Increase Face AF successful Rate in Lowlight and Backlit Scenes

1 Issue description

A、There may be some little blur images during Face AF consistency test.

B、From log, only one step difference during fine search leads to little blur image.

C、Tried different AF filter doesn’t help this scene  e.g. IIR[0.05 0.12] IIR[0.2 0.35]

2 Solution

Enable AF reserved parameter to extend Face ROI for AF to cover model’s neckwear or more details.

2.1 Tuning parameters

// FACE ROI Extension parameters 

// af_algo.reserve[47]: AF_CHROMATIX_EN_FD_ROI_TUNE 
// Default value: 0 
// Data range: [0 - 1] 
// Description: Unsigned Integer. 
// 1 to enable AF FD ROI adjustment, 0 to disable 
0.000000f, /* AF_CHROMATIX_EN_FD_ROI_TUNE */ 

// af_algo.reserve[91]: AF_ROI_FACE_EXTENSION 
// Default value: 0 // Data range: [0, 1] 
// Description: a switch to turn on face ROI extension which extend ROI region from face 
// detection region and gravity information to include hair region and neck region to 
// avoid flat focus value. 
// Constraints: enable to include hair region, but may including some background region.
0.000000f, /* AF_ROI_FACE_EXTENSION */ 

// af_algo.reserve[92]: AF_ROI_FACE_SCALE_X 
// Default value: 0.05 // Data range: [0 - 1] 
// Description: Face ROI width will be (1+scale) times of width of face detection region. 
// Constraints: set too large will include background region. Should not set too high due 
// to no face region in extra horizontal region. 
0.050000f, /* AF_ROI_FACE_SCALE_X */ 

// af_algo.reserve[93]: AF_ROI_FACE_SCALE_Y 
// Default value: 0.5
// Data range: [0 - 1] 
// Description: Face ROI height will be (1+scale) times of height of face detection region. 
// Constraints: this parameter enlarge vertical ROI region from face detection region and 
// include hair region and neck region. It will avoid flat focus region when face is too smooth. 
0.500000f, /* AF_ROI_FACE_SCALE_Y */ 

// af_algo.reserve[94]: AF_ROI_FACE_SYMMETIRC_X 
// Default value: 0.5 
// Data range: [0 - 1] 
// Description: decide how face ROI extend to left/right side. Value 0.5 means symmetric 
// extension. 0 means extend to left. 1 means extend to right. 
// Constraints: 
0.500000f, /* AF_ROI_FACE_SYMMETIRC_X */ 

// af_algo.reserve[95]: AF_ROI_FACE_SYMMETIRC_Y 
// Default value: 0.25 // Data range: [0 - 1] 
// Description: decide how face ROI extend to up/down side. Value 0.5 means symmetric 
// extension. 1 means extend to upside. 
// 0 means extend to downside. 
// Constraints: 
0.250000f, /* AF_ROI_FACE_SYMMETIRC_Y */

2.2 Tuning Example

For above case, red rectangle is original face ROI, blue rectangle is face ROI extension based on tuningparameters.

AF_CHROMATIX_EN_FD_ROI_TUNE = 1
AF_ROI_FACE_EXTENSION = 1
AF_ROI_FACE_SCALE_X = 0.05
AF_ROI_FACE_SCALE_Y = 0.5
AF_ROI_FACE_SYMMETIRC_X = 0.5
AF_ROI_FACE_SYMMETIRC_Y = 0.25

X extend ratio is 0.05, and X symmetry ratio is 0.5, thus the ROI is extended towards left to 0.025 timesof width, towards right to 0.025 times of width.

Y extend ratio is 0.5, and Y symmetry ratio is 0.25, thus the ROI is extended towards top to 0.125 timesof height, towards bottom to 0.375 times of height.

With extended FD ROI, FV curve for same scene become sharper since the model’s neckwear which hasmore details was included in the ROI, as shown in below diagram.