Reading and Writing Gigabyte-Sized Files

The two most important factors in image file formats that affect their ability to support gigabyte-sized images are as follows:

Maximum allowed image dimensions. This usually depends on the integer format used for storing image dimensions.
File size limitation. This usually depends on the integer format used for storing lengths or offsets to various data in the file. If a file is stored compressed, image size after decompression may be greater than the maximum supported file size.
- Some file formats have limitations on blocks (chunks, strips) of pixel data, but allow multiple such blocks to exist in a file, and thus avoid the limitation on the file size.

Although some file formats allow storing gigabyte-sized images, particular software may have difficulties with reading or writing them.

The table below lists some of the popular file formats and their capabilities for storing gigabyte-sized images.

Image Format	Max Available Image Dimensions (width x height, pixels)	Max Image Size, When Uncompressed, Approximately (for a 24-bit RGB image).	File Size Limit, Approximately
JPEG, EXIF JPEG	65535 x 65535	12 Gb	None
TIFF, EXIF TIFF	2^32-1 x 2^32-1	3 * 2^24 Tb	4 Gb 1
JP2, JPX	2^32-1 x 2^32-1	3 * 2^24 Tb	None
PSB	300 000 x 300 000	250 Gb	None
PSD	30 000 x 30 000	2,5 Gb	4 Gb
BMP	2^31-1 x 2^31-1	3 * 2^20 Tb	4 Gb
PNG	2^32-1 x 2^32 - 1	3* 2^24 Tb	None
DICOM	65535 x 65535	12 Gb	2 Gb
PBM / PGM / PPM / PNM	None	None	None
TGA	65535 x 65535	12 Gb	4 Gb

1) TIFF format uses 32-bit unsigned integers to store data offsets and sizes. As a result, a strip of pixel data in a TIFF image cannot be stored at an offset greater than 4 Gb, and its size formally cannot be greater than 4 Gb. Thus, the size of the largest compliant TIFF image can be a bit less than 8 Gb. This assumes that two strips of nearly 4 Gb size are used.

ImageGear supports the reading and writing of single-page, single-strip, single-tiled uncompressed TIFF images where strip byte counts are greater than 4 Gb. If the size of a strip exceeds 4 Gb, ImageGear writes 0 to StripBytes tag. The reader can calculate strip size from image dimensions in such a case.

Note though, that such files are formally incompliant and may not be supported by other readers.

When writing a gigabyte-sized TIFF image, make sure to keep the "IMAGE_BEFORE_IFD" TIFF control parameter set to its default value of False.