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Edge blending (for projectors)


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Edge blending is only available to licensed users of Racer.

Edge blending is used when running Racer in a multiple projector setup. Most often, you'll use 3 projectors. More expensive projectors support edge blending in hardware, as do some of the higher-end nVidia Quadro cards. However, it's not really required (it's easy enough to do it in software with Cg these days). Also, some tricks may be possible in the future to have edge blending affect only parts of the center render (for example, not edge-blending dials/dashboards).

Edge blending works by having multiple projections overlap eachother slightly, which smooths out any misaligned images. To avoid the area of the overlap becoming too bright, the edge is darkened towards black to make a nice transition from one projection to the other. This all results in a slow mix between the two images, much less prone to be noticed by the viewer.

A fabulous page on the internet about edge blending is located here. It described how the blend curve really works (it's not a linear degradation to black).

Edge blending requires multiple computers, rendering in concert. More information on hooking up multiple computers to provide one image can be found at the multiview page.

The image below shows edge blending editing in action, combined with projector distortion (this is all inside Racer; no external tools are needed). A test pattern is overlaid for easier editing of the parameters when aligning the overlap for 2 projectors (both with respect to the overlap being located in the right place and also for adjusting the blend region to correct for gamma, since light doesn't really add up in a nice linear way). Only the left side is edge blended (this would be the projected image on the right of the center projection).

edge blending racer

Edge blend parameters

Edge blending characteristics are decided in racer.ini under the 'renderer' section. Overlapping parameters are defined in the 'multiview' section though. The available parameters:

Calculating projection values

Calculating all these values is relative easy to do if you follow these steps:

All this can be easily put into a simple spreadsheet; download one here. It allows entering resolution, overlap (in pixels) and an optional car camera shift if the driver isn't centered with respect to the middle projector.

An example 3 projector setup

Say we have 3 projectors, all doing 1024x768 images. Now assume you want 25% overlap, meaning 256 pixels. Let's follow the steps:

Asymmetric center screen frustum

For some setups, the viewer isn't located at the centerline in front of the projection screen (for example, if you are seated in a car in the left seat). This means the center computer's display must be shifted. Racer has some options to correct for this (note that the driver on the right is less lucky and sees a slightly off picture).

Testing the setup

Testing can be done using the renderer.edge_blending.test_pattern setting. Set this to 1 to get a grid and some colors, which you can use to synchronize your projectors color balance, keystone, zoom etc. Use the 'edgetest on|off' script commands to turn the pattern on/off. Turn off the test pattern altogether in production phase, as it takes memory.

You can also execute the 'distort edit' script command (pressing SPACE will toggle the test pattern, even if set to 0 in racer.ini). You really need a distorting & edge blending Cg shader for that (fs_filter1 in racer.ini). This is explained in more detail in the distortion page.

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(last updated November 13, 2012 )