diff --git a/src/zm_image.cpp b/src/zm_image.cpp
index 095fc2ae9..3a5a7218f 100644
--- a/src/zm_image.cpp
+++ b/src/zm_image.cpp
@@ -3121,6 +3121,15 @@ void Image::Fill(Rgb colour, int density, const Polygon &polygon) {
 }
 
 namespace {
+// Ceiling-divide an unsigned dimension by 2^shift. AV_CEIL_RSHIFT must NOT
+// be used here: its runtime form is -((-(a)) >> (b)), which relies on
+// arithmetic shift of a negative value and silently produces 2^31 + a/2^b
+// when `a` is unsigned (logical shift), sending plane loops billions of
+// samples out of bounds.
+inline unsigned int ceil_rshift(unsigned int a, unsigned int shift) {
+  return (a + (1u << shift) - 1) >> shift;
+}
+
 // Rotate `src_w` × `src_h` plane (bpp bytes per sample, src_linesize stride)
 // into dst (dst_linesize stride) according to angle (90/180/270).
 // For 90/270: dst dims are src_h × src_w. For 180: dst dims are src_w × src_h.
@@ -3224,10 +3233,10 @@ void Image::Rotate(int angle) {
   if (planar) {
     // Each plane has 1 byte per sample. Plane 0 is luma at full resolution;
     // planes 1/2 are chroma subsampled by desc->log2_chroma_w/h. Use ceiling
-    // (AV_CEIL_RSHIFT) — flooring would drop the last chroma column/row for
-    // odd luma dimensions and leave part of U/V unrotated.
-    const unsigned int cw = AV_CEIL_RSHIFT(width,  desc->log2_chroma_w);
-    const unsigned int ch = AV_CEIL_RSHIFT(height, desc->log2_chroma_h);
+    // division — flooring would drop the last chroma column/row for odd luma
+    // dimensions and leave part of U/V unrotated.
+    const unsigned int cw = ceil_rshift(width,  desc->log2_chroma_w);
+    const unsigned int ch = ceil_rshift(height, desc->log2_chroma_h);
     rotate_plane(src_planes[0], src_strides[0], width, height,
                  dst_planes[0], dst_strides[0], 1, angle);
     rotate_plane(src_planes[1], src_strides[1], cw, ch,
@@ -3306,8 +3315,8 @@ void Image::Flip( bool leftright ) {
   if (planar) {
     // Ceiling division so odd luma dimensions don't drop the last chroma
     // column/row.
-    const unsigned int cw = AV_CEIL_RSHIFT(width,  desc->log2_chroma_w);
-    const unsigned int ch = AV_CEIL_RSHIFT(height, desc->log2_chroma_h);
+    const unsigned int cw = ceil_rshift(width,  desc->log2_chroma_w);
+    const unsigned int ch = ceil_rshift(height, desc->log2_chroma_h);
     flip_plane(src_planes[0], src_strides[0], width, height,
                dst_planes[0], dst_strides[0], 1, leftright);
     flip_plane(src_planes[1], src_strides[1], cw, ch,
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index c13a6f3d4..600ded72a 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -17,6 +17,7 @@ set(TEST_SOURCES
   zm_comms.cpp
   zm_crypt.cpp
   zm_font.cpp
+  zm_image.cpp
   zm_onvif_renewal.cpp
   zm_pixformat.cpp
   zm_poly.cpp
diff --git a/tests/zm_image.cpp b/tests/zm_image.cpp
new file mode 100644
index 000000000..3251628dd
--- /dev/null
+++ b/tests/zm_image.cpp
@@ -0,0 +1,162 @@
+/*
+ * This file is part of the ZoneMinder Project. See AUTHORS file for Copyright information
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "zm_catch2.h"
+
+#include "zm_config.h"
+#include "zm_image.h"
+#include "zm_rgb.h"
+
+extern "C" {
+#include <libavutil/imgutils.h>
+}
+
+#include <cstring>
+
+namespace {
+
+// Image::Initialise() loads the timestamp font from the DB-backed config,
+// which the test binary doesn't have; point it at the test fixture font.
+void bootstrap_image_config() {
+  config.font_file_location = "data/fonts/04_valid.zmfnt";
+}
+
+struct Planes {
+  uint8_t *data[4] = {nullptr, nullptr, nullptr, nullptr};
+  int stride[4] = {0, 0, 0, 0};
+};
+
+// View an Image's buffer with the same layout Image uses internally
+// (av_image_fill_arrays, align 32).
+Planes plane_view(Image &image, AVPixelFormat fmt, int w, int h) {
+  Planes planes;
+  REQUIRE(av_image_fill_arrays(planes.data, planes.stride, image.Buffer(),
+                               fmt, w, h, 32) > 0);
+  return planes;
+}
+
+}  // namespace
+
+// Regression: Image::Rotate/Flip computed chroma plane dimensions with
+// AV_CEIL_RSHIFT on unsigned operands. The macro's runtime form
+// -((-(a)) >> (b)) is only valid for signed types; with unsigned width it
+// yields 2^31 + width/2, sending the chroma loops billions of samples out
+// of bounds (segfault on every rotated planar image - decoder thread crash
+// on any monitor with a rotated orientation).
+TEST_CASE("Image::Rotate YUV420P", "[image]") {
+  bootstrap_image_config();
+  const int w = 1280, h = 720;
+  Image image(w, h, ZM_COLOUR_GRAY8, ZM_SUBPIX_ORDER_YUV420P);
+  REQUIRE(image.Buffer() != nullptr);
+
+  // Distinct fill per plane + a marker pixel in each
+  Planes src = plane_view(image, AV_PIX_FMT_YUV420P, w, h);
+  memset(src.data[0], 0x10, static_cast<size_t>(src.stride[0]) * h);
+  memset(src.data[1], 0x20, static_cast<size_t>(src.stride[1]) * (h / 2));
+  memset(src.data[2], 0x30, static_cast<size_t>(src.stride[2]) * (h / 2));
+  src.data[0][20 * src.stride[0] + 10] = 200;        // luma (10, 20)
+  src.data[1][30 * src.stride[1] + 40] = 210;        // U (40, 30)
+  src.data[2][50 * src.stride[2] + 60] = 220;        // V (60, 50)
+
+  SECTION("rotate 90") {
+    image.Rotate(90);
+    REQUIRE(image.Width() == static_cast<unsigned int>(h));
+    REQUIRE(image.Height() == static_cast<unsigned int>(w));
+
+    Planes dst = plane_view(image, AV_PIX_FMT_YUV420P, h, w);
+    // 90deg: (sx, sy) -> (dx, dy) = (src_h-1-sy, sx)
+    REQUIRE(dst.data[0][10 * dst.stride[0] + (h - 1 - 20)] == 200);
+    // chroma plane is (w/2 x h/2): (40, 30) -> (h/2-1-30, 40)
+    REQUIRE(dst.data[1][40 * dst.stride[1] + (h / 2 - 1 - 30)] == 210);
+    REQUIRE(dst.data[2][60 * dst.stride[2] + (h / 2 - 1 - 50)] == 220);
+  }
+
+  SECTION("rotate 180") {
+    image.Rotate(180);
+    REQUIRE(image.Width() == static_cast<unsigned int>(w));
+    REQUIRE(image.Height() == static_cast<unsigned int>(h));
+
+    Planes dst = plane_view(image, AV_PIX_FMT_YUV420P, w, h);
+    // 180deg: (sx, sy) -> (src_w-1-sx, src_h-1-sy)
+    REQUIRE(dst.data[0][(h - 1 - 20) * dst.stride[0] + (w - 1 - 10)] == 200);
+    REQUIRE(dst.data[1][(h / 2 - 1 - 30) * dst.stride[1] + (w / 2 - 1 - 40)] == 210);
+    REQUIRE(dst.data[2][(h / 2 - 1 - 50) * dst.stride[2] + (w / 2 - 1 - 60)] == 220);
+  }
+
+  SECTION("rotate 270") {
+    image.Rotate(270);
+    REQUIRE(image.Width() == static_cast<unsigned int>(h));
+    REQUIRE(image.Height() == static_cast<unsigned int>(w));
+
+    Planes dst = plane_view(image, AV_PIX_FMT_YUV420P, h, w);
+    // 270deg: (sx, sy) -> (dx, dy) = (sy, src_w-1-sx)
+    REQUIRE(dst.data[0][(w - 1 - 10) * dst.stride[0] + 20] == 200);
+    REQUIRE(dst.data[1][(w / 2 - 1 - 40) * dst.stride[1] + 30] == 210);
+    REQUIRE(dst.data[2][(w / 2 - 1 - 60) * dst.stride[2] + 50] == 220);
+  }
+}
+
+TEST_CASE("Image::Rotate YUV420P odd dimensions", "[image]") {
+  bootstrap_image_config();
+  // Odd luma dims exercise the ceiling in the chroma plane size: 101x57
+  // luma -> 51x29 chroma.
+  const int w = 101, h = 57;
+  Image image(w, h, ZM_COLOUR_GRAY8, ZM_SUBPIX_ORDER_YUV420P);
+
+  Planes src = plane_view(image, AV_PIX_FMT_YUV420P, w, h);
+  const int cw = (w + 1) / 2, ch = (h + 1) / 2;
+  memset(src.data[0], 0x10, static_cast<size_t>(src.stride[0]) * h);
+  memset(src.data[1], 0x20, static_cast<size_t>(src.stride[1]) * ch);
+  memset(src.data[2], 0x30, static_cast<size_t>(src.stride[2]) * ch);
+  // Marker in the LAST chroma column/row - the part flooring would drop
+  src.data[1][(ch - 1) * src.stride[1] + (cw - 1)] = 211;
+
+  image.Rotate(90);
+  REQUIRE(image.Width() == static_cast<unsigned int>(h));
+  REQUIRE(image.Height() == static_cast<unsigned int>(w));
+
+  Planes dst = plane_view(image, AV_PIX_FMT_YUV420P, h, w);
+  // (cw-1, ch-1) -> (ch-1-(ch-1), cw-1) = (0, cw-1)
+  REQUIRE(dst.data[1][(cw - 1) * dst.stride[1] + 0] == 211);
+}
+
+TEST_CASE("Image::Flip YUV420P", "[image]") {
+  bootstrap_image_config();
+  const int w = 640, h = 480;
+  Image image(w, h, ZM_COLOUR_GRAY8, ZM_SUBPIX_ORDER_YUV420P);
+
+  Planes src = plane_view(image, AV_PIX_FMT_YUV420P, w, h);
+  memset(src.data[0], 0x10, static_cast<size_t>(src.stride[0]) * h);
+  memset(src.data[1], 0x20, static_cast<size_t>(src.stride[1]) * (h / 2));
+  memset(src.data[2], 0x30, static_cast<size_t>(src.stride[2]) * (h / 2));
+  src.data[0][20 * src.stride[0] + 10] = 200;
+  src.data[1][30 * src.stride[1] + 40] = 210;
+
+  SECTION("horizontal") {
+    image.Flip(true);
+    Planes dst = plane_view(image, AV_PIX_FMT_YUV420P, w, h);
+    REQUIRE(dst.data[0][20 * dst.stride[0] + (w - 1 - 10)] == 200);
+    REQUIRE(dst.data[1][30 * dst.stride[1] + (w / 2 - 1 - 40)] == 210);
+  }
+
+  SECTION("vertical") {
+    image.Flip(false);
+    Planes dst = plane_view(image, AV_PIX_FMT_YUV420P, w, h);
+    REQUIRE(dst.data[0][(h - 1 - 20) * dst.stride[0] + 10] == 200);
+    REQUIRE(dst.data[1][(h / 2 - 1 - 30) * dst.stride[1] + 40] == 210);
+  }
+}