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This is a brief introduction to cv-cat new filters:


Filter: Accumulated

This filter is used to calculate pixel-wise (and channel-wise) average from the sequential series of input images.

As it relies on the sequential accumulated input images, this filter is run in serial mode in cv-cat. This as implications when used with 'forked' image processing.

However parallel processing is utilised on image rows dimension.

Please download the following file which contains a total of 8 images: images.bin: 8 images showing movement. Viewing the images:

cat images.bin | cv-cat "view=250;null" 


Calculating averages using all accumulated input images, the output is also 8 images.

cat images.bin | cv-cat "accumulated=average;view=250;null"

The 6th output image is the average of all 6 accumulated images, the 7th is the average of the 7 accumulated input images.


Exponential Moving Average (EMA):

Calculating the average using a sliding window of images. Here a sliding window of 3 images is used.

cat images.bin | cv-cat "accumulated=average,3;view=250;null"

The output is 8 images, the 6th image is the accumulation of image 1 to 6. Please research the simple EMA formula.


Forked Arithmetic Filters: Multiply, Divide, Add and Subtract

This group of filters work similar to the mask filer: Masking images with cv-cat, they both use a sub-filters to generate a mask or operand image. 

A mask has values of 0 or '> 0' mask file to apply to the image, a corresponding pixel in the mask with a value of 0 is masked. The arithmetic filters work on operand images where is pixel value is important.


This filter will do pixel-wise multiplication the operand image and the input image. It wraps cv::multiply function.

Please download this simple mask file: scaled-3f.bin

#Viewing the mask
cat scaled-3f.bin | cv-calc header
cat scaled-3f.bin| cv-cat "view=1000;null"

Applying a single scaled image to the input images:

cat images.bin | cv-cat "multiply=load:scaled-3f.bin;view=250;null"

You should see images similar to below. scaled-3f.bin has values in the range of 0 to 1.0, the command above will darken the images.

From the example above: cv-cat's multiply is run in parallel, multiple input images are applied the scaled-3f.bin file in parallel.

This is because the all sub-filter(s) can run in parallel mode, in this case there is only one sub-filter 'load'.

The example below also shows multiply running in parallel mode as as load and threshold are parallel-able filters.

cat images.bin | cv-cat "multiply=load:scaled-3f.bin|threshold:0.7,1;view=250;null"


This filter simply subtract the operand image from each input image. This is a wrapper to cv::subtract.

The operand image is derived from the sub-filters. In this example we shall use the accumulated filter mentioned earlier. This is a simple method for detecting moving objects in the image.

cat images.bin| cv-cat "subtract=accumulated:average,3;brightness=5;view=250;null"

Each input image is subtracted the EMA average, where the EMA window is 3.

You should see similar images shown below:

In the example above: the multiply filter is run in serial mode. This is because one of the sub-filter or sub-filters ('accumulated' in this case) can only be run in serial mode.

If you have a webcam handy or it is built into the laptop, try this command:

cv-cat --camera "subtract=accumulated:average,10;view;null"


This is a wrapper to cv::add

This filter is the opposite of subtract. In this case if you add the EMA average (the "background") to the input images. Any moving object becomes transparent.

cat images.bin| cv-cat "add=accumulated:average,3;view=250;null"

This is the result:

Of course you can always try this pipeline with a physical camera:

cv-cat --camera "subtract=accumulated:average,10;view;null"



This filter wraps cv::divide, divides the input images by the operand.

The file scaled-3f.bin has values in the range 0 to 1.0, hence dividing the image by scaled-3f.bin will brighten the image.

Arithmetic filters: the output image type is the same as the input image type.

cat images.bin| cv-cat "divide=load:scaled-3f.bin;view=250;null"



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