(Page 8 of 13 in this chapter)

2.7 Performing Image Format Conversions

NaturalFax uses TIFF-F image files only. TIFF-F files (Tagged Image File Format for Fax) have a graphics file format characterized by the use of "tags" to specify the format of the file and the end-of-line (EOL) indication is byte-aligned.
This section explains the important characteristics of an image's format and how the format may change as various NaturalFax operations are performed.

2.7.1 Image Characteristics

In a TIFF-F file, there are three basic characteristics of the image data: encoding resolution page width
NaturalFax supports two encoding formats: NFX_ENCODE_1D one-dimensional encoding NFX_ENCODE_2D two-dimensional encoding
A fax terminal may support only 1D or it may support both 1D and 2D encoding types. A terminal may not support only 2D encoding. The conversion between 1D and 2D encoding is a one-to-one-mapping; no information is lost either way and the two images appear identical.
A 1D image consumes more data space than does the equivalent 2D image. Encoding and decoding a 2D image consumes more processor execution time. A local error in a 1D image affects a single line. An error in a 2D image may affect several subsequent lines as well. The 1D and 2D differences are summarized in the following table: Uses less data space Requires less execution time Local error Image quality 1D X affects a single line identical 2D X may affect multiple lines identical
NaturalFax supports three resolution formats: NFX_RESOLUTION_LOW 3.85 scan lines per millimeter vertically NFX_RESOLUTION_HIGH 7.7 scan lines per millimeter vertically NFX_RESOLUTION_SUPER_HIGH 15.4 scan lines per millimeter vertically
A fax terminal may support only LOW resolution, both LOW and HIGH resolution, or all three resolution values. When converting from higher to lower resolution, information is lost. The resultant image is more compact which saves storage space and transmission time. When converting from higher to lower resolution, NaturalFax favors black 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.

NaturalFax supports the following page width formats: NFX_PAGE_WIDTH_A4 8.25 inches wide at 200 pixels per inch resolution NFX_PAGE_WIDTH_B4 10 inches wide at 200 pixels per inch resolution NFX_PAGE_WIDTH_A3 11.9 inches wide at 200 pixels per inch resolution
A fax terminal may support only A4, both A4 and B4, or all three page width values. When converting from a wider to a narrower page, information is lost. The resultant 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 is favored on shrinkage to prevent the loss of thin black lines.
NaturalFax can convert from narrower to wider by appending white space.

	Uses less data space	Requires less execution time	Local error	Image quality
1D		X	affects a single line	identical
2D	X		may affect multiple lines	identical

2.7.2 Image Conversions

Each image in a TIFF-F file has a specific set of values for the image characteristics. During transmission, the image characteristics of the file may be different from the values of the image being sent or received. There are several reasons why format conversion may occur: machine restrictions Not all fax terminals can handle the full range of values for each of the characteristics. A "more capable" machine will always support the values of a "less capable" machine. At the start of a fax session, the receiver announces its capabilities (DIS message) and the sender chooses the image format for the transmission (DCS message) within the limits of those capabilities. If the file to be sent has an image format beyond the capabilities of the receiver, an image conversion must be performed. file size concerns Certain image formats use less data space than others; sometimes with loss of image quality. Using a format requiring less data saves time (and telephone charges) during transmission, and saves storage space in a file. The trade-off between data space and image quality is an application decision. processing concerns Converting between one image format and another requires host processor execution time. In a system to be loaded with the maximum number of fax operations, the execution time required for conversions can become the limiting factor. The trade-off between transmission time and host execution load is an application decision. Offline conversion of an image from one format to another can be performed to help minimize the execution load. bad line replacement NaturalFax performs bad line analysis by decoding a line and comparing its length in pixels to the line length appropriate to the page width in use. During conversion bad lines may be eliminated or replaced. document compatibility NaturalFax requires a TIFF-F file to have the same image format for all of its pages. If multiple pages in a file have different formats, the file must be converted to the same format or split into multiple files before transmitting.

(Page 8 of 13 in this chapter)

tech_support@nmss.com