# Image format characteristics

This topic explains file storage formats and image encoding formats as specified in ITU T.4 and T.6. Use this information to choose the most effective combination of image format characteristics for your host system and your application.

NaturalFax uses TIFF-F or TIFF-S image files. TIFF-F files (tagged image file format for fax) and TIFF-S files (tagged image file format, profile S for T.37) use tags to specify the format of the graphics file. Refer to T.37 and TIFF-S for more information about using TIFF-S image formats.

The image data in a TIFF-F or TIFF-S file has three basic characteristics:

- Encoding format (always 1D for TIFF-S)

- Resolution format (always LOW for TIFF-S)

- Page width (always A4 for TIFF-S)

## Encoding formats

NaturalFax supports four encoding formats:

Encoding format | Description
--- | ---
NFX_ENCODE_1D | One-dimensional (MH) encoding
NFX_ENCODE_2D | Two-dimensional (MR) encoding
NFX_ENCODE_MMR | MMR (modified modified read) encoding
NFX_ENCODE_TIFF_S | Encoding used for T.37 file formats (which sets TIFF-S encoding, resolution, and page width)

A fax terminal can support any of the following combinations of encoding formats:

- 1D only

- 1D and 2D

- 1D, 2D, and MMR

Fax terminals are backwards-compatible: a fax terminal cannot support 2D or MMR encoding without also supporting 1D encoding.

All three encoding formats produce identical images. No information is lost during a conversion to a different encoding format. The final image appears identical to the original image.

1D, 2D, and MMR provide the same image quality, but differ in the following attributes:

Attribute | Description
Data compression | Affects how much disk space is required for file storage and how much transmission time is needed for image transfer.
CPU resource consumption for encoding | Affects application performance and availability of CPU resources for other needs.
Impact of errors | Includes whether a local error affects one line or multiple lines, and whether error correction mode (ECM) must be enabled.
Compatibility with other fax terminals | Affects the probability that an application will need to convert an image to a different encoding in order to transmit it successfully.

MMR encoding consumes the smallest amount of disk space and the least transmission time. An equivalent image in 2D encoding consumes more disk space and transmission time than MMR, and 1D encoding consumes more disk space and transmission time than 2D.

Encoding and decoding an MMR image consumes more CPU resources than encoding and decoding a 2D image. 1D images consume the least CPU resources for encoding and decoding.

A local error in a 1D image affects a single line in the document. An error in a 2D image may affect several subsequent lines as well. An error in an MMR image affects the remainder of the page. Therefore, MMR images can only be sent with error correction mode (ECM) enabled. 1D and 2D images can be sent with or without ECM enabled.

Files stored in MMR format may have to be converted to another encoding format before they can be transmitted to some fax machines. Use MMR to minimize disk storage requirements when the system has plenty of available CPU resources for format conversions.

Note: You can use MMR encoding to store fax image files and still transmit or receive faxes with ECM disabled by using on-the-fly conversion. To enable on-the-fly conversion, refer to Performing online image conversion.

1D encoding enables files to be transmitted directly to any fax machine without conversion. Files stored in 1D format use the lowest common denominator of encoding formats. Use 1D encoding to conserve CPU resources and minimize format conversion needs.

The following table summarizes the advantages and disadvantages of each image encoding format. The image quality for all three encoding formats is identical.

Encoding | Storage space | CPU resources | An error affects... | Transmit time | ECM requirement
--- | --- | --- | --- | --- | ---
1D | Most | Least | A single line | Most | Optional
2D | More | More | Multiple lines | More | Optional
MMR | Least | Most | Remainder of page, which forces retransmission of a frame | Least | Mandatory

## Resolution formats

NaturalFax supports three resolution formats:

Resolution format | Description
--- | ---
NFX_RESOLUTION_LOW | 3.85 scan lines/mm, vertically
NFX_RESOLUTION_HIGH | 7.7 scan lines/mm, vertically
NFX_RESOLUTION_SUPER_HIGH | 15.4 scan lines/mm, vertically

A fax terminal can support only LOW resolution, both LOW and HIGH resolution, or all three resolution values, LOW, HIGH, and SUPER_HIGH. Converting an image from higher to lower resolution modifies the image data, and information is lost. The resulting image is more compact, which saves storage space and transmission time. When converting from higher to lower resolution, NaturalFax favors black pixels to prevent the loss of thin black lines.

NaturalFax can convert from lower to higher resolution by duplicating lines. However, conversion from lower to higher makes little sense since the additional redundant information does not improve image quality and requires more storage space and transmission time.

## Page width formats

NaturalFax supports the following page width formats:

Page width format | Description
--- | ---
NFX_PAGE_WIDTH_A4 | 8.25 inches wide, 200 pixels/inch resolution
NFX_PAGE_WIDTH_B4 | 10 inches wide, 200 pixels/inch resolution
NFX_PAGE_WIDTH_A3 | 11.9 inches wide, 200 pixels/inch resolution

A fax terminal may support only A4, both A4 and B4, or all three page width values, A4, B4, and A3. Converting an image from a wider to a narrower page modifies the image data, and information is lost. The resulting image is more compact, which saves storage space and transmission time. NaturalFax shrinks lines to fit so that the image is not cut off. Black pixels are favored for shrinkage to prevent the loss of thin black lines.

NaturalFax can convert from a narrower page width to a wider page width by appending white space.