Alembic: added frame offset property

The offset is subtracted, so that it's intuitive (dragging slider pushes
the animation further that way on the timeline).

source/blender/blenkernel/intern/cachefile.c

@@ -211,8 +211,9 @@ bool BKE_cachefile_filepath_get(
 
 float BKE_cachefile_time_offset(CacheFile *cache_file, const float time, const float fps)
 {
+	const float time_offset = cache_file->frame_offset / fps;
 	const float frame = (cache_file->override_frame ? cache_file->frame : time);
-	return cache_file->is_sequence ? frame : frame / fps;
+	return cache_file->is_sequence ? frame : frame / fps - time_offset;
 }
 
 /* TODO(kevin): replace this with some depsgraph mechanism, or something similar. */

source/blender/editors/interface/interface_templates.c

@@ -4041,6 +4041,9 @@ void uiTemplateCacheFile(uiLayout *layout, bContext *C, PointerRNA *ptr, const c
 	uiLayoutSetEnabled(row, RNA_boolean_get(&fileptr, "override_frame"));
 	uiItemR(row, &fileptr, "frame", 0, "Frame", ICON_NONE);
 
+	row = uiLayoutRow(layout, false);
+	uiItemR(row, &fileptr, "frame_offset", 0, "Frame Offset", ICON_NONE);
+
 	row = uiLayoutRow(layout, false);
 	uiItemL(row, IFACE_("Manual Transform:"), ICON_NONE);
 

source/blender/makesdna/DNA_cachefile_types.h

@@ -75,9 +75,12 @@ typedef struct CacheFile {
 
 	float scale;
 	float frame;  /* The frame/time to lookup in the cache file. */
+	float frame_offset; /* The frame offset to subtract. */
 
 	short flag;  /* Animation flag. */
 	short draw_flag;
+
+	char padding[4];
 } CacheFile;
 
 #ifdef __cplusplus

source/blender/makesrna/intern/rna_cachefile.c

@@ -148,6 +148,15 @@ static void rna_def_cachefile(BlenderRNA *brna)
 	                                        " or to determine which file to use in a file sequence");
 	RNA_def_property_update(prop, 0, "rna_CacheFile_update");
 
+	prop = RNA_def_property(srna, "frame_offset", PROP_FLOAT, PROP_NONE);
+	RNA_def_property_float_sdna(prop, NULL, "frame_offset");
+	RNA_def_property_range(prop, -MAXFRAME, MAXFRAME);
+	RNA_def_property_ui_text(prop, "Frame Offset",
+	                         "Subtracted from the current frame to use for "
+	                         "looking up the data in the cache file, or to "
+	                         "determine which file to use in a file sequence");
+	RNA_def_property_update(prop, 0, "rna_CacheFile_update");
+
 	/* ----------------- Axis Conversion ----------------- */
 
 	prop = RNA_def_property(srna, "forward_axis", PROP_ENUM, PROP_NONE);

source/blender/modifiers/intern/MOD_meshsequencecache.c

@@ -109,7 +109,6 @@ static DerivedMesh *applyModifier(ModifierData *md, Object *ob,
 	Scene *scene = md->scene;
 	const float frame = BKE_scene_frame_get(scene);
 	const float time = BKE_cachefile_time_offset(mcmd->cache_file, frame, FPS);
-
 	const char *err_str = NULL;
 
 	CacheFile *cache_file = mcmd->cache_file;