Dont use extra memory for mirror operations

tools/utils/images/transforms.go

@@ -0,0 +1,55 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package images
+
+import (
+	"fmt"
+)
+
+var _ = fmt.Print
+
+func reverse_row(bytes_per_pixel int, pix []uint8) {
+	if len(pix) <= bytes_per_pixel {
+		return
+	}
+	i := 0
+	j := len(pix) - bytes_per_pixel
+	for i < j {
+		pi := pix[i : i+bytes_per_pixel : i+bytes_per_pixel]
+		pj := pix[j : j+bytes_per_pixel : j+bytes_per_pixel]
+		for x := 0; x < bytes_per_pixel; x++ {
+			pi[x], pj[x] = pj[x], pi[x]
+		}
+		i += bytes_per_pixel
+		j -= bytes_per_pixel
+	}
+}
+
+func FlipPixelsH(bytes_per_pixel, width, height int, pix []uint8) {
+	stride := bytes_per_pixel * width
+	parallel(0, height, func(ys <-chan int) {
+		for y := range ys {
+			i := y * stride
+			reverse_row(bytes_per_pixel, pix[i:i+stride])
+		}
+	})
+}
+
+func FlipPixelsV(bytes_per_pixel, width, height int, pix []uint8) {
+	stride := bytes_per_pixel * width
+	num := height / 2
+	parallel(0, num, func(ys <-chan int) {
+		for y := range ys {
+			upper := y
+			lower := height - 1 - y
+			a := upper * stride
+			b := lower * stride
+			as := pix[a : a+stride : a+stride]
+			bs := pix[b : b+stride : b+stride]
+			for i := range as {
+				as[i], bs[i] = bs[i], as[i]
+			}
+		}
+	})
+
+}