fbpx

Designing Advanced Image Filters in OpenCV | Creating Instagram Filters – Pt 3⁄3

By Taha Anwar and Rizwan Naeem

On February 22, 2022

Watch Video Here

In the previous tutorial of this series, we had covered Look Up Tables in-depth and utilized them to create some interesting lighting effects on images/videos. Now in this one, we are gonna level up the game by creating 10 very interesting and cool Instagram filters.

The Filters which are gonna be covered are; Warm Filter, Cold Filter, Gotham Filter, GrayScale Filter, Sepia Filter, Pencil Sketch Filter, Sharpening Filter, Detail Enhancing Filter, Invert Filter, and Stylization Filter.

You must have used at least one of these and maybe have wondered how these are created, what’s the magic (math) behind these. We are gonna cover all this in-depth in today’s tutorial and you will learn a ton of cool image transformation techniques with OpenCV so buckle up and keep reading the tutorial.

This is the last tutorial of our 3 part Creating Instagram Filters series. All three posts are titled as:

3-4 Filters in this tutorial use Look Up Tables (LUT) which were explained in the previous tutorial, so make sure to go over that one if you haven’t already. Also, we have used mouse events to switch between filters in real-time and had covered mouse events in the first post of the series, so go over that tutorial as well if you don’t know how to use mouse events in OpenCV.

The tutorial is pretty simple and straightforward, but for a detailed explanation you can check out the YouTube video above, although this blog post alone does have enough details to help you follow along.

Download Code:

Outline

We will be creating the following filters-like effects in this tutorial.

  1. Warm Filter
  2. Cold Filter
  3. Gotham Filter
  4. GrayScale Filter
  5. Sepia Filter
  6. Pencil Sketch Filter
  7. Sharpening Filter
  8. Detail Enhancing Filter
  9. Invert Filter
  10. Stylization Filter

Alright, so without further ado, let’s dive in.

Import the Libraries

We will start by importing the required libraries.

Creating Warm Filter-like Effect

The first filter is gonna be the famous Warm Effect, it absorbs blue cast in images, often caused by electronic flash or outdoor shade, and improves skin tones. This gives a kind of warm look to images that’s why it is called the Warm Effect. To apply this to images and videos, we will create a function applyWarm() that will decrease the pixel intensities of the blue channel and increase the intensities of the red channel of an image/frame by utilizing Look Up tables ( that we learned about in the previous tutorial).

So first, we will have to construct the Look Up Tables required to increase/decrease pixel intensities. For this purpose, we will be using the scipy.interpolate.UnivariateSpline() function to get the required input-output mapping.

Output:

First 10 elements from the increase table:
[7.32204295e-15 1.03827895e+00 2.08227359e+00 3.13191257e+00
4.18712454e+00 5.24783816e+00 6.31398207e+00 7.38548493e+00
8.46227539e+00 9.54428209e+00]

First 10 elements from the decrease table::
[-5.69492230e-15 7.24142824e-01 1.44669675e+00 2.16770636e+00
2.88721627e+00 3.60527107e+00 4.32191535e+00 5.03719372e+00
5.75115076e+00 6.46383109e+00]

Now that we have the Look Up Tables we need, we can move on to transforming the red and blue channel of the image/frame using the function cv2.LUT(). And to split and merge the channels of the image/frame, we will be using the function cv2.split() and cv2.merge() respectively. The applyWarm() function (like every other function in this tutorial) will display the resultant image along with the original image or return the resultant image depending upon the passed arguments.

Now, let’s utilize the applyWarm() function created above to apply this warm filter on a few sample images.

Woah! Got the same results as the Instagram warm filter, with just a few lines of code. Now let’s move on to the next one.

Creating Cold Filter-like Effect

This one is kind of the opposite of the above filter, it gives coldness look to images/videos by increasing the blue cast. To create this filter effect, we will define a function applyCold() that will increase the pixel intensities of the blue channel and decrease the intensities of the red channel of an image/frame by utilizing the same LookUp tables, we had constructed above.

For this one too, we will be using the cv2.split()cv2.LUT() and cv2.merge() functions to split, transform, and merge the channels.

Now we will test this cold filter effect utilizing the applyCold() function on some sample images.

Now we’ll use the look up table creat

Nice! Got the expected results for this one too.

Creating Gotham Filter-like Effect

Now the famous Gotham Filter comes in, you must have heard or used this one on Instagram, it gives a warm reddish type look to images. We will try to apply a similar effect to images and videos by creating a function applyGotham(), that will utilize LookUp tables to manipulate image/frame channels in the following manner.

  • Increase mid-tone contrast of the red channel
  • Boost the lower-mid values of the blue channel
  • Decrease the upper-mid values of the blue channel

But again first, we will have to construct the Look Up Tables required to perform the manipulation on the red and blue channels of the image. We will again utilize the scipy.interpolate.UnivariateSpline() function to get the required mapping.

First 10 elements from the midtone contrast increase table:
[0.09416024 0.75724879 1.39938782 2.02149343 2.62448172 3.20926878
3.77677071 4.32790362 4.8635836 5.38472674]

First 10 elements from the lowermids increase table:
[0.15030475 1.31080448 2.44957754 3.56865611 4.67007234 5.75585842
6.82804653 7.88866883 8.9397575 9.98334471]

First 10 elements from the uppermids decrease table::
[-0.27440589 0.8349419 1.93606131 3.02916902 4.11448171 5.19221607
6.26258878 7.32581654 8.38211602 9.4317039 ]

Now that we have the required mappings, we can move on to creating the function applyGotham() that will utilize these LookUp tables to apply the required effect.

Now, let’s test this Gotham effect utilizing the applyGotham() function on a few sample images and visualize the results.

Now w

Stunning results! Now, let’s move to a simple one.

Creating Grayscale Filter-like Effect

Instagram also has a Grayscale filter also known as 50s TV Effect, it simply converts a (RGB) color image into a Grayscale (black and white) image. We can easily create a similar effect in OpenCV by using the cv2.cvtColor() function. So let’s create a function applyGrayscale() that will utilize cv2.cvtColor() function to apply this Grayscale filter-like effect on images and videos.

Now let’s utilize this applyGrayscale() function to apply the grayscale effect on a few sample images and display the results.

Cool! Working as expected. Let’s move on to the next one.

Creating Sepia Filter-like Effect

I think this one is the most famous among all the filters we are creating today. This gives a warm reddish-brown vintage effect to images which makes the images look a bit ancient which is really cool. To apply this effect, we will create a function applySepia() that will utilize the cv2.transform() function and the fixed sepia matrix (standardized to create this effect, that you can easily find online) to serve the purpose.

Now let’s check this sepia effect by utilizing the applySepia() function on a few sample images.

Spectacular results! Reminds me of the movies, I used to watch in my childhood ( Yes, I am that old 😜 ).

Creating Pencil Sketch Filter-like Effect

The next one is the Pencil Sketch Filter, creating a Pencil Sketch manually requires hours of hard work but luckily in OpenCV, we can do this in just one line of code by using the function cv2.pencilSketch() that give a pencil sketch-like effect to images. So lets create a function applyPencilSketch() to convert images/videos into Pencil Sketches utilizing the cv2.pencilSketch() function.

We will use the following funciton to applythe pencil sketch filter, this function retruns a grayscale sketch and a colored sketch of the image

This filter is a type of edge preserving filter, these filters have 2 Objectives, one is to give more weightage to pixels closer so that the blurring can be meaningfull and second to average only the similar intensity valued pixels to avoid the edges, so in this both of these objectives are controled by the two following parameters.

sigma_s Just like sigma in other smoothing filters this sigma value controls the area of the neighbourhood (Has Range between 0-200)

sigma_r This param controls the how dissimilar colors within the neighborhood will be averaged. For example a larger value will restrcit color variation and it will enforce that constant color stays throughout. (Has Range between 0-1)

shade_factor This has range 0-0.1 and controls how bright the final output will be by scaling the intensity.

Now we will apply this pencil sketch effect by utilizing the applyPencilSketch() function on a few sample images and visualize the results.

Now let’s check how the changeIntensity() functi

Amazing right? we created this effect with just a single line of code. So now, instead of spending hours manually sketching someone or something, you can take an image and apply this effect on it to get the results in seconds. And you can further tune the parameters of the cv2.pencilSketch() function to get even better results.

Creating Sharpening Filter-like Effect

Now let’s try to create the Sharpening Effect, this enhances the clearness of an image/video and decreases the blurriness which gives a new interesting look to the image/video. For this we will create a function applySharpening() that will utilize the cv2.filter2D() function to give the required effect to an image/frame passed to it.

Now, let’s see this in action utilizing the applySharpening() function created above on a few sample images.

Nice! this filter makes the original images look as if they are out of focus (blur).

Creating a Detail Enhancing Filter

Now this Filter is another type of edge preserving fitler and has the same parameters as the pencil sketch filter.This filter intensifies the details in images/videos, we’ll be using the function called cv2.detailEnhance(). let’s start by creating the a wrapper function applyDetailEnhancing(), that will utilize the cv2.detailEnhance() function to apply the needed effect.

Now, let’s test the function applyDetailEnhancing() created above on a few sample images.

Satisfying results! let’s move on to the next one.

Creating Invert Filter-like Effect

This filter inverts the colors in images/videos meaning changes darkish colors into light and vice versa, which gives a very interesting look to images/videos. This can be accomplished using multiple approaches we can either utilize a LookUp table to perform the required transformation or subtract the image by 255 and take absolute of the results or just simply use the OpenCV function cv2.bitwise_not(). Let’s create a function applyInvert() to serve the purpose.

Let’s check this effect on a few sample images utilizing the applyInvert() function.

Looks a little scary, lets’s try it on a few landscape images.

Interesting effect! but I will definitely not recommend using this one on your own images, except if your intention is to scare someone xD.

Creating Stylization Filter-like Effect

Now let’s move on to the final one, which gives a painting-like effect to images. We will create a function applyStylization() that will utilize the cv2.stylization() function to apply this effect on images and videos. This one too will only need a single line of code.

Now, as done for every other filter, we will utilize the function applyStylization() to test this effect on a few sample images.

Again got fascinating results! Wasn’t that fun to see how simple it is to create all these effects?

Apply Instagram Filters On a Real-Time Web-cam Feed

Now that we have created the filters and have tested them on images, let’s move to apply these on a real-time webcam feed, first, we will have to create a mouse event callback function mouseCallback(), similar to the one we had created for the Color Filters in the previous tutorial, the function will allow us to select the filter to apply, and capture and store images into the disk by utilizing mouse events in real-time.

Now that we have a mouse event callback function mouseCallback() to select a filter to apply, we will create another function applySelectedFilter() that we will need, to check which filter is selected at the moment and apply that filter to the image/frame in real-time.

Now that we will the required functions, let’s test the filters on a real-time webcam feed, we will be switching between the filters by utilizing the mouseCallback() and applySelectedFilter() functions created above and will overlay a Camera ROI over the frame and allow the user to capture images with the selected filter applied, by clicking on the Camera ROI in real-time.