Photoshop

Creating an image stack (Photoshop Extended)

For best results, images contained in an image stack should have the same dimensions and mostly similar content, such as a set of still images taken from a fixed viewpoint, or a series of frames from a stationary video camera. The content of your images should be similar enough to allow you to register or align them to other images in the set.

  1. Combine the separate images into one multi-layered image. See Duplicate layers.
    Note: An image stack must contain at least two layers.

    You can also combine images using a script (File > Scripts > Load Files into Stack).

  2. Choose Select > All Layers.
    Note: To make the Background layer selectable with the All Layers command, you must first convert it to a regular layer.
  3. Choose Edit > Auto-Align Layers and select Auto as the alignment option. If Auto does not create good registration of your layers, try the Reposition option.
  4. Choose Layer > Smart Objects > Convert to Smart Object.
  5. Choose Layer > Smart Objects > Stack Mode and select a stack mode from the submenu.

    • For noise reduction, use the Mean or Median plug‑ins.

    • For removing objects from the image, use the Median plug‑in.

      The output is a composite image the same size as the original image stack. You may need to experiment with different plug‑ins to get the best enhancement for a particular image.

      To change the rendering effect, choose a different Stack Mode from the submenu. Stack rendering is not cumulative—each render effect operates on the original image data in the stack and replaces previous effects.

Stack modes

Stack modes operate on a per-channel basis only, and only on non-transparent pixels. For example, the Maximum mode returns the maximum red, green, and blue channel values for a pixel cross-section and merges those into one composite pixel value in the rendered image.

Rendering plug‑in name

Result

Comments

Entropy

entropy = - sum( (probability of value) * log2( probability of value) )

Probability of value = (number of occurrences of value) / (total number of non-transparent pixels)

The binary entropy (or zero order entropy) defines a lower bound on how many bits would be necessary to losslessly encode the information in a set.

Kurtosis

kurtosis = ( sum( (value - mean)4 ) over non-transparent pixels ) / ( ( number of non-transparent pixels - 1 ) * (standard deviation)4 ).

A measure of peakedness or flatness compared to a normal distribution. The kurtosis for a standard normal distribution is 3.0. Kurtosis greater than 3 indicates a peaked distribution, and kurtosis less than 3 indicates a flat distribution (compared to a normal distribution).

Maximum

The maximum channel values for all non-transparent pixels

  

Mean

The mean channel values for all non-transparent pixels

Effective for noise reduction

Median

The median channel values for all non-transparent pixels

Effective for noise reduction and removal of unwanted content from the image

Minimum

The minimum channel values for all non-transparent pixels

  

Range

Maximum minus the minimum of the non-transparent pixel values

  

Skewness

skewness = (sum( (value - mean)3 ) over non-transparent pixels ) / ( ( number of non-transparent pixels - 1 ) * (standard deviation)3 )

Skewness is a measure of symmetry or asymmetry around the statistical mean

Standard Deviation

standard deviation = Square Root(variance)

 

Summation

The sum channel values for all non-transparent pixels

  

Variance

variance = (sum( (value-mean)2 ) over non-transparent pixels ) / ( number of non-transparent pixels - 1)

 

Remove stack rendering

 Choose Layers > Smart Objects > Stack Mode > None to remove any rendering from an image stack and convert it back to a regular Smart Object.

Edit an image stack

Because an image stack is a Smart Object, you can edit the original images that make up the stack layers at any time.

 Choose Layer > Smart Objects > Edit Contents, or double‑click the layer thumbnail. After you save the edited Smart Object, the stack is automatically rendered with the last rendering option applied to the stack.

Convert an image stack

To preserve rendering effects on an image stack, convert the Smart Object to a regular layer. (You may want to copy the Smart Object before converting, in case you want to later re‑render the image stack.)

 Choose Layer > Smart Objects > Rasterize.