From 63de99a5b3fa9212c5dfd7020fd6bce6f40af3b6 Mon Sep 17 00:00:00 2001
From: John Cupitt <jcupitt@gmail.com>
Date: Fri, 31 Jan 2025 18:54:32 +0000
Subject: [PATCH] fix invertlut in some cases

If the measurements filled the entire x range, we were not writing the
final value.

For example:

```
2 2
0 0
1 1
```

```
$ vips invertlut linear.mat x2.v
$ vips getpoint x2.v 255 0
-nan
```

With this PR you get 1, as expected.
---
 ChangeLog                  | 1 +
 libvips/create/invertlut.c | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/ChangeLog b/ChangeLog
index ae0098ca47..9fa1247943 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -19,6 +19,7 @@
 - fill_nearest: fix a leak
 - colour: use suggested rendering intent as fallback [kleisauke]
 - morph: fix Orc path with large masks [kleisauke]
+- invertlut: fix final value in some cases
 
 10/10/24 8.16.0
 
diff --git a/libvips/create/invertlut.c b/libvips/create/invertlut.c
index ff24aab7eb..02a89baa4c 100644
--- a/libvips/create/invertlut.c
+++ b/libvips/create/invertlut.c
@@ -212,7 +212,7 @@ vips_invertlut_build_create(VipsInvertlut *lut)
 
 		/* Interpolate the data sections.
 		 */
-		for (k = first; k < last; k++) {
+		for (k = first; k <= last; k++) {
 			/* Where we're at in the [0,1] range.
 			 */
 			double ki = (double) k / (lut->size - 1);