From fbf026653b117f8363f9b49ce0d803669c45d026 Mon Sep 17 00:00:00 2001
From: Pekka Paalanen <pekka.paalanen@collabora.com>
Date: Thu, 13 Mar 2025 16:43:52 +0200
Subject: [PATCH] color-lcms: cmsMAT3 is row-major after all
Found out in https://github.com/mm2/Little-CMS/issues/483
This should be a complete no-op change, but there is one difference.
Previously matrix_inf_norm() summed over rows. Now it sums over columns
as it should be. This shouldn't hurt because it is only used to identify
identity matrices. The bug was actually in is_identity_matrix_stage()
because it deliberately passed what was assumed to be a transpose of the
matrix.
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
---
libweston/color-lcms/color-transform.c | 18 +++++-------------
tests/color_util.h | 3 +++
2 files changed, 8 insertions(+), 13 deletions(-)
diff --git a/libweston/color-lcms/color-transform.c b/libweston/color-lcms/color-transform.c
index 643186a97e..3da96958b3 100644
--- a/libweston/color-lcms/color-transform.c
+++ b/libweston/color-lcms/color-transform.c
@@ -192,7 +192,7 @@ matrix_inf_norm(const cmsMAT3 *mat)
double sum = 0.0;
for (col = 0; col < 3; col++)
- sum += fabs(mat->v[col].n[row]);
+ sum += fabs(mat->v[row].n[col]);
if (infnorm < sum)
infnorm = sum;
@@ -222,9 +222,9 @@ matrix_is_identity(const cmsMAT3 *mat, int bits_precision)
}
static const cmsMAT3 *
-stage_matrix_transpose(const _cmsStageMatrixData *smd)
+stage_matrix(const _cmsStageMatrixData *smd)
{
- /* smd is row-major, cmsMAT3 is column-major */
+ /* Both are row-major. */
return (const cmsMAT3 *)smd->Double;
}
@@ -259,7 +259,7 @@ is_identity_matrix_stage(const cmsStage *stage)
return false;
data = cmsStageData(stage);
- return matrix_is_identity(stage_matrix_transpose(data),
+ return matrix_is_identity(stage_matrix(data),
MATRIX_PRECISION_BITS);
}
@@ -274,15 +274,7 @@ multiply_matrix_stages(cmsContext context_id, cmsStage *next, cmsStage *prev)
prev_ = cmsStageData(prev);
next_ = cmsStageData(next);
- /* res = prev^T * next^T */
- _cmsMAT3per(&res, stage_matrix_transpose(next_),
- stage_matrix_transpose(prev_));
-
- /*
- * res is column-major while Alloc function takes row-major;
- * the cast effectively transposes the matrix.
- * We return (prev^T * next^T)^T = next * prev.
- */
+ _cmsMAT3per(&res, stage_matrix(next_), stage_matrix(prev_));
ret = cmsStageAllocMatrix(context_id, 3, 3,
(const cmsFloat64Number*)&res, NULL);
abort_oom_if_null(ret);
diff --git a/tests/color_util.h b/tests/color_util.h
index 46817dfc13..52a765c3fc 100644
--- a/tests/color_util.h
+++ b/tests/color_util.h
@@ -53,6 +53,9 @@ struct lcmsVEC3 {
float n[3];
};
+/*
+ * While LittleCMS' cmsMAT3 is row-major, this here is colum-major.
+ */
struct lcmsMAT3 {
/* array of columns */
struct lcmsVEC3 v[3];
--
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