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Adding Background Motion Blur

26th Feb 2006, sjh


1) Overview

This tutorial steps through a process of adding a background motion blur behind a subject. The idea is to simulate the action of a longer shutter time while panning the camera with the subject. That leaves a motion blur on the background, and the foreground subject is sharp.

The basic approach is to blur the image, and then place a masked copy of the original on top to restore the sharpness. The main complication is in blur from the foreground leeching into the background

Before → After

Procedure Summary

  1. Duplicate original layer and apply motion blur.
  2. Add another dupliate of the original image on top
  3. Apply a layer mask to the sharp top layer to allow blur to show through
  4. Add another intermediate layer with blur in opposite direction.
  5. Mask opposing blur layer
  6. Add wheel rotation.

Currently this tutorial is only available for The GIMP. A Photoshop version may come soon, although mostly the same approach can be followed.

2) Procedure

Adding Blur

Duplicate the background layer, and on the new top layer, apply the basic desired motion blur. In this case the blur is horizontal for horizontal movement, and so has angle zero.

Restoring Foreground

This is the most time-consuming step. Start by duplicating the background again, and placing this new layer on top of the layer stack. Add a layer mask to this layer (Right-click on the layer in the layers dialog). Now you need to paint white in the mask to make the foreground appear sharp again, while leaving black in the mask to allow the blur to show through.

This is a standard masking task, and it can benefit from starting with the channel mixer on another duplicated layer in order to get the mask information from the image itself. Other approaches include just painting it in, or using a path contour make the selection

Fixing the Foreground Blur

At this stage you can see the blur of the car extending past the back. As effective as that may be in some way, it isn't correct. One way of fixing this is to reverse the direction of the blur on another layer. So again duplicate the background, and again blur, but this time in the opposite direction. Now the blurring is to the front.

Combining Blurs

That just as bad, but now we can add a mask to the top blur layer and only make it active for sections of the image where it is appropriate, only using the new blur around the back areas of the car.

There are still some areas that may not be reached, like between the two front wheels, because it is affected in both directions from the blur. These will require manual touchup. In this case they can just be masked back in on the top detail layer, and optionally some cloning or smudge/blur to match.

Select a Wheel

The image is fairly close now, but the stationary wheels are a giveaway. Here we need to add some radial blur. To do this, copy a wheel and paste it as a new layer. As of Gimp 2.2, you then set this layer to image size, and then find the pixel coordinates of the rim centre my mousing over it.

Where a wheel isn't viewed from directly flat-on, it becomes necessary to here use the scale/shear/perspective tools to turn the wheel into a circle. After blurring, the reverse transformation is applied before combining back into the image. Here it is close enough not to have to do that.

Spin the Wheel

Now go to the motion blur filter again, and this time select radial blur, with the centre coordinates of the wheel. You can fine-tune the centre by adjusting it so that there is the least blur added to the rim outline.

Complete

At this stage, it is basically complete. Here we can erase out the wheel rim itself on the rotated layer, as it doesn't blur on real rotation. Then the process is repeated for the other wheel, or in this case simply copied and overlaid.


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