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fastkp.go
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fastkp.go
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package keypoints
import (
"encoding/json"
"image"
"math"
"os"
"path/filepath"
"go.viam.com/utils"
)
// Implementation of FAST keypoint detector
// From paper :
// Rosten, Edward; Tom Drummond (2005). Fusing points and lines for high performance tracking.
// IEEE International Conference on Computer Vision. Vol. 2. pp. 1508–1511.
// Available at: http://www.edwardrosten.com/work/rosten_2005_tracking.pdf
// Resources:
// - https://medium.com/data-breach/introduction-to-fast-features-from-accelerated-segment-test-4ed33dde6d65
// - https://homepages.inf.ed.ac.uk/rbf/CVonline/LOCAL_COPIES/AV1011/AV1FeaturefromAcceleratedSegmentTest.pdf
// FASTConfig holds the parameters necessary to compute the FAST keypoints.
type FASTConfig struct {
NMatchesCircle int `json:"n_matches"`
NMSWinSize int `json:"nms_win_size_px"`
Threshold int `json:"threshold"` // between 0 and 255, represents a unit of grayscale intensity of the pixel
Oriented bool `json:"oriented"`
Radius int `json:"radius_px"`
}
// PixelType stores 0 if a pixel is darker than center pixel, and 1 if brighter.
type PixelType int
const (
darker PixelType = iota // 0
brighter // 1
)
// FASTPixel stores coordinates of an image point in a neighborhood and its PixelType.
type FASTPixel struct {
Point image.Point
Type PixelType
}
// FASTKeypoints stores keypoint locations and orientations (nil if not oriented).
type FASTKeypoints OrientedKeypoints
// NewFASTKeypointsFromImage returns a pointer to a FASTKeypoints struct containing keypoints locations and
// orientations if Oriented is set to true in the configuration.
func NewFASTKeypointsFromImage(img *image.Gray, cfg *FASTConfig) *FASTKeypoints {
kps := ComputeFAST(img, cfg)
var orientations []float64
if cfg.Oriented {
orientations = ComputeKeypointsOrientations(img, kps, cfg.Radius)
}
return &FASTKeypoints{
kps,
orientations,
}
}
// IsOriented returns true if FASTKeypoints contains orientations.
func (kps *FASTKeypoints) IsOriented() bool {
return !(kps.Orientations == nil)
}
var (
// CrossIdx contains the neighbors coordinates in a 3-cross neighborhood.
CrossIdx = []image.Point{{0, 3}, {3, 0}, {0, -3}, {-3, 0}}
// CircleIdx contains the neighbors coordinates in a circle of radius 3 neighborhood.
CircleIdx = []image.Point{
{0, -3},
{1, -3},
{2, -2},
{3, -1},
{3, 0},
{3, 1},
{2, 2},
{1, 3},
{0, 3},
{-1, 3},
{-2, 2},
{-3, 1},
{-3, 0},
{-3, -1},
{-2, -2},
{-1, -3},
}
)
// LoadFASTConfiguration loads a FASTConfig from a json file.
func LoadFASTConfiguration(file string) *FASTConfig {
var config FASTConfig
filePath := filepath.Clean(file)
configFile, err := os.Open(filePath)
defer utils.UncheckedErrorFunc(configFile.Close)
if err != nil {
return nil
}
jsonParser := json.NewDecoder(configFile)
err = jsonParser.Decode(&config)
if err != nil {
return nil
}
return &config
}
// GetPointValuesInNeighborhood returns a slice of floats containing the values of neighborhood pixels in image img.
func GetPointValuesInNeighborhood(img *image.Gray, coords image.Point, neighborhood []image.Point) []float64 {
vals := make([]float64, len(neighborhood))
for i := 0; i < len(neighborhood); i++ {
c := img.GrayAt(coords.X+neighborhood[i].X, coords.Y+neighborhood[i].Y).Y
vals[i] = float64(c)
}
return vals
}
func isValidSliceVals(vals []float64, n int) bool {
cnt := 0
for _, val := range vals {
// if value is positive, increment count of consecutive darker or brighter pixels in neighborhood
if val > 0 {
cnt++
} else {
// otherwise, reset count
cnt = 0
}
if cnt > n {
return true
}
}
return false
}
func sumOfPositiveValuesSlice(s []float64) float64 {
sum := 0.
for _, it := range s {
if it > 0 {
sum += it
}
}
return sum
}
func sumOfNegativeValuesSlice(s []float64) float64 {
sum := 0.
for _, it := range s {
if it < 0 {
sum += it
}
}
return sum
}
func computeNMSScore(img *image.Gray, pix FASTPixel) float64 {
val := float64(img.GrayAt(pix.Point.X, pix.Point.Y).Y)
circleValues := GetPointValuesInNeighborhood(img, pix.Point, CircleIdx)
diffValues := make([]float64, len(circleValues))
for i, v := range circleValues {
diffValues[i] = v - val
}
if pix.Type == brighter {
return sumOfPositiveValuesSlice(diffValues)
}
return -1. * sumOfNegativeValuesSlice(diffValues)
}
func canDeleteNMS(pix FASTPixel, pix2Score map[image.Point]float64, winSize int) bool {
for dx := -winSize; dx < winSize+1; dx++ {
for dy := -winSize; dy < winSize+1; dy++ {
neighbor := image.Point{pix.Point.X + dx, pix.Point.Y + dy}
if neighborScore, ok := pix2Score[neighbor]; ok && neighborScore > pix2Score[pix.Point] {
return true
}
}
}
return false
}
// nonMaximumSuppression returns maximal keypoints in a window of size 2*winSize+1 around keypoints.
func nonMaximumSuppression(img *image.Gray, kps []FASTPixel, winSize int) KeyPoints {
// compute score map
pix2score := make(map[image.Point]float64)
for _, kp := range kps {
pix2score[kp.Point] = computeNMSScore(img, kp)
}
// initialize keypoints
nmsKps := make([]image.Point, 0)
for _, kp := range kps {
if !canDeleteNMS(kp, pix2score, winSize) {
nmsKps = append(nmsKps, kp.Point)
}
}
return nmsKps
}
func computeAngle(img *image.Gray, kp image.Point, radius int, halfWidthMax []int) float64 {
w, h := img.Bounds().Max.X, img.Bounds().Max.Y
m10, m01 := 0, 0
for y := -radius + 1; y < radius; y++ {
if y+kp.Y < 0 || y+kp.Y >= h {
continue
}
currentWidth := halfWidthMax[int(math.Abs(float64(y)))]
for x := 0; x < currentWidth; x++ {
if x+kp.X < w {
m10 += x * int(img.GrayAt(x+kp.X, y+kp.Y).Y)
m01 += y * int(img.GrayAt(x+kp.X, y+kp.Y).Y)
}
}
}
return math.Atan2(float64(m01), float64(m10))
}
// ComputeKeypointsOrientations compute keypoints orientations in image.
func ComputeKeypointsOrientations(img *image.Gray, kps KeyPoints, radius int) []float64 {
halfWidthMax := make([]int, radius)
for i := 0; i < radius; i++ {
halfWidthMax[i] = int(math.Sqrt(float64(radius*radius - i*i)))
}
orientations := make([]float64, len(kps))
for i, kp := range kps {
orientations[i] = computeAngle(img, kp, radius, halfWidthMax)
}
return orientations
}
func getBrighterValues(s []float64, t float64) []float64 {
brighterValues := make([]float64, len(s))
for i, v := range s {
if v > t {
brighterValues[i] = 1
} else {
brighterValues[i] = 0
}
}
return brighterValues
}
func getDarkerValues(s []float64, t float64) []float64 {
darkerValues := make([]float64, len(s))
for i, v := range s {
if v < t {
darkerValues[i] = 1
} else {
darkerValues[i] = 0
}
}
return darkerValues
}
// ComputeFAST computes the location of FAST keypoints.
// The configuration should contain the following parameters
// - nMatchCircle - Minimum number of consecutive pixels out of 16 pixels on the
// circle that should all be either brighter or darker w.r.t
// test-pixel. A point c on the circle is darker w.r.t test pixel p
// if “Ic < Ip - threshold“ and brighter if
// “Ic > Ip + threshold“.
// - nmsWin - int, size of window to perform non-maximum suppression
// - threshold - int, Threshold used to decide whether the pixels on the
// circle are brighter, darker or similar w.r.t. the test pixel. Given in absolute units.
// Decrease the threshold when more corners are desired and
// vice-versa.
func ComputeFAST(img *image.Gray, cfg *FASTConfig) KeyPoints {
kps := make([]FASTPixel, 0)
w, h := img.Bounds().Max.X, img.Bounds().Max.Y
for y := 3; y < h-3; y++ {
for x := 3; x < w-3; x++ {
pixel := float64(img.GrayAt(x, y).Y)
circleValues := GetPointValuesInNeighborhood(img, image.Point{x, y}, CircleIdx)
brighterValues := getBrighterValues(circleValues, pixel+float64(cfg.Threshold))
darkerValues := getDarkerValues(circleValues, pixel-float64(cfg.Threshold))
if isValidSliceVals(brighterValues, cfg.NMatchesCircle) {
kps = append(kps, FASTPixel{image.Point{x, y}, brighter})
} else if isValidSliceVals(darkerValues, cfg.NMatchesCircle) {
kps = append(kps, FASTPixel{image.Point{x, y}, darker})
}
}
}
// nonMaximumSuppression
nmsKps := nonMaximumSuppression(img, kps, cfg.NMSWinSize)
return nmsKps
}