just for test, and after then rewrite concat's At function

concat.go

@@ -1,4 +1,5 @@
 //go:build todo
+
 package imageutils
 
 import (
@@ -7,10 +8,21 @@ import (
 	"image/draw"
 )
 
+type Side int
+
+const (
+	Left Side = iota
+	Right 
+	Up
+	Down
+)
+
 type pair struct {
 	first, second image.Image
+	side          Side
 }
 
+// Takes color.Model of the first Image.
 func (p pair) ColorModel() color.Model { return p.first.ColorModel() }
 
 func (p pair) Bounds() image.Rectangle {
@@ -19,27 +31,59 @@ func (p pair) Bounds() image.Rectangle {
 		b2 = p.second.Bounds()
 	)
 
-	return image.Rectangle{
+	rect := image.Rectangle{}
+	switch p.side {
+	case Left:
+		fallthrough
+	case Right:
+		rect = image.Rectangle{
 			image.ZP,
 			image.Point{
 				X: b1.Dx() + b2.Dx(),
 				Y: max(b1.Dy(), b2.Dy()),
 			},
 		}
+	case Up:
+		fallthrough
+	case Down:
+		rect = image.Rectangle{
+			image.ZP,
+			image.Point{
+				X: max(b1.Dx(), b2.Dx()),
+				Y: b1.Dy() + b2.Dy(),
+			},
+		}
+	}
+	return rect
 }
 
 func (p pair) At(x, y int) color.Color {
 	rgbaImg := image.NewRGBA(p.Bounds())
+
+	switch p.side {
+	case Left:
 		draw.Draw(
-		rgbaImg,
-		rgbaImg.Bounds(),
-		p.first,
-		image.ZP,
-		draw.Src,
+			rgbaImg, rgbaImg.Bounds(),
+			p.second,
+			image.ZP, draw.Src,
 		)
 		draw.Draw(
-		rgbaImg,
-		p.Bounds(),
+			rgbaImg, p.Bounds(),
+			p.first,
+			image.Point{
+				p.second.Bounds().Dx(),
+				0,
+			},
+			draw.Src,
+		)
+	case Right:
+		draw.Draw(
+			rgbaImg, rgbaImg.Bounds(),
+			p.first,
+			image.ZP, draw.Src,
+		)
+		draw.Draw(
+			rgbaImg, p.Bounds(),
 			p.second,
 			image.Point{
 				p.first.Bounds().Dx(),
@@ -47,10 +91,41 @@ func (p pair) At(x, y int) color.Color {
 			},
 			draw.Src,
 		)
-	return rgbaImg
+	case Up:
+		draw.Draw(
+			rgbaImg, rgbaImg.Bounds()
+			p.first,
+			image.ZP, draw.Src
+		)
+		draw.Draw(
+			rgbaImg, p.Bounds(),
+			p.second,
+			image.Point{
+				0,
+				p.first.Bounds().Dy()
+			},
+			draw.Src,
+		)
+	case Down:
+		draw.Draw(
+			rgbaImg, rgbaImg.Bounds()
+			p.second,
+			image.ZP, draw.Src
+		)
+		draw.Draw(
+			rgbaImg, p.Bounds(),
+			p.first,
+			image.Point{
+				0,
+				p.second.Bounds().Dy()
+			},
+			draw.Src,
+		)
+	}
+	return rgbaImg.At(x, y)
 }
 
-// Concat concatenates second image to the right side of first.
-func Concat(i1, i2 image.Image) image.Image {
-	return pair{i1, i2}
+// Concat concatenates second image on a given side.
+func Concat(i1, i2 image.Image, s Side) image.Image {
+	return pair{i1, i2, s}
 }