Breaking the 4GB Bitmap Size Barrier
Both the BMP format and the TIFF format are limited to a maximum file size of 4 GBytes. When these file formats were developed no one could imagine a need to support larger files but modern imaging machines must support panels of 3 x 3 meters with pixel sizes of less than 1 um or 300 x 300 mm regions with pixel size of 0.01 um.Header Limitation
The basic bitmap file structure is not the cause of this 4GB limit. Instead, there is a header in both file formats that informs any parser the size of the file. The bytes used for this header can only represent a maximum of 4 GBytes.
One of Artwork's clients wanted to use gbr_rip but also needed to produce BMP style bitmaps that were as large as 50 GByte. What to do? These files were so large because monochrome BMP does not support compression.Solution - Ignore the Header
We agreed that the file size information in the header was not really necessary and that we would simply set those bytes = 0. We simply went ahead and provided the rest of the data in BMP format and were able to support these 50 GBytes file sizes.
Implications of Setting the File Header to Zero
The obvious implication of producing such as BMP file is that most BMP parsers won't read it. They will likely balk when they come to the header section and see that the file size = 0. Of course, our client modified his parser to support such files.
In order to enable our clients to view and analyze such large files, we have upgraded Artwork's Very Large Bitmap Viewer (VLBV) to support these files. We've also added optimizations to the loading/compressing approach.
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