Creating Content: File Formats
After creating your own Deep Zoom Image, you end up with a tiered file structure that mimics the levels of the pyramid. You'll notice that the thumbnails below get larger and larger as they go from folder 0 to folder 10.
Deep Zoom Image (DZI)
A Deep Zoom Image consists of a DZI file and a subdirectory of image files. The DZI file may have either an .xml or .dzi extension, as the .xml extension is more compatible with web server requirements. This image subdirectory contains multiple subdirectories that mimics the levels of resolution of the image pyramid. If you look at the example folder below, you'll notice that the thumbnails get larger and larger as they go from folder 0 (lowest resolution), to folder 10 (highest resolution). Each of these subdirectories then contains the individual tile image files for that resolution level (shown below to the right).
For those of you paying close attention, I actually transposed the row and column IDs in the file name to help my puzzle fall together. Sneaky, I know, but it made for a better story.
Sparse Images
Sparse images are a type of Deep Zoom Image that has different resolutions in different areas and is typically created by merging together multiple source photos of different sizes. If you go back to the home page, “The Contoso Fixster” is a great example of a sparse image. The file structure for a sparse image is no different from a typical Deep Zoom Image and can be extended to include as many source images as are needed.
Deep Zoom Collection (DZC)
Deep Zoom Collections allow you to load a large number of images simultaneously. It wouldn’t be possible to render such a large amount of data if a network round trip was required to load each individual image. Deep Zoom Collections store these images in texture atlases, and by placing image thumbnails into the image tiles introduced in the Deep Zoom Image file format, these texture atlases are stored as a tiled image pyramid (shown in the image below). Much like the Deep Zoom Image, a Deep Zoom Collection consists of a .dzc or .xml file for the collection definition and a subdirectory of Deep Zoom Images. These individual subdirectories map to the DZI file structure, each with a .dzi or .xml file, image file subdirectories, and the corresponding nested image tiles themselves.
For more detail, check out A Deep Zoom Blog: Deep Zoom Collections Explained
Image File Formats
The tiled pyramids for Deep Zoom Images and Deep Zoom Collections can be encoded as JPEG, PNG or HD Photo files*. For most purposes, especially photographs, JPEG is the appropriate file format. Experiment with the JPEG compression, ranging from 1-100, to find the best compromise between file size and image quality.
For simple diagrams, text, or images with single color backgrounds where size is not critical, PNG is typically the best file format. PNG also supports an optional alpha channel, making it possible to present images with transparency.
* (HD Photo, recently standardized as JPEG XR, is a new image format from Microsoft that offers the versatility and efficient compression of JPEGs combined with the transparency option traditionally restricted to PNGs. JPEG XR is not yet supported by Silverlight Deep Zoom or Seadragon Ajax.)
