From 6fbdefa21f7c603bf57ff3f0b07eab1626e0b8d2 Mon Sep 17 00:00:00 2001
From: Elias Naur <mail@eliasnaur.com>
Date: Mon, 2 Dec 2019 13:58:12 +0100
Subject: [PATCH] app/headless: compensate for OpenGL's lower left origin

The origin of image.Images is the upper left corner. Convert the
ReadPixels result by flipping the image the y-direction.

Signed-off-by: Elias Naur <mail@eliasnaur.com>
---
 app/headless/headless.go | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/app/headless/headless.go b/app/headless/headless.go
index 88f4d8c5..0628ecbf 100644
--- a/app/headless/headless.go
+++ b/app/headless/headless.go
@@ -111,6 +111,17 @@ func (w *Window) Screenshot() (*image.RGBA, error) {
 		}
 		return nil
 	})
+	// Flip image in y-direction. OpenGL's origin is in the lower
+	// left corner.
+	row := make([]uint8, img.Stride)
+	for y := 0; y < w.size.Y/2; y++ {
+		y1 := w.size.Y - y - 1
+		dest := img.PixOffset(0, y1)
+		src := img.PixOffset(0, y)
+		copy(row, img.Pix[dest:])
+		copy(img.Pix[dest:], img.Pix[src:src+len(row)])
+		copy(img.Pix[src:], row)
+	}
 	return img, nil
 }