Image stacks

Last updated on 14 October 2024

As of July 2024, Photoshop’s legacy 3D features have been removed. However, now in the Photoshop (beta) app you can seamlessly add 3D objects to your 2D Photoshop design using the Adobe Substance 3D Viewer (beta) app. Click here for more details.

About image stacks

An image stack combines a group of images with a similar frame of reference, but differences of quality or content across the set. Once combined in a stack, you can process the multiple images to produce a composite view that eliminates unwanted content or noise.

You can use image stacks to enhance images in number of ways:

To reduce image noise and distortion in forensic, medical, or astrophotographic images.
To remove unwanted or accidental objects from a series of stationary photos or a series of video frames. For example, you want to remove a figure walking through an image, or remove a car passing in front of the main subject matter.

Image stacks are stored as Smart Objects. The processing options you can apply to the stack are called stack modes. Applying a stack mode to an image stack is a non-destructive edit. You can change stack modes to produce different effects; the original image information in the stack remains unchanged. To preserve changes after you apply the stack mode, save the result as a new image, or rasterize the Smart Object. You can create an image stack manually or using a script.

Create an image stack

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.

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).
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.
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.
Choose Layer > Smart Objects > Convert to Smart Object.
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 them 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 can copy the Smart Object before converting, in case you want to later re‑render the image stack.)

Choose Layer > Smart Objects > Rasterize.

Use a script to create an image stack

You can use the Statistics script to automate creating and rendering an image stack.

Choose File > Scripts > Statistics.
Choose a stack mode from the Choose Stack Mode menu.
Apply the stack mode to currently open files, or browse to select a folder or individual files.

Files you select are listed in the dialog box.
If desired, select Attempt To Automatically Align Source Images (equivalent to choosing Edit > Auto-Align Layers). Then click OK.

Photoshop combines the multiple images into a single multilayered image, converts the layers into a Smart Object, and applies the selected stack mode.

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