Block map

Block map, a part of algorithm transparency, divides scaled image into blocks of approximately the same size. It is computed during feature extraction. Two block grids are defined, called primary and secondary, that are shifted by half a block relative to each other.

Block map is used extensively throughout the algorithm. It is used to compute clipped contrast and derived mask data, equalized image, block orientation and smoothed orientation, parallel smoothing and orthogonal smoothing, and binarized image and derived data. It is used in many places to optimize out calculations over areas outside of fingerprint mask.

KeywordSuffixZip
block-map.json005-block-map.json

Visualization

The following two visualizations were created from this stage's json data overlaid on top of the original fingerprint image. Visualizations themselves are not part of transparency data.

Visualization of primary block grid overlaid on top of the original fingerprint image
Primary block grid is pictured blue. Secondary block grid is in the background as light gray lines.
Visualization of secondary block grid overlaid on top of the original fingerprint image
Secondary block grid is pictured green. Primary block grid is in the background as light gray lines. Note that edge blocks are half the normal size. No block extends beyond image edges.

Json

Json data contains three fields.

Both primary and secondary block grids have the same structure.

Positions are measured in pixels of the scaled image. Axis y starts at the top of the image and increases towards bottom. Axis x starts at the left edge of the image and increases to the right. This orientation of axes applies to both pixels and higher level blocks.

Example: 005-block-map.json

{
  "pixels": {
    "x": 388,
    "y": 374
  },
  "primary": {
    "blocks": {
      "x": 26,
      "y": 25
    },
    "corners": {
      "x": 27,
      "y": 26
    },
    "x": [
      0,
      14,
      29,
      44,
      59,
      74,
      89,
      104,
      119,
      134,
      149,
      164,
      179,
      194,
      208,

... skipped 82 lines ...

      0,
      7,
      21,
      36,
      51,
      66,
      81,
      96,
      111,
      126,
      141,
      156,
      171,
      186,
      201,
      216,
      231,
      246,
      261,
      276,
      291,
      306,
      321,
      336,
      351,
      366,
      374
    ]
  }
}