Added frame offset slider to clip datablocks

In contrast to start_frame (which affects on where footage actually
starts to play and also affects on all data associated with a clip
such as motion tracking, reconstruction and so on) this slider only
affects on a way how frame number is mapping to a filename, without
touching any kind of tracking data.

The formula is:
  file_name = clip_file_name + frame_offset - (start_frame - 1)

release/scripts/startup/bl_ui/space_clip.py

@@ -1013,6 +1013,7 @@ class CLIP_PT_footage(CLIP_PT_clip_view_panel, Panel):
         col = layout.column()
         col.template_movieclip(sc, "clip", compact=True)
         col.prop(clip, "start_frame")
+        col.prop(clip, "frame_offset")
 
 
 class CLIP_PT_tools_clip(CLIP_PT_clip_view_panel, Panel):

source/blender/blenkernel/intern/movieclip.c

@@ -159,7 +159,7 @@ static void get_sequence_fname(MovieClip *clip, int framenr, char *name)
 	offset = sequence_guess_offset(clip->name, strlen(head), numlen);
 
 	if (numlen)
-		BLI_stringenc(name, head, tail, numlen, offset + framenr - clip->start_frame);
+		BLI_stringenc(name, head, tail, numlen, offset + framenr - clip->start_frame + clip->frame_offset);
 	else
 		BLI_strncpy(name, clip->name, sizeof(clip->name));
 
@@ -171,7 +171,7 @@ static void get_proxy_fname(MovieClip *clip, int proxy_render_size, int undistor
 {
 	int size = rendersize_to_number(proxy_render_size);
 	char dir[FILE_MAX], clipdir[FILE_MAX], clipfile[FILE_MAX];
-	int proxynr = framenr - clip->start_frame + 1;
+	int proxynr = framenr - clip->start_frame + 1 + clip->frame_offset;
 
 	BLI_split_dirfile(clip->name, clipdir, clipfile, FILE_MAX, FILE_MAX);
 
@@ -250,7 +250,7 @@ static ImBuf *movieclip_load_movie_file(MovieClip *clip, MovieClipUser *user, in
 		int fra;
 
 		dur = IMB_anim_get_duration(clip->anim, tc);
-		fra = framenr - clip->start_frame;
+		fra = framenr - clip->start_frame + clip->frame_offset;
 
 		if (fra < 0)
 			fra = 0;
@@ -446,6 +446,7 @@ static MovieClip *movieclip_alloc(const char *name)
 	clip->proxy.quality = 90;
 
 	clip->start_frame = 1;
+	clip->frame_offset = 1;
 
 	return clip;
 }

source/blender/editors/space_clip/clip_editor.c

@@ -523,7 +523,7 @@ typedef struct SpaceClipDrawContext {
 	unsigned last_texture;		/* ID of previously used texture, so it'll be restored after clip drawing */
 
 	/* fields to check if cache is still valid */
-	int framenr, start_frame;
+	int framenr, start_frame, frame_offset;
 	short render_size, render_flag;
 } SpaceClipDrawContext;
 
@@ -565,6 +565,7 @@ int ED_space_clip_load_movieclip_buffer(SpaceClip *sc, ImBuf *ibuf)
 	need_rebind |= context->render_size != sc->user.render_size;
 	need_rebind |= context->render_flag != sc->user.render_flag;
 	need_rebind |= context->start_frame != clip->start_frame;
+	need_rebind |= context->frame_offset != clip->frame_offset;
 
 	if (need_rebind) {
 		int width = ibuf->x, height = ibuf->y;
@@ -622,6 +623,7 @@ int ED_space_clip_load_movieclip_buffer(SpaceClip *sc, ImBuf *ibuf)
 		context->render_size = sc->user.render_size;
 		context->render_flag = sc->user.render_flag;
 		context->start_frame = clip->start_frame;
+		context->frame_offset = clip->frame_offset;
 	}
 	else {
 		/* displaying exactly the same image which was loaded t oa texture,

source/blender/makesdna/DNA_movieclip_types.h

@@ -86,7 +86,14 @@ typedef struct MovieClip {
 
 	int len;    /* length of movie */
 
-	int start_frame, pad;
+	int start_frame;    /* scene frame number footage starts playing at */
+	                    /* affects all data which is associated with a clip */
+	                    /* such as motion tracking, camera reconstruciton and so */
+
+	int frame_offset;   /* offset which is adding to a file number when reading frame */
+	                    /* from a file. affects only a way how scene frame is mapping */
+	                    /* to a file name and not touches other data associated with */
+	                    /* a clip */
 } MovieClip;
 
 typedef struct MovieClipScopes {

source/blender/makesrna/intern/rna_movieclip.c

@@ -286,10 +286,16 @@ static void rna_def_movieclip(BlenderRNA *brna)
 	RNA_def_property_ui_text(prop, "Grease Pencil", "Grease pencil data for this movie clip");
 	RNA_def_property_update(prop, NC_MOVIECLIP | ND_DISPLAY, NULL);
 
-	/* frame offset */
+	/* start_frame */
 	prop = RNA_def_property(srna, "start_frame", PROP_INT, PROP_NONE);
 	RNA_def_property_int_sdna(prop, NULL, "start_frame");
-	RNA_def_property_ui_text(prop, "Start Frame", "Global scene frame number at which this movie starts playing");
+	RNA_def_property_ui_text(prop, "Start Frame", "Global scene frame number at which this movie starts playing. Affects all data associated with a clip");
+	RNA_def_property_update(prop, NC_MOVIECLIP | ND_DISPLAY, "rna_MovieClip_reload_update");
+
+	/* frame_offset */
+	prop = RNA_def_property(srna, "frame_offset", PROP_INT, PROP_NONE);
+	RNA_def_property_int_sdna(prop, NULL, "frame_offset");
+	RNA_def_property_ui_text(prop, "Frame Offset", "Offset of footage first frame relative to it's file name. Affects only how footage is loaing, not changes data associated with a clip");
 	RNA_def_property_update(prop, NC_MOVIECLIP | ND_DISPLAY, "rna_MovieClip_reload_update");
 }