Driver Image operations: libfprint Reference Manual

## Driver Image operations Driver Image operations — Driver image operation functions |

## Functions

struct fp_img * | fpi_img_new ()
struct fp_img * | fpi_img_new_for_imgdev ()
struct fp_img * | fpi_img_realloc ()
struct fp_img * | fpi_img_resize ()
int | fpi_std_sq_dev ()
int | fpi_mean_sq_diff_norm ()

## Types and Values

enum | FpiImgFlags

## Includes

    #include <fpi-img.h>

## Description

Those are the driver-specific helpers for fp_img manipulation. See fp_img's documentation for more information about data formats, and their uses in front-end applications.

## Functions

### fpi_img_new ()

    struct fp_img *
    fpi_img_new (size_t length);

Creates a new fp_img structure with `length` bytes of data allocated to hold the image.

#### Parameters

length | the length of data to allocate |

#### Returns

a new fp_img to free with `fp_img_free()`

### fpi_img_new_for_imgdev ()

    struct fp_img *
    fpi_img_new_for_imgdev (struct fp_img_dev *imgdev);

Creates a new fp_img structure, like `fpi_img_new()`, but uses the driver's advertised height and width to calculate the size of the length of data to allocate.

#### Parameters

imgdev | a fp_img_dev imaging fingerprint device |

#### Returns

a new fp_img to free with `fp_img_free()`

### fpi_img_realloc ()

    struct fp_img *
    fpi_img_realloc (struct fp_img *img,
                     size_t newsize);

Changes the size of the data part of the fp_img.

#### Parameters

img | an fp_img image |
newsize | the new length of the image |

#### Returns

the modified fp_img, the same as the first argument to this function

### fpi_img_resize ()

    struct fp_img *
    fpi_img_resize (struct fp_img *img,
                    unsigned int w_factor,
                    unsigned int h_factor);

Resizes the fp_img image by scaling it by `w_factor` times horizontally and `h_factor` times vertically.

#### Parameters

img | an fp_img image |
w_factor | horizontal factor to resize the image by |
h_factor | vertical factor to resize the image by |

#### Returns

a newly allocated fp_img, the original `img` will not be modified and will also need to be freed

### fpi_std_sq_dev ()

    int
    fpi_std_sq_dev (const unsigned char *buf,
                    int size);

Calculates the squared standard deviation of the individual pixels in the buffer, as per the following formula:

    1
    2
    mean = sum (buf[0..size]) / size
    sq_dev = sum ((buf[0.size] - mean) ^ 2)

This function is usually used to determine whether image is empty.

#### Parameters

buf | buffer (usually bitmap, one byte per pixel) |
size | size of `buffer` |

#### Returns

the squared standard deviation for `buffer`

### fpi_mean_sq_diff_norm ()

    int
    fpi_mean_sq_diff_norm (unsigned char *buf1,
                           unsigned char *buf2,
                           int size);

This function calculates the normalized mean square difference of two buffers, usually two lines, as per the following formula:

    1
    sq_diff = sum ((buf1[0..size] - buf2[0..size]) ^ 2) / size

This functions is usually used to get numerical difference between two images.

#### Parameters

buf1 | buffer (usually bitmap, one byte per pixel) |
buf2 | buffer (usually bitmap, one byte per pixel) |
size | buffer size of smallest buffer |

#### Returns

the normalized mean squared difference between `buf1` and `buf2`

## Types and Values

### enum FpiImgFlags

Flags used in the fp_img structure to describe the image contained into the structure. Note that a number of functions will refuse to handle images which haven't been standardised through `fp_img_standardize()` (meaning the `FP_IMG_V_FLIPPED` , `FP_IMG_H_FLIPPED` and `FP_IMG_COLORS_INVERTED` should all be unset when the image needs to be analysed).

#### Members

FP_IMG_V_FLIPPED | the image is vertically flipped |
FP_IMG_H_FLIPPED | the image is horizontally flipped |
FP_IMG_COLORS_INVERTED | the colours are inverted |
FP_IMG_BINARIZED_FORM | binarised image, see `fp_img_binarize()` |
FP_IMG_PARTIAL | the image is partial, useful for driver to keep track of incomplete captures |