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# Warped Tile Sorting

It is occasionaly useful to be able to specify non-rectangular tiles. This is commonly the case when working with an array of projectors which are not precisely aligned. By determining the distortion of one projector relative to another, we can distort the tile such that when it is projected, neighboring projectors are aligned.

This document does not discuss how to determine the transformation to be used to warp the tiles. There have been several papers published on how to recover this transformation, which can be referred to as needed.

## The Big Picture

Assume that we have two tiles as shown above, with one rotated 45 degrees and shifted off to one side. Units are given with respect to the default GL transformation. We will walk through the steps needed to setup the Tilesort SPU and CRServer to see a world of this nature. For this example, refer to warp_demo.conf. Once all tiles have been specified, the projection matrix will be scaled such that the mural viewport is a rectangle that roughly fits within the union of the tiles.

## Step 1: Determine the Transformations

The first step is to determine the linear transform which maps a tile to its approprate spot on the mural. In order to avoid problems with potential pixel size mismatches, we will specify our tiles in a space which can be thought of as the normalized device coordinates of the mural. That is, specifing the identity as a transform will yield a tile from (-1, -1) to (1, 1). This is what we will use for our first tile above.

For our second tile, things will be a bit more interesting. First, rotate the tile by 45 degrees, then translate it 2 units along the x axis. This yields the transformation shown in the image above.

Specifing every tile in our mural in this manner is somewhat overkill. Usually, tiles will be rectangular in the framebuffer space of the system they are to be displayed upon. With this, we can simplify the specification.

We define a display to be a collection of tiles which need to be warped by the same transformation. In the case of projector alignment, we may have several tiles which are displayed on a given projector. We will specify a transformation only for each display in the system. Tiles which fall within a display are specified in a familiar fashion. In our example above, we only use one tile per display. However, the specification of more tiles on a given display is not difficult and does not need additional transformations to be determined.

NOTE: It is also necessary to determine the inverses of the above transformations. This is usually trivial to recover when computing projector (mis)alignment, thus we have not included matrix inversion code with Chromium.

## Step 2: Configure the Tilesort SPU

We now need to report the displays and their transformations to the Tilesort SPU. In our Python config file, we add the following:

       tilesort_spu.AddDisplay(0, DISPLAY_WIDTH, DISPLAY_HEIGHT, inverse_transform_0, transform_0)

Where the first argument is the display id, the second and third specify the resolution of the display, followed by the transformation and its inverse. For our first tile, we would use the following:

       ident_matrix       = [1, 0, 0,  0, 1, 0,  0, 0, 1]
       tilesort_spu.AddDisplay(0, TILE_WIDTH, TILE_HEIGHT, ident_matrix, ident_matrix);

We also need to pass the Tilesort SPU a few options so that it knows to use the warped tiles.

       tilesort_spu.Conf('local_tile_spec', 1)
       tilesort_spu.Conf('bucket_mode',     'Warped Grid')

## Step 3: Configure the CRServer

Finally, we need to add tiling information to the CRServers where they reside. The format for this is:

       render_node.AddTileToDisplay(display_id, start_x, start_y, TILE_WIDTH, TILE_HEIGHT )

Additional tiles can be added to the display at will. As with the Tilesort SPU, we must set an option to make use of the tiling:

       render_node.Conf('local_tile_spec', 1)