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add something on rounding, tile sie, and the meaning of overlap

John Cupitt 2017-11-14 14:21:04 +00:00
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The-DZI-File-Format.md

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For a general overview, see https://msdn.microsoft.com/en-us/library/cc645077(VS.95).aspx.
When creating a DZI image, you specify a size, format, and overlap. The [conversion tool](http://openseadragon.github.io/examples/creating-zooming-images/) then goes through the image, breaking it into tiles. It starts with the full resolution image, and starts creating tiles from the upper left corner, with the first tile being 0_0, the next in the top row being 1_0, etc. The first tile on the next row is 0_1, etc. This becomes the highest level.
When creating a DZI image, you specify a size, format, and overlap. The [conversion tool](http://openseadragon.github.io/examples/creating-zooming-images/) then goes through the image, breaking it into tiles. It starts with the full resolution image, and starts creating tiles from the upper left corner, with the first tile being 0_0, the next in the top row being 1_0, etc. The first tile on the next row is 0_1, etc.
The tool then scales the image down to half width and runs through and tiles it again, using the same numbering system. This becomes the next level.
Note that tile overlaps are on both sides of the join, so a one pixel overlap (for example) means tiles 0_0 and 1_0 have two columns of pixels in common.
The tool continues scaling down by half, outputting new levels, until the image is down to approximately 1px wide. This is the lowest level, and it's designated level 0. The next level above it is level 1, etc.
The tile size is usually chosen so that non-edge tiles are a power of two along the side. With overlap one and tile size 254, tile 0_0 will be 255x255, but all no-edge tiles will be 256x256.
Metadata about the file is stored in an XML file using either the .dzi or .xml file extension, e.g. example.dzi. The tiles are stored in a folder next to it, named e.g. example_files. The tile of the lowest level in this example would be `example_files/0/0_0.jpg`.
This becomes the highest-numbered layer. The tool then scales the image down to half width, rounding odd-sized image sizes up, and runs through and tiles it again, using the same numbering system. This becomes the next level.
The tool continues scaling down by half and outputting new levels until the image is down to approximately 1px wide. This is the lowest level, and it's designated level 0. The next level above it is level 1, etc.
Metadata about the file is stored in an XML file using either the .dzi or .xml file extension, e.g. example.dzi. The tiles are stored in a folder next to it, named e.g. example_files. The tile of the lowest level in this example would be `example_files/0/0_0.jpg`.