Patch #36622, by Henrik Aarnio: Fit backdrop image to the area dimensions.

A new operator to alter the backdrop zoom level so that it fits fully within the node editor area, and centers the image.
Shortcut alt-home, as home is used for fitting stuff into the view everywhere.

release/scripts/startup/bl_ui/space_node.py

@@ -152,6 +152,7 @@ class NODE_MT_view(Menu):
             layout.operator("node.backimage_move", text="Backdrop move")
             layout.operator("node.backimage_zoom", text="Backdrop zoom in").factor = 1.2
             layout.operator("node.backimage_zoom", text="Backdrop zoom out").factor = 0.833
+            layout.operator("node.backimage_fit", text="Fit backdrop to available space")
 
         layout.separator()
 
@@ -361,6 +362,8 @@ class NODE_PT_backdrop(Panel):
         col.prop(snode, "backdrop_x", text="X")
         col.prop(snode, "backdrop_y", text="Y")
         col.operator("node.backimage_move", text="Move")
+        
+        layout.operator("node.backimage_fit", text="Fit")
 
 
 class NODE_PT_quality(bpy.types.Panel):

source/blender/editors/space_node/node_intern.h

@@ -129,6 +129,7 @@ void NODE_OT_view_selected(struct wmOperatorType *ot);
 
 void NODE_OT_backimage_move(struct wmOperatorType *ot);
 void NODE_OT_backimage_zoom(struct wmOperatorType *ot);
+void NODE_OT_backimage_fit(struct wmOperatorType *ot);
 void NODE_OT_backimage_sample(struct wmOperatorType *ot);
 
 /* drawnode.c */

source/blender/editors/space_node/node_ops.c

@@ -98,6 +98,7 @@ void node_operatortypes(void)
 	
 	WM_operatortype_append(NODE_OT_backimage_move);
 	WM_operatortype_append(NODE_OT_backimage_zoom);
+	WM_operatortype_append(NODE_OT_backimage_fit);
 	WM_operatortype_append(NODE_OT_backimage_sample);
 	
 	WM_operatortype_append(NODE_OT_add_file);
@@ -249,6 +250,7 @@ void node_keymap(struct wmKeyConfig *keyconf)
 	RNA_float_set(kmi->ptr, "factor", 0.83333f);
 	kmi = WM_keymap_add_item(keymap, "NODE_OT_backimage_zoom", VKEY, KM_PRESS, KM_ALT, 0);
 	RNA_float_set(kmi->ptr, "factor", 1.2f);
+	WM_keymap_add_item(keymap, "NODE_OT_backimage_fit", HOMEKEY, KM_PRESS, KM_ALT, 0);
 	WM_keymap_add_item(keymap, "NODE_OT_backimage_sample", ACTIONMOUSE, KM_PRESS, KM_ALT, 0);
 
 	kmi = WM_keymap_add_item(keymap, "NODE_OT_link_make", FKEY, KM_PRESS, 0, 0);

source/blender/editors/space_node/node_view.c

@@ -327,6 +327,60 @@ void NODE_OT_backimage_zoom(wmOperatorType *ot)
 	RNA_def_float(ot->srna, "factor", 1.2f, 0.0f, 10.0f, "Factor", "", 0.0f, 10.0f);
 }
 
+static int backimage_fit_exec(bContext *C, wmOperator *op)
+{
+	SpaceNode *snode = CTX_wm_space_node(C);
+	ARegion *ar = CTX_wm_region(C);
+
+	Image *ima;
+	ImBuf *ibuf;
+
+	const float pad = 32.0f;
+
+	void *lock;
+
+	float facx, facy;
+
+	ima = BKE_image_verify_viewer(IMA_TYPE_COMPOSITE, "Viewer Node");
+	ibuf = BKE_image_acquire_ibuf(ima, NULL, &lock);
+
+	if (ibuf == NULL) {
+		BKE_image_release_ibuf(ima, ibuf, lock);
+		return OPERATOR_CANCELLED;
+	}
+
+	facx = 1.0f * (ar->sizex - pad) / (ibuf->x * snode->zoom);
+	facy = 1.0f * (ar->sizey - pad) / (ibuf->y * snode->zoom);
+
+	BKE_image_release_ibuf(ima, ibuf, lock);
+
+	snode->zoom *= min_ff(facx, facy);
+
+	snode->xof = 0;
+	snode->yof = 0;
+
+	ED_region_tag_redraw(ar);
+
+	return OPERATOR_FINISHED;
+}
+
+void NODE_OT_backimage_fit(wmOperatorType *ot)
+{
+
+	/* identifiers */
+	ot->name = "Background Image Fit";
+	ot->idname = "NODE_OT_backimage_fit";
+	ot->description = "Zoom in/out the background image";
+
+	/* api callbacks */
+	ot->exec = backimage_fit_exec;
+	ot->poll = composite_node_active;
+
+	/* flags */
+	ot->flag = OPTYPE_BLOCKING;
+
+}
+
 /******************** sample backdrop operator ********************/
 
 typedef struct ImageSampleInfo {