Partial cleanup of timing system, with some guidance from Joshua:

* Fractional frames support has been changed to use a new var, scene->r.subframe. 
This is a 0.0-1.0 float representing a subframe interval, used in generating a final float 
frame number to evaluate animation system etc.
* Changed frame_to_float() and some instances of bsystem_time() into a convenience function:
float BKE_curframe(scene) which retrieves the floating point current frame, after subframe
and frame length corrections.
* Removed blur_offs and field_offs globals. These are now stored in render, used to 
generate a scene->r.subframe before render database processing.

source/blender/blenkernel/BKE_scene.h

@@ -80,6 +80,8 @@ void scene_select_base(struct Scene *sce, struct Base *selbase);
 /* checks for cycle, returns 1 if it's all OK */
 int scene_check_setscene(struct Scene *sce);
 
+float BKE_curframe(struct Scene *scene);
+
 void scene_update_tagged(struct Scene *sce);
 void scene_update_for_newframe(struct Scene *sce, unsigned int lay);
 

source/blender/blenkernel/intern/action.c

@@ -1418,7 +1418,7 @@ static void do_nla(Scene *scene, Object *ob, int blocktype)
 	bActionStrip *strip, *striplast=NULL, *stripfirst=NULL;
 	float striptime, frametime, length, actlength;
 	float blendfac, stripframe;
-	float scene_cfra= frame_to_float(scene, scene->r.cfra); 
+	float scene_cfra= BKE_curframe(scene);
 	int	doit, dostride;
 	
 	if(blocktype==ID_AR) {

source/blender/blenkernel/intern/object.c

@@ -1605,20 +1605,8 @@ void object_make_proxy(Object *ob, Object *target, Object *gob)
 
 /* there is also a timing calculation in drawobject() */
 
-float bluroffs= 0.0f, fieldoffs= 0.0f;
 int no_speed_curve= 0;
 
-/* ugly calls from render */
-void set_mblur_offs(float blur)
-{
-	bluroffs= blur;
-}
-
-void set_field_offs(float field)
-{
-	fieldoffs= field;
-}
-
 void disable_speed_curve(int val)
 {
 	no_speed_curve= val;
@@ -1628,10 +1616,8 @@ void disable_speed_curve(int val)
 /* ob can be NULL */
 float bsystem_time(struct Scene *scene, Object *ob, float cfra, float ofs)
 {
-	/* returns float ( see frame_to_float in ipo.c) */
-	
-	/* bluroffs and fieldoffs are ugly globals that are set by render */
-	cfra+= bluroffs+fieldoffs;
+	/* returns float ( see BKE_curframe in scene.c) */
+	cfra += scene->r.subframe;
 	
 	/* global time */
 	if (scene)

source/blender/blenkernel/intern/particle_system.c

@@ -3925,7 +3925,7 @@ void particle_system_update(Scene *scene, Object *ob, ParticleSystem *psys)
 	if(!psys_check_enabled(ob, psys))
 		return;
 
-	cfra= bsystem_time(scene, ob, (float)scene->r.cfra, 0.0f);
+	cfra= BKE_curframe(scene);
 	sim.psmd= psys_get_modifier(ob, psys);
 
 	/* system was already updated from modifier stack */

source/blender/blenkernel/intern/scene.c

@@ -879,20 +879,14 @@ int scene_check_setscene(Scene *sce)
 	return 1;
 }
 
-/* This (evil) function is needed to cope with two legacy Blender rendering features
-* mblur (motion blur that renders 'subframes' and blurs them together), and fields 
-* rendering. Thus, the use of ugly globals from object.c
-*/
-// BAD... EVIL... JUJU...!!!!
-// XXX moved here temporarily
-float frame_to_float (Scene *scene, int cfra)		/* see also bsystem_time in object.c */
-{
-	extern float bluroffs;	/* bad stuff borrowed from object.c */
-	extern float fieldoffs;
-	float ctime;
+/* This function is needed to cope with fractional frames - including two Blender rendering features
+* mblur (motion blur that renders 'subframes' and blurs them together), and fields rendering. */
 
-	ctime= (float)cfra;
-	ctime+= bluroffs+fieldoffs;
+/* see also bsystem_time in object.c */
+float BKE_curframe(Scene *scene)
+{
+	float ctime = scene->r.cfra;
+	ctime+= scene->r.subframe;
 	ctime*= scene->r.framelen;	
 
 	return ctime;
@@ -929,7 +923,7 @@ void scene_update_tagged(Scene *scene)
 
 	/* recalc scene animation data here (for sequencer) */
 	{
-		float ctime = frame_to_float(scene, scene->r.cfra); 
+		float ctime = BKE_curframe(scene); 
 		AnimData *adt= BKE_animdata_from_id(&scene->id);
 
 		if(adt && (adt->recalc & ADT_RECALC_ANIM))
@@ -946,7 +940,7 @@ void scene_update_tagged(Scene *scene)
 /* applies changes right away, does all sets too */
 void scene_update_for_newframe(Scene *sce, unsigned int lay)
 {
-	float ctime = frame_to_float(sce, sce->r.cfra);
+	float ctime = BKE_curframe(sce);
 	Scene *sce_iter;
 	
 	/* clear animation overrides */

source/blender/editors/animation/anim_ops.c

@@ -69,6 +69,7 @@ static void change_frame_apply(bContext *C, wmOperator *op)
 	/* set the new frame number */
 	CFRA= RNA_int_get(op->ptr, "frame");
 	FRAMENUMBER_MIN_CLAMP(CFRA);
+	SUBFRA = 0.f;
 	
 	/* do updates */
 	sound_seek_scene(C);

source/blender/editors/include/ED_keyframing.h

@@ -257,7 +257,7 @@ short fcurve_frame_has_keyframe(struct FCurve *fcu, float frame, short filter);
  * Checks whether a keyframe exists for the given ID-block one the given frame.
  *  - It is recommended to call this method over the other keyframe-checkers directly,
  * 	  in case some detail of the implementation changes...
- *	- frame: the value of this is quite often result of frame_to_float(CFRA)
+ *	- frame: the value of this is quite often result of BKE_curframe()
  */
 short id_frame_has_keyframe(struct ID *id, float frame, short filter);
 

source/blender/editors/screen/screen_ops.c

@@ -1504,6 +1504,7 @@ static int frame_offset_exec(bContext *C, wmOperator *op)
 	delta = RNA_int_get(op->ptr, "delta");
 
 	CTX_data_scene(C)->r.cfra += delta;
+	CTX_data_scene(C)->r.subframe = 0.f;
 	
 	sound_seek_scene(C);
 

source/blender/editors/space_action/action_edit.c

@@ -1138,6 +1138,7 @@ static int actkeys_framejump_exec(bContext *C, wmOperator *op)
 	if (ked.i1) {
 		Scene *scene= ac.scene;
 		CFRA= (int)floor((ked.f1 / ked.i1) + 0.5f);
+		SUBFRA= 0.f;
 	}
 	
 	/* set notifier that things have changed */

source/blender/editors/space_graph/graph_edit.c

@@ -1582,6 +1582,7 @@ static int graphkeys_framejump_exec(bContext *C, wmOperator *op)
 		
 		/* take the average values, rounding to the nearest int for the current frame */
 		CFRA= (int)floor((ked.f1 / ked.i1) + 0.5f);
+		SUBFRA= 0.f;
 		sipo->cursorVal= ked.f2 / (float)ked.i1;
 	}
 	

source/blender/editors/space_graph/graph_ops.c

@@ -71,6 +71,7 @@ static void graphview_cursor_apply(bContext *C, wmOperator *op)
 	 * NOTE: sync this part of the code with ANIM_OT_change_frame
 	 */
 	CFRA= RNA_int_get(op->ptr, "frame");
+	SUBFRA=0.f;
 	sound_seek_scene(C);
 	
 	/* set the cursor value */

source/blender/editors/space_view3d/view3d_draw.c

@@ -820,7 +820,7 @@ static void draw_selected_name(Scene *scene, Object *ob, View3D *v3d)
 		}
 		
 		/* colour depends on whether there is a keyframe */
-		if (id_frame_has_keyframe((ID *)ob, /*frame_to_float(scene, CFRA)*/(float)(CFRA), v3d->keyflags))
+		if (id_frame_has_keyframe((ID *)ob, /*BKE_curframe(scene)*/(float)(CFRA), v3d->keyflags))
 			UI_ThemeColor(TH_VERTEX_SELECT);
 		else
 			UI_ThemeColor(TH_TEXT_HI);

source/blender/makesdna/DNA_scene_types.h

@@ -209,12 +209,12 @@ typedef struct RenderData {
 	struct FFMpegCodecData ffcodecdata;
 	
 	int cfra, sfra, efra;	/* frames as in 'images' */
+	float subframe;			/* subframe offset from cfra, in 0.0-1.0 */
 	int psfra, pefra;		/* start+end frames of preview range */
 
 	int images, framapto;
 	short flag, threads;
 
-	float ctime;			/* use for calcutions */
 	float framelen, blurfac;
 
 	/** For UR edge rendering: give the edges this color */
@@ -1022,6 +1022,7 @@ typedef struct Scene {
 #define ID_NEW_US(a)	if( (a)->id.newid) {(a)= (void *)(a)->id.newid; (a)->id.us++;}
 #define ID_NEW_US2(a)	if( ((ID *)a)->newid) {(a)= ((ID *)a)->newid; ((ID *)a)->us++;}
 #define	CFRA			(scene->r.cfra)
+#define SUBFRA			(scene->r.subframe)
 #define	F_CFRA			((float)(scene->r.cfra))
 #define	SFRA			(scene->r.sfra)
 #define	EFRA			(scene->r.efra)

source/blender/modifiers/intern/MOD_build.c

@@ -41,6 +41,7 @@
 #include "BKE_modifier.h"
 #include "BKE_object.h"
 #include "BKE_particle.h"
+#include "BKE_scene.h"
 
 
 static void initData(ModifierData *md)
@@ -106,7 +107,7 @@ static DerivedMesh *applyModifier(ModifierData *md, Object *ob,
 		frac = bsystem_time(md->scene, ob, md->scene->r.cfra,
 					bmd->start - 1.0f) / bmd->length;
 	} else {
-		frac = md->scene->r.cfra - bmd->start / bmd->length;
+		frac = BKE_curframe(md->scene) - bmd->start / bmd->length;
 	}
 	CLAMP(frac, 0.0, 1.0);
 

source/blender/modifiers/intern/MOD_collision.c

@@ -40,6 +40,7 @@
 #include "BKE_modifier.h"
 #include "BKE_object.h"
 #include "BKE_pointcache.h"
+#include "BKE_scene.h"
 
 
 static void initData(ModifierData *md) 
@@ -119,7 +120,7 @@ static void deformVerts(
 		CDDM_apply_vert_coords(dm, vertexCos);
 		CDDM_calc_normals(dm);
 		
-		current_time = bsystem_time (md->scene,  ob, ( float ) md->scene->r.cfra, 0.0 );
+		current_time = BKE_curframe(md->scene);
 		
 		if(G.rt > 0)
 			printf("current_time %f, collmd->time %f\n", current_time, collmd->time);

source/blender/modifiers/intern/MOD_explode.c

@@ -39,13 +39,14 @@
 #include "BLI_edgehash.h"
 
 #include "BKE_cdderivedmesh.h"
+#include "BKE_deform.h"
 #include "BKE_lattice.h"
 #include "BKE_mesh.h"
 #include "BKE_modifier.h"
 #include "BKE_object.h"
 #include "BKE_particle.h"
+#include "BKE_scene.h"
 #include "BKE_utildefines.h"
-#include "BKE_deform.h"
 
 #include "MEM_guardedalloc.h"
 
@@ -682,7 +683,7 @@ static DerivedMesh * explodeMesh(ExplodeModifierData *emd,
 	timestep= psys_get_timestep(&sim);
 
 	//if(part->flag & PART_GLOB_TIME)
-		cfra=bsystem_time(scene, 0,(float)scene->r.cfra,0.0);
+		cfra= BKE_curframe(scene);
 	//else
 	//	cfra=bsystem_time(scene, ob,(float)scene->r.cfra,0.0);
 

source/blender/modifiers/intern/MOD_wave.c

@@ -38,6 +38,7 @@
 #include "BKE_DerivedMesh.h"
 #include "BKE_object.h"
 #include "BKE_deform.h"
+#include "BKE_scene.h"
 
 #include "depsgraph_private.h"
 
@@ -247,7 +248,7 @@ static void waveModifier_do(WaveModifierData *md,
 	MVert *mvert = NULL;
 	MDeformVert *dvert = NULL;
 	int defgrp_index;
-	float ctime = bsystem_time(scene, ob, (float)scene->r.cfra, 0.0);
+	float ctime = BKE_curframe(scene);
 	float minfac =
 			(float)(1.0 / exp(wmd->width * wmd->narrow * wmd->width * wmd->narrow));
 	float lifefac = wmd->height;

source/blender/render/intern/include/render_types.h

@@ -190,6 +190,7 @@ struct Render
 	
 	/* use this instead of R.r.cfra */
 	float cfra;
+	float mblur_offs, field_offs;
 	
 	/* render database */
 	int totvlak, totvert, tothalo, totstrand, totlamp;

source/blender/render/intern/source/convertblender.c

@@ -1520,7 +1520,7 @@ static int render_new_particle_system(Render *re, ObjectRen *obr, ParticleSystem
 	RNG *rng= 0;
 	float loc[3],loc1[3],loc0[3],mat[4][4],nmat[3][3],co[3],nor[3],time;
 	float strandlen=0.0f, curlen=0.0f;
-	float hasize, pa_size, r_tilt, r_length, cfra=bsystem_time(re->scene, ob, (float)re->scene->r.cfra, 0.0);
+	float hasize, pa_size, r_tilt, r_length, cfra= BKE_curframe(re->scene);
 	float pa_time, pa_birthtime, pa_dietime;
 	float random, simplify[2];
 	int i, a, k, max_k=0, totpart, dosimplify = 0, dosurfacecache = 0;
@@ -1639,7 +1639,7 @@ static int render_new_particle_system(Render *re, ObjectRen *obr, ParticleSystem
 
 	if(part->flag & PART_GLOB_TIME)
 #endif // XXX old animation system
-	cfra = bsystem_time(re->scene, 0, (float)re->scene->r.cfra, 0.0);
+	cfra = BKE_curframe(re->scene);
 
 ///* 2.4 setup reactors */
 //	if(part->type == PART_REACTOR){

source/blender/render/intern/source/pipeline.c

@@ -1302,6 +1302,8 @@ void RE_InitState(Render *re, Render *source, RenderData *rd, SceneRenderLayer *
 	/* we clip faces with a minimum of 2 pixel boundary outside of image border. see zbuf.c */
 	re->clipcrop= 1.0f + 2.0f/(float)(re->winx>re->winy?re->winy:re->winx);
 	
+	re->mblur_offs = re->field_offs = 0.f;
+	
 	RE_init_threadcount(re);
 }
 
@@ -1762,6 +1764,7 @@ static void do_render_3d(Render *re)
 	/* internal */
 	
 //	re->cfra= cfra;	/* <- unused! */
+	re->scene->r.subframe = re->mblur_offs + re->field_offs;
 	
 	/* make render verts/faces/halos/lamps */
 	if(render_scene_needs_vector(re))
@@ -1778,6 +1781,8 @@ static void do_render_3d(Render *re)
 	
 	/* free all render verts etc */
 	RE_Database_Free(re);
+	
+	re->scene->r.subframe = 0.f;
 }
 
 /* called by blur loop, accumulate RGBA key alpha */
@@ -1877,7 +1882,7 @@ static void do_render_blur_3d(Render *re)
 	
 	/* do the blur steps */
 	while(blur--) {
-		set_mblur_offs( re->r.blurfac*((float)(re->r.mblur_samples-blur))/(float)re->r.mblur_samples );
+		re->mblur_offs = re->r.blurfac*((float)(re->r.mblur_samples-blur))/(float)re->r.mblur_samples;
 		
 		re->i.curblur= re->r.mblur_samples-blur;	/* stats */
 		
@@ -1895,7 +1900,7 @@ static void do_render_blur_3d(Render *re)
 	re->result= rres;
 	BLI_rw_mutex_unlock(&re->resultmutex);
 	
-	set_mblur_offs(0.0f);
+	re->mblur_offs = 0.0f;
 	re->i.curblur= 0;	/* stats */
 	
 	/* weak... the display callback wants an active renderlayer pointer... */
@@ -1975,15 +1980,17 @@ static void do_render_fields_3d(Render *re)
 		re->i.curfield= 2;	/* stats */
 		
 		re->flag |= R_SEC_FIELD;
-		if((re->r.mode & R_FIELDSTILL)==0) 
-			set_field_offs(0.5f);
+		if((re->r.mode & R_FIELDSTILL)==0) {
+			re->field_offs = 0.5f;
+		}
 		RE_SetCamera(re, re->scene->camera);
 		if(re->r.mode & R_MBLUR)
 			do_render_blur_3d(re);
 		else
 			do_render_3d(re);
 		re->flag &= ~R_SEC_FIELD;
-		set_field_offs(0.0f);
+		
+		re->field_offs = 0.0f;
 		
 		rr2= re->result;
 	}
@@ -2448,7 +2455,7 @@ static void do_render_seq(Render * re)
 
 	if(recurs_depth==0) {
 		/* otherwise sequencer animation isnt updated */
-		BKE_animsys_evaluate_all_animation(G.main, (float)cfra); // XXX, was frame_to_float(re->scene, cfra)
+		BKE_animsys_evaluate_all_animation(G.main, (float)cfra); // XXX, was BKE_curframe(re->scene)
 	}
 
 	recurs_depth++;

source/blender/render/intern/source/pointdensity.c

@@ -39,6 +39,7 @@
 #include "BKE_main.h"
 #include "BKE_object.h"
 #include "BKE_particle.h"
+#include "BKE_scene.h"
 #include "BKE_texture.h"
 
 #include "DNA_meshdata_types.h"
@@ -95,7 +96,7 @@ static void pointdensity_cache_psys(Render *re, PointDensity *pd, Object *ob, Pa
 	ParticleKey state;
 	ParticleSimulationData sim = {re->scene, ob, psys, NULL};
 	ParticleData *pa=NULL;
-	float cfra = bsystem_time(re->scene, ob, (float)re->scene->r.cfra, 0.0);
+	float cfra = BKE_curframe(re->scene);
 	int i, childexists;
 	int total_particles, offset=0;
 	int data_used = point_data_used(pd);

source/blender/render/intern/source/texture.c

@@ -98,7 +98,7 @@ void init_render_texture(Render *re, Tex *tex)
 	if(tex->type==TEX_PLUGIN) {
 		if(tex->plugin && tex->plugin->doit) {
 			if(tex->plugin->cfra) {
-				*(tex->plugin->cfra)= (float)cfra; //frame_to_float(re->scene, cfra); // XXX old animsys - timing stuff to be fixed 
+				*(tex->plugin->cfra)= (float)cfra; //BKE_curframe(re->scene); // XXX old animsys - timing stuff to be fixed 
 			}
 		}
 	}