Tracking: Store lens principal point in normalized space

This avoids need to do special trickery detecting whether the principal
point is to be changed when reloading movie clip. This also allows to
transfer the optical center from high-res footage to possibly its lower
resolution proxy without manual adjustment.

On a user level the difference is that the principal point is exposed in
the normalized coordinates: frame center has coordinate of (0, 0), left
bottom corner of a frame has coordinate of (-1, -1) and the right top
corner has coordinate of (1, 1).

Another user-visible change is that there is no more operator for setting
the principal point to center: use backspace on the center sliders will
reset values to 0 which corresponds to the center.

The code implements versioning in both directions, so it should be
possible to open file in older Blender versions without loosing
configuration.

For the Python API there are two ways to access the property:
- `tracking.camera.principal_point` which is measured in the normalized
  space.
- `tracking.camera.principal_point_pixels` to access the pixel-space
  principal point.

Both properties are not animatable, so there will by no conflict coming.

Differential Revision: https://developer.blender.org/D16573

release/scripts/startup/bl_ui/space_clip.py

@@ -920,8 +920,7 @@ class CLIP_PT_tracking_lens(Panel):
         col.prop(camera, "units", text="Units")
 
         col = layout.column()
-        col.prop(clip.tracking.camera, "principal", text="Optical Center")
-        col.operator("clip.set_center_principal", text="Set Center")
+        col.prop(clip.tracking.camera, "principal_point", text="Optical Center")
 
         col = layout.column()
         col.prop(camera, "distortion_model", text="Lens Distortion")
@@ -1353,7 +1352,6 @@ class CLIP_MT_clip(Menu):
 
         if clip:
             layout.operator("clip.set_scene_frames")
-            layout.operator("clip.set_center_principal")
             layout.operator("clip.prefetch")
             layout.operator("clip.reload")
             layout.menu("CLIP_MT_proxy")

source/blender/blenkernel/BKE_blender_version.h

@@ -25,7 +25,7 @@ extern "C" {
 
 /* Blender file format version. */
 #define BLENDER_FILE_VERSION BLENDER_VERSION
-#define BLENDER_FILE_SUBVERSION 1
+#define BLENDER_FILE_SUBVERSION 2
 
 /* Minimum Blender version that supports reading file written with the current
  * version. Older Blender versions will test this and show a warning if the file

source/blender/blenkernel/BKE_tracking.h

@@ -457,6 +457,12 @@ void BKE_tracking_camera_get_reconstructed_interpolate(struct MovieTracking *tra
                                                        float framenr,
                                                        float mat[4][4]);
 
+/* Access the principal point in pixels space. */
+void BKE_tracking_camera_principal_point_pixel_get(struct MovieClip *clip,
+                                                   float r_principal_point_pixel[2]);
+void BKE_tracking_camera_principal_point_pixel_set(struct MovieClip *clip,
+                                                   const float principal_point_pixel[2]);
+
 /* --------------------------------------------------------------------
  * (Un)distortion.
  */

source/blender/blenkernel/intern/movieclip.c

@@ -69,6 +69,8 @@
 
 #include "BLO_read_write.h"
 
+#include "tracking_private.h"
+
 /* Convert camera object to legacy format where the camera tracks are stored in the MovieTracking
  * structure when saving .blend file. */
 #define USE_LEGACY_CAMERA_OBJECT_FORMAT_ON_SAVE 1
@@ -221,6 +223,16 @@ static void movieclip_blend_write(BlendWriter *writer, ID *id, const void *id_ad
   }
 #endif
 
+  /* Assign the pixel-space principal point for forward compatibility. */
+  /* TODO(sergey): Remove with the next major version update when forward compatibility is allowed
+   * to be broken. */
+  if (!is_undo && clip->lastsize[0] != 0 && clip->lastsize[1] != 0) {
+    tracking_principal_point_normalized_to_pixel(tracking->camera.principal_point,
+                                                 clip->lastsize[0],
+                                                 clip->lastsize[1],
+                                                 tracking->camera.principal_legacy);
+  }
+
   BLO_write_id_struct(writer, MovieClip, id_address, &clip->id);
   BKE_id_blend_write(writer, &clip->id);
 
@@ -725,7 +737,7 @@ typedef struct MovieClipCache {
 
     /* cache for undistorted shot */
     float focal_length;
-    float principal[2];
+    float principal_point[2];
     float polynomial_k[3];
     float division_k[2];
     float nuke_k[2];
@@ -981,17 +993,6 @@ static void movieclip_load_get_size(MovieClip *clip)
   }
 }
 
-static void movieclip_principal_to_center(MovieClip *clip)
-{
-  MovieClipUser user = *DNA_struct_default_get(MovieClipUser);
-
-  int width, height;
-  BKE_movieclip_get_size(clip, &user, &width, &height);
-
-  clip->tracking.camera.principal[0] = ((float)width) / 2.0f;
-  clip->tracking.camera.principal[1] = ((float)height) / 2.0f;
-}
-
 static void detect_clip_source(Main *bmain, MovieClip *clip)
 {
   ImBuf *ibuf;
@@ -1041,7 +1042,6 @@ MovieClip *BKE_movieclip_file_add(Main *bmain, const char *name)
     clip->tracking.camera.focal = 24.0f * width / clip->tracking.camera.sensor_width;
   }
 
-  movieclip_principal_to_center(clip);
   movieclip_calc_length(clip);
 
   return clip;
@@ -1154,7 +1154,7 @@ static bool check_undistortion_cache_flags(const MovieClip *clip)
   }
 
   /* check for distortion model changes */
-  if (!equals_v2v2(camera->principal, cache->postprocessed.principal)) {
+  if (!equals_v2v2(camera->principal_point, cache->postprocessed.principal_point)) {
     return false;
   }
 
@@ -1279,7 +1279,7 @@ static void put_postprocessed_frame_to_cache(
   if (need_undistortion_postprocess(user, flag)) {
     cache->postprocessed.distortion_model = camera->distortion_model;
     cache->postprocessed.focal_length = camera->focal;
-    copy_v2_v2(cache->postprocessed.principal, camera->principal);
+    copy_v2_v2(cache->postprocessed.principal_point, camera->principal_point);
     copy_v3_v3(cache->postprocessed.polynomial_k, &camera->k1);
     copy_v2_v2(cache->postprocessed.division_k, &camera->division_k1);
     copy_v2_v2(cache->postprocessed.nuke_k, &camera->nuke_k1);
@@ -1719,26 +1719,12 @@ void BKE_movieclip_reload(Main *bmain, MovieClip *clip)
   /* update clip source */
   detect_clip_source(bmain, clip);
 
-  const int old_width = clip->lastsize[0];
-  const int old_height = clip->lastsize[1];
-
   /* Tag for re-calculation of the actual size. */
   clip->lastsize[0] = clip->lastsize[1] = 0;
 
   movieclip_load_get_size(clip);
   movieclip_calc_length(clip);
 
-  int width, height;
-  MovieClipUser user = *DNA_struct_default_get(MovieClipUser);
-  BKE_movieclip_get_size(clip, &user, &width, &height);
-
-  /* If the resolution changes then re-initialize the principal point.
-   * Ideally the principal point will be in some sort of relative space, but this is not how it is
-   * designed currently. The code should cover the most of the common cases. */
-  if (width != old_width || height != old_height) {
-    movieclip_principal_to_center(clip);
-  }
-
   BKE_ntree_update_tag_id_changed(bmain, &clip->id);
 }
 

source/blender/blenkernel/intern/tracking.c

@@ -14,6 +14,7 @@
 
 #include "DNA_anim_types.h"
 #include "DNA_camera_types.h"
+#include "DNA_defaults.h"
 #include "DNA_gpencil_types.h"
 #include "DNA_movieclip_types.h"
 #include "DNA_object_types.h" /* SELECT */
@@ -51,7 +52,7 @@
 typedef struct MovieDistortion {
   struct libmv_CameraIntrinsics *intrinsics;
   /* Parameters needed for coordinates normalization. */
-  float principal[2];
+  float principal_px[2];
   float pixel_aspect;
   float focal;
 } MovieDistortion;
@@ -2103,10 +2104,14 @@ static void reconstructed_camera_scale_set(const MovieTrackingObject *tracking_o
 void BKE_tracking_camera_shift_get(
     MovieTracking *tracking, int winx, int winy, float *shiftx, float *shifty)
 {
+  float principal_px[2];
+  tracking_principal_point_normalized_to_pixel(
+      tracking->camera.principal_point, winx, winy, principal_px);
+
   /* Indeed in both of cases it should be winx -
    * it's just how camera shift works for blender's camera. */
-  *shiftx = (0.5f * winx - tracking->camera.principal[0]) / winx;
-  *shifty = (0.5f * winy - tracking->camera.principal[1]) / winx;
+  *shiftx = (0.5f * winx - principal_px[0]) / winx;
+  *shifty = (0.5f * winy - principal_px[1]) / winx;
 }
 
 void BKE_tracking_camera_to_blender(
@@ -2167,6 +2172,32 @@ void BKE_tracking_camera_get_reconstructed_interpolate(MovieTracking *UNUSED(tra
   reconstructed_camera_scale_set(tracking_object, mat);
 }
 
+void BKE_tracking_camera_principal_point_pixel_get(struct MovieClip *clip,
+                                                   float r_principal_point_pixel[2])
+{
+  const MovieTrackingCamera *camera = &clip->tracking.camera;
+
+  int frame_width, frame_height;
+  MovieClipUser user = *DNA_struct_default_get(MovieClipUser);
+  BKE_movieclip_get_size(clip, &user, &frame_width, &frame_height);
+
+  tracking_principal_point_normalized_to_pixel(
+      camera->principal_point, frame_width, frame_height, r_principal_point_pixel);
+}
+
+void BKE_tracking_camera_principal_point_pixel_set(struct MovieClip *clip,
+                                                   const float principal_point_pixel[2])
+{
+  MovieTrackingCamera *camera = &clip->tracking.camera;
+
+  int frame_width, frame_height;
+  MovieClipUser user = *DNA_struct_default_get(MovieClipUser);
+  BKE_movieclip_get_size(clip, &user, &frame_width, &frame_height);
+
+  tracking_principal_point_pixel_to_normalized(
+      principal_point_pixel, frame_width, frame_height, camera->principal_point);
+}
+
 /* --------------------------------------------------------------------
  * (Un)distortion.
  */
@@ -2185,7 +2216,10 @@ MovieDistortion *BKE_tracking_distortion_new(MovieTracking *tracking,
   distortion->intrinsics = libmv_cameraIntrinsicsNew(&camera_intrinsics_options);
 
   const MovieTrackingCamera *camera = &tracking->camera;
-  copy_v2_v2(distortion->principal, camera->principal);
+  tracking_principal_point_normalized_to_pixel(tracking->camera.principal_point,
+                                               calibration_width,
+                                               calibration_height,
+                                               distortion->principal_px);
   distortion->pixel_aspect = camera->pixel_aspect;
   distortion->focal = camera->focal;
 
@@ -2203,7 +2237,10 @@ void BKE_tracking_distortion_update(MovieDistortion *distortion,
       tracking, calibration_width, calibration_height, &camera_intrinsics_options);
 
   const MovieTrackingCamera *camera = &tracking->camera;
-  copy_v2_v2(distortion->principal, camera->principal);
+  tracking_principal_point_normalized_to_pixel(tracking->camera.principal_point,
+                                               calibration_width,
+                                               calibration_height,
+                                               distortion->principal_px);
   distortion->pixel_aspect = camera->pixel_aspect;
   distortion->focal = camera->focal;
 
@@ -2296,8 +2333,8 @@ void BKE_tracking_distortion_distort_v2(MovieDistortion *distortion,
 
   /* Normalize coords. */
   float inv_focal = 1.0f / distortion->focal;
-  double x = (co[0] - distortion->principal[0]) * inv_focal,
-         y = (co[1] - distortion->principal[1] * aspy) * inv_focal;
+  double x = (co[0] - distortion->principal_px[0]) * inv_focal,
+         y = (co[1] - distortion->principal_px[1] * aspy) * inv_focal;
 
   libmv_cameraIntrinsicsApply(distortion->intrinsics, x, y, &x, &y);
 
@@ -2314,8 +2351,8 @@ void BKE_tracking_distortion_undistort_v2(MovieDistortion *distortion,
   libmv_cameraIntrinsicsInvert(distortion->intrinsics, x, y, &x, &y);
 
   const float aspy = 1.0f / distortion->pixel_aspect;
-  r_co[0] = (float)x * distortion->focal + distortion->principal[0];
-  r_co[1] = (float)y * distortion->focal + distortion->principal[1] * aspy;
+  r_co[0] = (float)x * distortion->focal + distortion->principal_px[0];
+  r_co[1] = (float)y * distortion->focal + distortion->principal_px[1] * aspy;
 }
 
 void BKE_tracking_distortion_free(MovieDistortion *distortion)
@@ -2336,9 +2373,13 @@ void BKE_tracking_distort_v2(
       tracking, image_width, image_height, &camera_intrinsics_options);
   libmv_CameraIntrinsics *intrinsics = libmv_cameraIntrinsicsNew(&camera_intrinsics_options);
 
+  float principal_px[2];
+  tracking_principal_point_normalized_to_pixel(
+      tracking->camera.principal_point, image_width, image_height, principal_px);
+
   /* Normalize coordinates. */
-  double x = (co[0] - camera->principal[0]) / camera->focal,
-         y = (co[1] - camera->principal[1] * aspy) / camera->focal;
+  double x = (co[0] - principal_px[0]) / camera->focal,
+         y = (co[1] - principal_px[1] * aspy) / camera->focal;
 
   libmv_cameraIntrinsicsApply(intrinsics, x, y, &x, &y);
   libmv_cameraIntrinsicsDestroy(intrinsics);
@@ -2363,8 +2404,12 @@ void BKE_tracking_undistort_v2(
   libmv_cameraIntrinsicsInvert(intrinsics, x, y, &x, &y);
   libmv_cameraIntrinsicsDestroy(intrinsics);
 
-  r_co[0] = (float)x * camera->focal + camera->principal[0];
-  r_co[1] = (float)y * camera->focal + camera->principal[1] * aspy;
+  float principal_px[2];
+  tracking_principal_point_normalized_to_pixel(
+      tracking->camera.principal_point, image_width, image_height, principal_px);
+
+  r_co[0] = (float)x * camera->focal + principal_px[0];
+  r_co[1] = (float)y * camera->focal + principal_px[1] * aspy;
 }
 
 ImBuf *BKE_tracking_undistort_frame(MovieTracking *tracking,

source/blender/blenkernel/intern/tracking_util.c

@@ -373,6 +373,30 @@ void tracking_set_marker_coords_from_tracking(int frame_width,
   marker->pos[1] += marker_unified[1];
 }
 
+void tracking_principal_point_normalized_to_pixel(const float principal_point_normalized[2],
+                                                  const int frame_width,
+                                                  const int frame_height,
+                                                  float r_principal_point_pixel[2])
+{
+  const float frame_center_x = ((float)frame_width) / 2;
+  const float frame_center_y = ((float)frame_height) / 2;
+
+  r_principal_point_pixel[0] = frame_center_x + principal_point_normalized[0] * frame_center_x;
+  r_principal_point_pixel[1] = frame_center_y + principal_point_normalized[1] * frame_center_y;
+}
+
+void tracking_principal_point_pixel_to_normalized(const float principal_point_pixel[2],
+                                                  const int frame_width,
+                                                  const int frame_height,
+                                                  float r_principal_point_normalized[2])
+{
+  const float frame_center_x = ((float)frame_width) / 2;
+  const float frame_center_y = ((float)frame_height) / 2;
+
+  r_principal_point_normalized[0] = (principal_point_pixel[0] - frame_center_x) / frame_center_x;
+  r_principal_point_normalized[1] = (principal_point_pixel[1] - frame_center_y) / frame_center_y;
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */
@@ -487,19 +511,23 @@ static void distortion_model_parameters_from_options(
 
 void tracking_cameraIntrinscisOptionsFromTracking(
     MovieTracking *tracking,
-    int calibration_width,
-    int calibration_height,
+    const int calibration_width,
+    const int calibration_height,
     libmv_CameraIntrinsicsOptions *camera_intrinsics_options)
 {
   MovieTrackingCamera *camera = &tracking->camera;
-  float aspy = 1.0f / tracking->camera.pixel_aspect;
+  const float aspy = 1.0f / tracking->camera.pixel_aspect;
+
+  float principal_px[2];
+  tracking_principal_point_normalized_to_pixel(
+      camera->principal_point, calibration_width, calibration_height, principal_px);
 
   camera_intrinsics_options->num_threads = BLI_system_thread_count();
 
   camera_intrinsics_options->focal_length = camera->focal;
 
-  camera_intrinsics_options->principal_point_x = camera->principal[0];
-  camera_intrinsics_options->principal_point_y = camera->principal[1] * aspy;
+  camera_intrinsics_options->principal_point_x = principal_px[0];
+  camera_intrinsics_options->principal_point_y = principal_px[1] * aspy;
 
   distortion_model_parameters_from_tracking(camera, camera_intrinsics_options);
 
@@ -515,8 +543,13 @@ void tracking_trackingCameraFromIntrinscisOptions(
 
   camera->focal = camera_intrinsics_options->focal_length;
 
-  camera->principal[0] = camera_intrinsics_options->principal_point_x;
-  camera->principal[1] = camera_intrinsics_options->principal_point_y / (double)aspy;
+  const float principal_px[2] = {camera_intrinsics_options->principal_point_x,
+                                 camera_intrinsics_options->principal_point_y / (double)aspy};
+
+  tracking_principal_point_pixel_to_normalized(principal_px,
+                                               camera_intrinsics_options->image_width,
+                                               camera_intrinsics_options->image_height,
+                                               camera->principal_point);
 
   distortion_model_parameters_from_options(camera_intrinsics_options, camera);
 }

source/blender/blenkernel/tracking_private.h

@@ -79,6 +79,26 @@ void tracking_set_marker_coords_from_tracking(int frame_width,
                                               const double search_pixel_x[5],
                                               const double search_pixel_y[5]);
 
+/**
+ * Convert the lens principal point (optical center) between normalized and pixel spaces.
+ *
+ * The normalized space stores principal point relative to the frame center which has normalized
+ * princibal coordinate of (0, 0). The right top corder of the frame corresponds to a notmalized
+ * principal coordinate of (1, 1), and the left bottom cornder corresponds to coordinate of
+ * (-1, -1).
+ *
+ * The pixel space is measured in pixels, with the reference being the left bottom cornder of
+ * the frame.
+ */
+void tracking_principal_point_normalized_to_pixel(const float principal_point_normalized[2],
+                                                  int frame_width,
+                                                  int frame_height,
+                                                  float r_principal_point_pixel[2]);
+void tracking_principal_point_pixel_to_normalized(const float principal_point_pixel[2],
+                                                  int frame_width,
+                                                  int frame_height,
+                                                  float r_principal_point_normalized[2]);
+
 /*********************** General purpose utility functions *************************/
 
 /**

source/blender/blenloader/intern/versioning_300.cc

@@ -34,9 +34,11 @@
 #include "DNA_material_types.h"
 #include "DNA_mesh_types.h"
 #include "DNA_modifier_types.h"
+#include "DNA_movieclip_types.h"
 #include "DNA_screen_types.h"
 #include "DNA_space_types.h"
 #include "DNA_text_types.h"
+#include "DNA_tracking_types.h"
 #include "DNA_workspace_types.h"
 
 #include "BKE_action.h"
@@ -3700,6 +3702,22 @@ void blo_do_versions_300(FileData *fd, Library * /*lib*/, Main *bmain)
     }
   }
 
+  if (!MAIN_VERSION_ATLEAST(bmain, 305, 2)) {
+    LISTBASE_FOREACH (MovieClip *, clip, &bmain->movieclips) {
+      MovieTracking *tracking = &clip->tracking;
+
+      const float frame_center_x = ((float)clip->lastsize[0]) / 2;
+      const float frame_center_y = ((float)clip->lastsize[1]) / 2;
+
+      tracking->camera.principal_point[0] = (tracking->camera.principal_legacy[0] -
+                                             frame_center_x) /
+                                            frame_center_x;
+      tracking->camera.principal_point[1] = (tracking->camera.principal_legacy[1] -
+                                             frame_center_y) /
+                                            frame_center_y;
+    }
+  }
+
   /**
    * Versioning code until next subversion bump goes here.
    *

source/blender/editors/space_clip/clip_intern.h

@@ -229,8 +229,6 @@ void CLIP_OT_set_scale(struct wmOperatorType *ot);
 void CLIP_OT_set_solution_scale(struct wmOperatorType *ot);
 void CLIP_OT_apply_solution_scale(struct wmOperatorType *ot);
 
-void CLIP_OT_set_center_principal(struct wmOperatorType *ot);
-
 void CLIP_OT_slide_marker(struct wmOperatorType *ot);
 
 void CLIP_OT_frame_jump(struct wmOperatorType *ot);

source/blender/editors/space_clip/space_clip.c

@@ -442,9 +442,6 @@ static void clip_operatortypes(void)
   /* navigation */
   WM_operatortype_append(CLIP_OT_frame_jump);
 
-  /* set optical center to frame center */
-  WM_operatortype_append(CLIP_OT_set_center_principal);
-
   /* selection */
   WM_operatortype_append(CLIP_OT_select);
   WM_operatortype_append(CLIP_OT_select_all);

source/blender/editors/space_clip/tracking_ops.c

@@ -964,47 +964,6 @@ void CLIP_OT_disable_markers(wmOperatorType *ot)
 
 /** \} */
 
-/* -------------------------------------------------------------------- */
-/** \name Set Principal Center Operator
- * \{ */
-
-static int set_center_principal_exec(bContext *C, wmOperator *UNUSED(op))
-{
-  SpaceClip *sc = CTX_wm_space_clip(C);
-  MovieClip *clip = ED_space_clip_get_clip(sc);
-  int width, height;
-
-  BKE_movieclip_get_size(clip, &sc->user, &width, &height);
-
-  if (width == 0 || height == 0) {
-    return OPERATOR_CANCELLED;
-  }
-
-  clip->tracking.camera.principal[0] = ((float)width) / 2.0f;
-  clip->tracking.camera.principal[1] = ((float)height) / 2.0f;
-
-  WM_event_add_notifier(C, NC_MOVIECLIP | NA_EDITED, clip);
-
-  return OPERATOR_FINISHED;
-}
-
-void CLIP_OT_set_center_principal(wmOperatorType *ot)
-{
-  /* identifiers */
-  ot->name = "Set Principal to Center";
-  ot->description = "Set optical center to center of footage";
-  ot->idname = "CLIP_OT_set_center_principal";
-
-  /* api callbacks */
-  ot->exec = set_center_principal_exec;
-  ot->poll = ED_space_clip_tracking_poll;
-
-  /* flags */
-  ot->flag = OPTYPE_REGISTER | OPTYPE_UNDO;
-}
-
-/** \} */
-
 /* -------------------------------------------------------------------- */
 /** \name Hide Tracks Operator
  * \{ */

source/blender/makesdna/DNA_tracking_types.h

@@ -48,8 +48,17 @@ typedef struct MovieTrackingCamera {
   /** Units of focal length user is working with. */
   short units;
   char _pad1[2];
-  /** Principal point. */
-  float principal[2];
+
+  /* Principal point (optical center) stored in normalized coordinates.
+   *
+   * The normalized space stores principal point relative to the frame center which has normalized
+   * princibal coordinate of (0, 0). The right top corder of the frame corresponds to a notmalized
+   * principal coordinate of (1, 1), and the left bottom cornder corresponds to coordinate of
+   * (-1, -1). */
+  float principal_point[2];
+
+  /** Legacy principal point in pixel space. */
+  float principal_legacy[2];
 
   /* Polynomial distortion */
   /** Polynomial radial distortion. */

source/blender/makesdna/intern/dna_rename_defs.h

@@ -95,6 +95,7 @@ DNA_STRUCT_RENAME_ELEM(MovieTracking, plane_tracks, plane_tracks_legacy)
 DNA_STRUCT_RENAME_ELEM(MovieTracking, reconstruction, reconstruction_legacy)
 DNA_STRUCT_RENAME_ELEM(MovieTracking, act_track, act_track_legacy)
 DNA_STRUCT_RENAME_ELEM(MovieTracking, act_plane_track, act_plane_track_legacy)
+DNA_STRUCT_RENAME_ELEM(MovieTrackingCamera, principal, principal_legacy)
 DNA_STRUCT_RENAME_ELEM(NodeCryptomatte, num_inputs, inputs_num)
 DNA_STRUCT_RENAME_ELEM(Object, col, color)
 DNA_STRUCT_RENAME_ELEM(Object, dup_group, instance_collection)

source/blender/makesrna/intern/rna_tracking.c

@@ -387,6 +387,20 @@ static void rna_trackingCamera_focal_mm_set(PointerRNA *ptr, float value)
   }
 }
 
+static void rna_trackingCamera_principal_point_pixels_get(PointerRNA *ptr,
+                                                          float *r_principal_point_pixels)
+{
+  MovieClip *clip = (MovieClip *)ptr->owner_id;
+  BKE_tracking_camera_principal_point_pixel_get(clip, r_principal_point_pixels);
+}
+
+static void rna_trackingCamera_principal_point_pixels_set(PointerRNA *ptr,
+                                                          const float *principal_point_pixels)
+{
+  MovieClip *clip = (MovieClip *)ptr->owner_id;
+  BKE_tracking_camera_principal_point_pixel_set(clip, principal_point_pixels);
+}
+
 static char *rna_trackingStabilization_path(const PointerRNA *UNUSED(ptr))
 {
   return BLI_strdup("tracking.stabilization");
@@ -1198,13 +1212,26 @@ static void rna_def_trackingCamera(BlenderRNA *brna)
   RNA_def_property_ui_text(prop, "Units", "Units used for camera focal length");
 
   /* Principal Point */
-  prop = RNA_def_property(srna, "principal", PROP_FLOAT, PROP_PIXEL);
+  prop = RNA_def_property(srna, "principal_point", PROP_FLOAT, PROP_NONE);
   RNA_def_property_array(prop, 2);
-  RNA_def_property_float_sdna(prop, NULL, "principal");
+  RNA_def_property_float_sdna(prop, NULL, "principal_point");
+  RNA_def_property_range(prop, -1, 1);
+  RNA_def_property_ui_range(prop, -1, 1, 0.1, 3);
   RNA_def_property_clear_flag(prop, PROP_ANIMATABLE);
   RNA_def_property_ui_text(prop, "Principal Point", "Optical center of lens");
   RNA_def_property_update(prop, NC_MOVIECLIP | NA_EDITED, NULL);
 
+  /* Principal Point, in pixels */
+  prop = RNA_def_property(srna, "principal_point_pixels", PROP_FLOAT, PROP_PIXEL);
+  RNA_def_property_array(prop, 2);
+  RNA_def_property_clear_flag(prop, PROP_ANIMATABLE);
+  RNA_def_property_float_funcs(prop,
+                               "rna_trackingCamera_principal_point_pixels_get",
+                               "rna_trackingCamera_principal_point_pixels_set",
+                               NULL);
+  RNA_def_property_ui_text(prop, "Principal Point", "Optical center of lens in pixels");
+  RNA_def_property_update(prop, NC_MOVIECLIP | NA_EDITED, NULL);
+
   /* Radial distortion parameters */
   prop = RNA_def_property(srna, "k1", PROP_FLOAT, PROP_NONE);
   RNA_def_property_float_sdna(prop, NULL, "k1");