From d9c6a724e97141db9cbaa66226e0410a5535ae28 Mon Sep 17 00:00:00 2001
From: Nick White <git@njw.name>
Date: Thu, 23 Jul 2020 15:01:41 +0100
Subject: Add Set and NewImage functions to implement image/draw.Image
 interface

Now the integralimg can be created directly from an image
using image/draw.Draw.
---
 integralimg.go      | 40 ++++++++++++++++++++++++++++++++++------
 integralimg_test.go |  5 ++---
 2 files changed, 36 insertions(+), 9 deletions(-)

diff --git a/integralimg.go b/integralimg.go
index 17d3332..a0cb565 100644
--- a/integralimg.go
+++ b/integralimg.go
@@ -28,22 +28,50 @@ func (i I) At(x, y int) color.Color {
 		return color.Gray{}
 	}
 
-	var oldx, oldy, oldxy uint64
-	oldx, oldy, oldxy = 0, 0, 0
+	var prevx, prevy, prevxy uint64
+	prevx, prevy, prevxy = 0, 0, 0
 	if x > 0 {
-		oldx = i[y][x-1]
+		prevx = i[y][x-1]
 	}
 	if y > 0 {
-		oldy = i[y-1][x]
+		prevy = i[y-1][x]
 	}
 	if x > 0 && y > 0 {
-		oldxy = i[y-1][x-1]
+		prevxy = i[y-1][x-1]
 	}
-	orig := i[y][x] + oldxy - oldx - oldy
+	orig := i[y][x] + prevxy - prevx - prevy
 
 	return color.Gray{uint8(orig)}
 }
 
+func (i I) Set(x, y int, c color.Color) {
+	var prevx, prevy, prevxy uint64
+	prevx, prevy, prevxy = 0, 0, 0
+	if x > 0 {
+		prevx = i[y][x-1]
+	}
+	if y > 0 {
+		prevy = i[y-1][x]
+	}
+	if x > 0 && y > 0 {
+		prevxy = i[y-1][x-1]
+	}
+	gray := color.GrayModel.Convert(c).(color.Gray).Y
+	final := uint64(gray) + prevx + prevy - prevxy
+	i[y][x] = final
+}
+
+// NewImage returns a new Integral Image with the given bounds.
+func NewImage(r image.Rectangle) *I {
+	w, h := r.Dx(), r.Dy()
+	var rows I
+	for i := 0; i < h; i++ {
+		col := make([]uint64, w)
+		rows = append(rows, col)
+	}
+	return &rows
+}
+
 // Sq contains an Integral Image and its Square
 type WithSq struct {
 	Img I
diff --git a/integralimg_test.go b/integralimg_test.go
index 3bc8dd6..df97caa 100644
--- a/integralimg_test.go
+++ b/integralimg_test.go
@@ -23,10 +23,9 @@ func TestFromPNG(t *testing.T) {
 		t.Fatalf("Could not decode image: %v\n", err)
 	}
 	b := img.Bounds()
-	gray := image.NewGray(image.Rect(0, 0, b.Dx(), b.Dy()))
-	draw.Draw(gray, b, img, b.Min, draw.Src)
 
-	integral := ToIntegralImg(gray)
+	integral := NewImage(image.Rect(0, 0, b.Dx(), b.Dy()))
+	draw.Draw(integral, b, img, b.Min, draw.Src)
 
 	if !imgsequal(img, integral) {
 		t.Errorf("Read png image differs to integral\n")
-- 
cgit v1.2.1-24-ge1ad