# HG changeset patch
# User Rik <rik@octave.org>
# Date 1381712256 25200
# Node ID a433244dd697ae58274b486798cb738d38b9b62f
# Parent  7ec70c575ad64c410f18544605edf64745afe3b8
image.m: Implement better check for linearity (bug #37410)

* scripts/image/image.m: Use the percentage change from the average spacing as
a unitless measure for linearity.  If it deviates by more than 1%, print a
warning.

diff -r 7ec70c575ad6 -r a433244dd697 scripts/image/image.m
--- a/scripts/image/image.m	Sun Oct 13 19:57:35 2013 +0200
+++ b/scripts/image/image.m	Sun Oct 13 17:57:36 2013 -0700
@@ -121,12 +121,13 @@
 
   if (numel (x) > 2 && numel (y) > 2)
     ## Test data for non-linear spacing which is unsupported
-    ## FIXME: Need a better check on linearity
-    tol = 100*eps;
+    tol = .01;  # 1% tolerance.  FIXME: this value was chosen without thought.
     dx = diff (x);
+    dxmean = (max (x) - min (x)) / (numel (x) - 1);
+    dx = abs ((dx - dxmean) / dxmean);
     dy = diff (y);
-    dx = std (dx) / mean (abs (dx));
-    dy = std (dy) / mean (abs (dy));
+    dymean = (max (y) - min (y)) / (numel (y) - 1);
+    dy = abs ((dy - dymean) / dymean);
     if (any (dx > tol) || any (dy > tol))
       warning ("image: non-linear X, Y data is ignored.  IMG will be shown with linear mapping");
     endif