ANSI INCITS-381-2009 pdf download Information Technology – Finger Image Based Data Interchange Format
7.2.3 Dynamic range The image grayscale shall be encoded using the precision necessary to meet the dynamic range requirement of the specified level in table 1. 7.3 Pixel aspect ratio For all parameter setting levels, the finger image shall be represented using square pixels, in which the horizontal and vertical dimensions of the pixels are equal. Any difference between these two dimensions should be within 1%. That is, the ratio of horizontal to vertical pixel dimensions should be between .99 and 1.01.
7.4 Grayscale data Grayscale finger image data may be stored, recorded, or transmitted in either compressed or uncompressed form. The grayscale value for each pixel shall be within the allowable range specified by the pixel depth parameter of the appropriate level in table 1. The image data portion of a record for an uncompressed grayscale image shall contain a set of pixels. Using a pixel depth of 8 bits (256 grayscale levels) each pixel shall be contained in a single byte. Pixel values with a depth of less than eight bits can be stored and transmitted in a packed or unpacked binary format. Using the packed format, bits shall be sequentially packed, left justified, starting with the first value. Pixel values not able to fit in one byte shall be split across byte boundaries. Only the last byte of the image shall be padded with zeroes in the low order bit positions. For example, assume a finite series of 3-bit pixel values are represented as “aaa bbb ccc ddd eee fff ggg hhh iii” where each character represents a single bit. The resulting byte stream containing the packed data would be: “aaabbbcc cdddeeef ffggghhh iii00000”. Data in the unpacked format shall be right justified on byte boundaries. Increased precision for pixel values greater than 255 shall use two unsigned bytes to hold up to sixteen-bit pixels with values in the range of 0-65535. These data can be stored or transmitted in either a packed or unpacked binary format. The encoding of a compressed grayscale image shall be the output of the appropriate grayscale compression algorithm specified. Upon decompression the grayscale value for each pixel shall be represented in the same manner as pixels in an uncompressed image.
7.5 Image resolution The resolution of the image data formatted and recorded for interchange may be the scan resolution of the image or it may have been subsampled, scaled, interpolated, or otherwise processed to produce a form for representing the ridge and valley structure areas of the fingerprint. This value shall be less than or equal to the scan resolution.
7.6 Fingerprint image location Some fingerprint matching systems perform better when the fingerprint core area is included in the image. This is particularly true of systems that use core-referenced features for indexing or matching. For such systems the fingerprint image should be located in the approximate middle of the image capture area. The image from the captured finger should be placed such that the core of the print is within 25% of the image dimension from the center pixel. This standard is designed to accommodate both plain (flat) or rolled images.