother image, the result is the intended pixel value. This is readilyachieved by any function f(x) whose value lies between 0 and 1, thepixel from the right image say is multiplied by f(x) and the correspondingpixel from the left image multiplied by 1-f(x). The function used hereis given below, there are lots of possible choices but this one was chosenbecause it allowed experimentation of the exact form of the functionfrom linear to highly curved.

f(x) =

0.5 (2 x) p 1 - 0.5 (2 (1 - x)) p

for 0 <= x <= 0.5 for 0.5 <= x <= 1

Graphically the function looks as follows, note the coordinates are normalisedto 0 (refers to the start of the edge blend region) and 1 (the end of the edgeblend region). So consider the right image, a pixel in the first column is multipliedby 0 (it has no effect) a pixel on the right edge of the blend region (column 255)is multiplied by 1. The blend function for the left hand image is 1 - the curve below. In thecenter of the blend region the curve passes through 0.5 which means each image pixelcontributes the same amount to the final pixel value. The exact curvature is controlledby the parameter 'p', the blending is linear for p=1, the transition around 0.5 becomessteeper as p increases. p=1 tends to result in a visible step at the edges of the blendingregion, p=2 is used for all examples in this document.

The edge blend is accomplished by placing a polygon with the edge blend functionabove represented as a grey scale 1 dimensional texture. This textured polygonis blended (OpenGL speak) appropriately so it scales the pixels in the imagebuffer. The nice thing about this method is that it can be trivially appliedto any OpenGL application since it is a post processing stage applied afterthe normal geometry drawing is performed. The following image shows the maskregion texture across the blend region.

The image below is a screen dump of what this looks like on the computer (whichhas a dual display), that is, the panoramic images multiplied by the mask above.