tchigher / imagecli

A command line image processing tool

imagecli

A command line image processing tool, built on top of image and imageproc.

• Installation
• Contributing
• Basic usage
• Multi-stage pipelines
• The image stack
• User-defined functions
• Operations

Installation

Either install using cargo, or clone this repo and build it locally.

cargo install imagecli

There are currently no pre-built binaries available.

Contributing

See CONTRIBUTING.md.

Basic usage

The simplest possible usage loads an image and then saves an identical copy of it.

imagecli --input robin.png --output ex0_0.png

To be slightly more useful, we can specify a different format for the output image. (See image for a list of supported formats.)

imagecli --input robin.png --output ex1_0.jpg

To make the remaining examples less verbose, we'll switch to using the short forms -i and -o of --input and --output.

To do anything more interesting than changing image formats, we need to define an image processing pipeline via --pipeline, or -p. This chains together a series of one or more operations.

For example, the following command line converts an image to grayscale.

imagecli -i robin.png -o ex2_0.png -p 'gray'

And the following rotates an image about its center by 45 degrees.

imagecli -i robin.png -o ex3_0.png -p 'rotate 45'

Multi-stage pipelines

You can apply multiple operations in a row by chaining them together using >. For example, the following pipeline converts an image to grayscale, rotates it by 30 degrees, and then computes its gradient using the Sobel filter.

imagecli -i robin.png -o ex0_0.png -p 'gray > rotate 30 > sobel'

The image stack

All of the pipelines shown thus far have taken a single image as input and produced a single image as output. However, we also support operations mapping multiple inputs to a single output, as well as operations mapping a single input to multiple outputs. This is handled via an implicit image stack: all input images are pushed onto the top of an image stack, and each operation pops one or more images from the top of the stack, applies some transformation, and pushes one or more output images back onto the stack. All images provided via the command line are pushed onto the image stack before we start running the pipeline, and when the pipeline completes we save the contents of the image stack to the --output paths.

Multiple inputs, single output

The following command line takes two images as input and applies the hcat operation, which horizontally concatenates a pair of images.

imagecli -i robin.png robin_gray.png -o ex0_0.png -p 'hcat'

The diagram below shows the state of the image stack as this pipeline is run.

    |
    | input images are pushed onto the stack
    v
 --------------------
 * robin
   robin_gray
 --------------------
    |
    | hcat pops two images
    v
 --------------------
 --------------------
    |
    | hcat computes result and pushes it onto the stack
    v
 --------------------
 * result
 --------------------
    |
    | result is saved to the specified output path
    v

Single input, multiple outputs

There aren't currently any image processing operations that produce multiple outputs from a single input. However, pipelines can also contain stack operations - operations that directly manipulate the image stack. These all have upper case names to make it easier to distinguish between image processing operations and stack operations.

For example, the DUP operation duplicates the top element of the stack. The following example loads a single image and then saves two copies of it.

imagecli -i robin.png -o ex0_0.png ex0_1.png -p 'DUP'

    |
    | input image is pushed onto the stack
    v
 --------------------
 * robin
 --------------------
    |
    | DUP duplicates the top of the stack
    v
 --------------------
 * robin
   robin
 --------------------
    |
    | results are saved to the specified output paths
    v

Multiple inputs, multiple outputs

As described above, each operation in a pipeline pops a fixed number of images from the top of the stack, applies a transformation to these images and pushes the results back onto the stack. This means that the following example only applies the specified blur function to the first input image.

imagecli -i robin.png robin_gray.png -o ex0_0.png ex0_1.png -p 'gaussian 5.0'

(Note that in the following diagrams we combine popping from the stack, applying a transformation, and pushing the result into a single step.)

    |
    | input images are pushed onto the stack
    v
 --------------------
 * robin
   robin_gray
 --------------------
    |
    | gaussian pops the top of the stack, transforms it, and pushes the result
    v
 --------------------
 * results
   robin_gray
 --------------------
    |
    | results are saved to the specified output paths
    v

This may not be what you wanted! If you want to apply the gaussian operation to both of the two images in the stack you have two options. The verbose option uses the SWAP stack operation to manually swap the order of the two elements in the stack. (The less verbose option uses an operation array, which is described later). SWAP is an alias for ROT 2, where the ROT operation rotates the positions of the top n elements of the stack - the top element moves n positions down the stack and the other the other top elements on the stack move up one.

 --------------------
 * first
   second
   third
 --------------------
    |
    | ROT 2
    v
 --------------------
 * second
   first
   third
 --------------------
    |
    | ROT 3
    v
 --------------------
 * first
   third
   second
 --------------------
    |
    | ROT 3
    v
 --------------------
 * third
   second
   first
 --------------------

The following command line uses SWAP to apply a Gaussian blur to both input images. Notice the second SWAP operation, which ensures that the outputs are in the same order as the inputs.

imagecli -i robin.png robin_gray.png -o ex1_0.png ex1_1.png -p 'gaussian 5.0 > SWAP > gaussian 5.0 > SWAP'

    |
    | push input images
    v
 --------------------
 * robin
   robin_gray
 --------------------
    |
    | the first gaussian operation transforms the top of the stack
    v
 --------------------
 * robin_blurred
   robin_gray
 --------------------
    |
    | SWAP swaps the order of the two stack elements
    v
 --------------------
 * robin_gray
   robin_blurred
 --------------------
    |
    | the second gaussian operation transforms the top of the stack
    v
 --------------------
 * robin_gray_blurred
   robin_blurred
 --------------------
    |
    | SWAP swaps the order of the two stack elements
    v
 --------------------
 * robin_blurred
   robin_gray_blurred
 --------------------
    |
    | save result
    v

As manually rotating through the image stack can be a bit verbose, we also support an array syntax which applies the nth in a series of operations to the nth element in the stack. For example, the following command line applies a Gaussian blur to the first image, and a blur with larger radius to the second.

imagecli -i robin.png robin_gray.png -o ex2_0.png ex2_1.png -p '[gaussian 2.0, gaussian 6.0]'

The description above assumes that each operation in the array consumes a single input and produces a single result. Array operations are actually more general than this, as the operations within them may consume more than one input or produce more than one result. In this case each operation is applied to the stack in turn, and the results pushed by each operation are popped into temporary storage before applying the next. Finally, all of the results are pushed to the stack. DUP n is treated as consuming 1 image and creating n + 1 results, and ROT n is always treated as producing no outputs.

If this explanation isn't clear then look through the stack diagram for the example below. Or don't - you'll probably never have cause to use this behaviour!

imagecli -i yellow.png robin.png robin_gray.png -o ex3_0.png -p '[DUP, hcat] > [vcat, id] > hcat'

    |
    | push input images
    v
 --------------------
 * yellow
   robin
   robin_gray
 --------------------
    |
    | DUP is applied to the first image in the stack, and hcat to the remaining two
    v
 --------------------
 * yellow
   yellow
   robins
 --------------------
    |
    | vcat is applied to the first two images in the stack, and id to the last image
    v
 --------------------
 * yellows
   robins
 --------------------
    |
    | the two images are horizontally concatenated
    v
 --------------------
 * yellows_robins
 --------------------
    |
    | save results
    v

User-defined functions

We provide limited support for user-defined functions via the func, func2 and func3 operations. These operations allow you to specify a function to run on each subpixel of an image. Functions are arithmetic expressions defined in terms of the following components:

• Binary arithmetic operators +, -, /, * and ^.
• Parentheses '(' and ')'.
• Numerical constants, e.g. 4.0.
• Coordinate variables x and y. x increases from left to right and y from top to bottom.
• Variables p, q and r. p is the value of the current subpixel in the first image, q and r in the second and third images.
• Comparison operators <, > and =. These evaluate to 1.0 if true and 0.0 if false.

The following function applies a diagonal gradient to an image, increasing its brightness towards its bottom right.

imagecli -i robin.png -o ex0_0.png -p 'func { p + x / 2 + y / 2 }'

The following example converts an image to grayscale and then applies a binary threshold. (This is equivalent to thresh 120, but takes longer to run and requires first converting to grayscale - otherwise the threshold would be applied independently to each channel.)

imagecli -i robin.png -o ex1_0.png -p 'gray > func { 255 * (p > 120) }'

Our final example uses func2 to apply a user-defined function to a pair of images.

imagecli -i robin.png -o ex2_0.png -p 'DUP 2 > const 184 268 (255, 255, 0) > DUP > ROT 3 > func2 { (p + q) / 2 } > ROT 3 > hcat 3'

Current limitations:

• All input images to func2 and func3 are required to be the same size.
• Applying different functions to each subpixel (or a function from pixels to pixels) is not yet supported.
• All images are converted to RGBA before applying the function.
• There is not yet support for calling out to other functions (e.g. min, max, sin).

Operations

The following conventions are used to describe the arguments taken by each operation.

• <foo>: a required positional argument. For example, rotate <angle> accepts the input rotate 17.
• [foo]: an optional positional argument. For example, hcat [count] accepts both hcat and hcat 5.
• , ..: repetition - one or more occurrences of the preceding argument, separated by commas. For example, op <val>, .. accepts op 1 and op 1, 2.
• (LEFT|RIGHT): something that matches either LEFT, or RIGHT. For example, resize (<width> <height>|w=<width>) accepts both resize 100 100 and resize w=100.
• 'T': the literal character T, if T is given special meaning above. For example, op '[' <val> ']' accepts the input op [ 10 ].

Follow the links for a more detailed description, including any restrictions on the inputs (for example that a value must be an integer, or must be strictly positive).

Operation	Usage	Description
Array	[IMAGE_OP, .. ]	Applies a series of image operations to the stack.
AdaptiveThreshold	athresh <block_radius>	Binarises an image using adaptive thresholding.
Blue	blue	Extracts the blue channel from an image as a grayscale image.
Carve	carve <width_ratio>	Shrinks an image's width using seam carving.
Circle	circle <filltype> <cx> <cy> <radius> '('COLOR')'	Draws a circle on an image.
Const	const <width> <height> '('COLOR')'	Creates an image with a single constant value.
Crop	crop <left> <top> <width> <height>	Extracts a rectangular region from an image.
Dup	DUP [count]	Duplicates the top element of the image stack count times. count defaults to 1 if not provided.
Func	func { EXPR }	Applies a user-provided function to each subpixel in an image.
Func2	func2 { EXPR }	Applies a user-provided function pairwise to the subpixels in two images.
Func3	func3 { EXPR }	Applies a user-provided function pairwise to the subpixels in three images.
Gaussian	gaussian <standard_deviation>	Applies a Gaussian blur to an image.
Gray	gray	Converts an image to grayscale.
Green	green	Extracts the green channel from an image as a grayscale image.
Grid	grid <columns> <rows>	Arranges a series of images into a grid.
HFlip	hflip	Flips an image horizontally.
Id	id	Applies the identity function, i.e. does nothing.
Median	median <x_radius> <y_radius>	Applies a median filter to an image.
OtsuThreshold	othresh	Binarises an image using Otsu thresholding.
Overlay	overlay <left> <top>	Overlays the second image on the stack onto the first.
Red	red	Extracts the red channel from an image as a grayscale image.
Resize	resize (<width> <height>|w=<width>|h=<height>|w=<width> h=<height>)	Resizes an image to the given dimensions.
Rot	ROT [count]	Rotates the top count elements of the stack by 1.
Rotate	rotate <angle>	Rotates an image clockwise about its center.angle gives the angle of rotation in degrees.
Scale	scale <ratio>	Scales image width and height by ratio.
Sobel	sobel	Computes image gradients using the Sobel filter.
Threshold	thresh	Binarises an image using a user-defined threshold.
Translate	translate <tx> <ty>	Translates an image by (tx, ty).
VFlip	vflip	Flips an image vertically.

Array

Usage: [IMAGE_OP, .. ]

Applies a series of image operations to the stack.

If each operation consumes a single input and produces a single image as a result then the nth operation is applied to the nth image in the stack.

In the more general case we first walk through each operation, apply it to the stack and pop all of its results. We then push all the results to the stack.

Examples

imagecli -i robin.png -o Array_0_0.png -p 'DUP 3 > [id, red, green, blue] > hcat 4'

AdaptiveThreshold

Usage: athresh <block_radius>

Binarises an image using adaptive thresholding.

block_radius is required to be an integer >= 0. Each pixel is compared to those in the block around it with side length 2 * block_radius + 1.

Examples

imagecli -i robin.png -o AdaptiveThreshold_0_0.png -p 'athresh 10'

Blue

Usage: blue

Extracts the blue channel from an image as a grayscale image.

Examples

imagecli -i robin.png -o Blue_0_0.png -p 'DUP > [id, blue] > hcat'

Carve

Usage: carve <width_ratio>

Shrinks an image's width using seam carving.

width_ratio is required to be a floating point number <= 1.0. The output image has width width_ratio * input_image_width.

Examples

imagecli -i robin.png -o Carve_0_0.png -p 'carve 0.85'

Circle

Usage: circle <filltype> <cx> <cy> <radius> '('COLOR')'

Draws a circle on an image.

filltype can be either hollow or filled. color can be: grayscale: (12), grayscale with alpha: (12, 255), RGB: (255, 0, 255), or RGBA: (128, 128, 0, 255).

Examples

imagecli -i robin.png -o Circle_0_0.png -p 'circle filled 80 40 50 (255, 255, 0)'

Const

Usage: const <width> <height> '('COLOR')'

Creates an image with a single constant value.

color can be grayscale: (12), grayscale with alpha: (12, 255), RGB: (255, 0, 255), or RGBA: (128, 128, 0, 255). Note that this consumes an image from the stack.

Examples

imagecli -i robin.png -o Const_0_0.png -p 'const 300 250 (255, 255, 0)'

Crop

Usage: crop <left> <top> <width> <height>

Extracts a rectangular region from an image.

Returns a copy of the image region with inclusive top left point (left, top) and dimensions (width, height).

Examples

imagecli -i robin.png -o Crop_0_0.png -p 'crop 10 50 100 150'

Dup

Usage: DUP [count]

Duplicates the top element of the image stack count times. count defaults to 1 if not provided.

Func

Usage: func { EXPR }

Applies a user-provided function to each subpixel in an image.

See the user-defined functions section of the user guide for more information.

Examples

imagecli -i robin.png -o Func_0_0.png -p 'func { p + x / 3 + y / 3 }'

imagecli -i robin.png -o Func_1_0.png -p 'gray > func { 255 * (p > 100) }'

Func2

Usage: func2 { EXPR }

Applies a user-provided function pairwise to the subpixels in two images.

See the user-defined functions section of the user guide for more information.

Examples

imagecli -i robin.png -o Func2_0_0.png -p 'DUP 2 > const 184 268 (255, 255, 0) > DUP > ROT 3 > func2 { (p + q) / 2 } > ROT 3 > hcat 3'

Func3

Usage: func3 { EXPR }

Applies a user-provided function pairwise to the subpixels in three images.

See the user-defined functions section of the user guide for more information.

Gaussian

Usage: gaussian <standard_deviation>

Applies a Gaussian blur to an image.

Examples

imagecli -i robin.png -o Gaussian_0_0.png -p 'gaussian 10.0'

Gray

Usage: gray

Converts an image to grayscale.

Examples

imagecli -i robin.png -o Gray_0_0.png -p 'gray'

Green

Usage: green

Extracts the green channel from an image as a grayscale image.

Examples

imagecli -i robin.png -o Green_0_0.png -p 'DUP > [id, green] > hcat'

Grid

Usage: grid <columns> <rows>

Arranges a series of images into a grid.

Aliases: hcat is equivalent to Grid 2 1, hcat n is equivalent to Grid n 1, vcat is equivalent to Grid 1 2, vcat n is equivalent to Grid 1 n.

Examples

imagecli -i robin.png -o Grid_0_0.png -p 'DUP 3 > [gaussian 1.0, gaussian 3.0, gaussian 5.0,gaussian 7.0] > grid 2 2'

imagecli -i robin.png -o Grid_1_0.png -p 'scale 0.5 > DUP 5 > [scale 1.0, scale 0.9, scale 0.8, scale 0.7, scale 0.6, scale 0.5] > grid 3 2'

HFlip

Usage: hflip

Flips an image horizontally.

Examples

imagecli -i robin.png -o HFlip_0_0.png -p 'DUP > [id, hflip] > hcat'

Id

Usage: id

Applies the identity function, i.e. does nothing.

This makes some pipelines more concise to write.

Median

Usage: median <x_radius> <y_radius>

Applies a median filter to an image.

The filter applied has width 2 * x_radius + 1 and height 2 * y_radius + 1.

Examples

imagecli -i robin.png -o Median_0_0.png -p 'median 4 4'

OtsuThreshold

Usage: othresh

Binarises an image using Otsu thresholding.

Examples

imagecli -i robin.png -o OtsuThreshold_0_0.png -p 'othresh'

Overlay

Usage: overlay <left> <top>

Overlays the second image on the stack onto the first.

Places the second image with its top left corner at (left, top ) on the first image, cropping if it does not fit.

Examples

imagecli -i robin.png -o Overlay_0_0.png -p 'DUP > const 184 268 (255, 255, 0) > overlay 10 50'

Red

Usage: red

Extracts the red channel from an image as a grayscale image.

Examples

imagecli -i robin.png -o Red_0_0.png -p 'DUP > [id, red] > hcat'

Resize

Usage: resize (<width> <height>|w=<width>|h=<height>|w=<width> h=<height>)

Resizes an image to the given dimensions.

If only one of width or height is provided then the target for the other dimension is chosen to preserve the image's aspect ratio.

Examples

imagecli -i robin.png -o Resize_0_0.png -p 'resize w=100'

Rot

Usage: ROT [count]

Rotates the top count elements of the stack by 1.

count defaults to 3 if not provided. Aliases: SWAP is equivalent to ROT 2.

Rotate

Usage: rotate <angle>

Rotates an image clockwise about its center.angle gives the angle of rotation in degrees.

Examples

imagecli -i robin.png -o Rotate_0_0.png -p 'rotate 45'

Scale

Usage: scale <ratio>

Scales image width and height by ratio.

Examples

imagecli -i robin.png -o Scale_0_0.png -p 'scale 0.7'

Sobel

Usage: sobel

Computes image gradients using the Sobel filter.

Examples

imagecli -i robin.png -o Sobel_0_0.png -p 'sobel'

Threshold

Usage: thresh

Binarises an image using a user-defined threshold.

Images are first converted to grayscale. Thresholds should be >=0 and < 256.

Examples

imagecli -i robin.png -o Threshold_0_0.png -p 'thresh 120'

Translate

Usage: translate <tx> <ty>

Translates an image by (tx, ty).

Positive values of tx move the image to the right, and positive values of ty move it downwards.

Examples

imagecli -i robin.png -o Translate_0_0.png -p 'DUP > [translate 10 20, translate -10 -20] > hcat'

VFlip

Usage: vflip

Flips an image vertically.

Examples

imagecli -i robin.png -o VFlip_0_0.png -p 'DUP > [id, vflip] > hcat'