Can frames with no animation be automatically skipped?

Question

I have an animation with lots of text objects flying about.

I'd like to render it with motion blur and stuff, but there are quite a few long sections where nothing is moving so that the text can be read. In these sections the render output (should) remain exactly the same.

Since it's pretty pointless to render the same image over and over, is there some way to automatically detect and skip those sections, only rendering one frame until there are more keyframes?

Answer 1

I had the same problem, solved it with a bit of code. This script renders your animation frame for frame, skipping frames with no animation and adding them to render destination as copies of their equivalents.

This gives you the same result as a normal animation render, and saves you a lot of processing time.

The script does the following:

• Take each animated object and store its F-Curve values at each frame in the animation.

• Create list of still frames, by comparing differences in neighbouring F-Curve values for each frame in each animated object.

• Render frames in desired range, skipping each still frame.

• Fill in skipped frames with copies of their equivalents.

    import bpy
    import subprocess
  
    def render_with_skips(start, stop):
        """
        Take start and stop, and render animation only for animated
        frames. Still frames, are substituted into the output folder
        as copies of their equivalents.
        """
  
        render_range = list(range(start, stop))
  
        # create JSON like dictionary to store each
        # animated object's fcurve data at each frame.
        all_obj_fcurves = {}
        for obj in bpy.data.objects:    
            obj_fcurves = {}
  
            try:
                obj.animation_data.action.fcurves
            except AttributeError:
                print("--|'%s' is not animated" % obj.name)
                continue
  
            print("\n--> '%s' is animated at frames:" % obj.name)
  
            for fr in list(range(start,stop+1)):
                fc_evals = [c.evaluate(fr) for c in obj.animation_data.action.fcurves]
                obj_fcurves.update({int(fr): fc_evals})
                print(fr, end=", ")
            print()
  
            all_obj_fcurves.update({obj.name: obj_fcurves})
  
  
        # loop through each animated object and find its
        # animated frames. then remove those frames from
        # a set containing all frames, to get still frames.
        still_frames = set(render_range)
        for obj in all_obj_fcurves.keys():
            obj_animated_frames = []
            for i, fr in enumerate(sorted(all_obj_fcurves[obj].keys())):
                if i != 0:
                    if all_obj_fcurves[obj][fr] != all_obj_fcurves[obj][fr_prev]:
                        obj_animated_frames.append(fr)
                fr_prev = fr
  
            still_frames = still_frames - set(obj_animated_frames)
  
        print("\nFound %d still frames" % len(still_frames))
        print(sorted(still_frames), end="\n\n")
  
  
        # render animation, skipping the still frames and
        # filling them in as copies of their equivalents
        filepath = bpy.context.scene.render.filepath
  
        for fr in render_range:
            if fr not in still_frames or fr == render_range[0]:
                bpy.context.scene.frame_set(fr)
                bpy.context.scene.render.filepath = filepath + '%04d' % fr
                bpy.ops.render.render(write_still=True)
            else:
                scene = bpy.context.scene
                abs_filepath = scene.render.frame_path(scene.frame_current)
                abs_path = '/'.join(abs_filepath.split('/')[:-1]) + '/'
                print("Frame %d is still, copying from equivalent" % fr)
                subprocess.call(['cp', abs_path + '%04d.png' % (fr-1), abs_path + '%04d.png' % fr])
  
        bpy.context.scene.render.filepath = filepath
  
    start = bpy.data.scenes['Scene'].frame_start
    end = bpy.data.scenes['Scene'].frame_end
    render_with_skips(start,end)
  

I should stress that this will only work if you do not have anything else moving around at the times when everything else in your camera view is still.

NOTE At this moment the code is only Linux/OSX compatible because it uses a subprocess to copy frames which is not Windows compatible. MarcHorstmanns's answer below implements my code so it works with Windows.

Answer 2

For Windows:

import bpy
import subprocess
import os
import shutil

def render_with_skips(start, stop):
    """
    Take start and stop, and render animation only for animated
    frames. Still frames, are substituted into the output folder
    as copies of their equivalents.
    """

    render_range = list(range(start, stop))

    # create JSON like dictionary to store each
    # animated object's fcurve data at each frame.
    all_obj_fcurves = {}
    for obj in bpy.data.objects:    
        obj_fcurves = {}

        try:
            obj.animation_data.action.fcurves
        except AttributeError:
            print("--|'%s' is not animated" % obj.name)
            continue

        print("\n--> '%s' is animated at frames:" % obj.name)

        for fr in list(range(start,stop+1)):
            fc_evals = [c.evaluate(fr) for c in obj.animation_data.action.fcurves]
            obj_fcurves.update({int(fr): fc_evals})
            print(fr, end=", ")
        print()

        all_obj_fcurves.update({obj.name: obj_fcurves})


    # loop through each animated object and find its
    # animated frames. then remove those frames from
    # a set containing all frames, to get still frames.
    still_frames = set(render_range)
    for obj in all_obj_fcurves.keys():
        obj_animated_frames = []
        for i, fr in enumerate(sorted(all_obj_fcurves[obj].keys())):
            if i != 0:
                if all_obj_fcurves[obj][fr] != all_obj_fcurves[obj][fr_prev]:
                    obj_animated_frames.append(fr)
            fr_prev = fr

        still_frames = still_frames - set(obj_animated_frames)

    print("\nFound %d still frames" % len(still_frames))
    print(sorted(still_frames), end="\n\n")


    # render animation, skipping the still frames and
    # filling them in as copies of their equivalents
    filepath = bpy.context.scene.render.filepath

    for fr in render_range:
        if fr not in still_frames or fr == render_range[0]:
            bpy.context.scene.frame_set(fr)
            bpy.context.scene.render.filepath = filepath + '%04d' % fr
            bpy.ops.render.render(write_still=True)
        else:
            scene = bpy.context.scene
            abs_filepath = scene.render.frame_path(scene.frame_current)
            #abs_path = '/'.join(abs_filepath.split('/')[:-1]) + '/'
            print("Frame %d is still, copying from equivalent" % fr)
            scn = bpy.context.scene
            shutil.copyfile(filepath + '%04d.png' % (fr-1), filepath + '%04d.png' % fr)

    bpy.context.scene.render.filepath = filepath

start = bpy.data.scenes['Scene'].frame_start
end = bpy.data.scenes['Scene'].frame_end
render_with_skips(start,end)

Answer 3

Automatically, no. You might want to duplicate the scene with Linked objects. Then have each scene render a different frame range which you determine manually. You can then stitch the rendered frames together using yet another scene that uses the VSE (with no 3D objects) and Image Strips.