Reading an image from a file is fairly straightforward in OpenCV-Python. But a lot of times, we would like to read an image from a URL and process it in OpenCV. One way to do it is to download the image, save it as a jpeg file, and then read it in OpenCV. But that’s too tedious! Who wants to do manual labor? Not me! Another way would be to automatically download the image as a jpeg file using wget and then reading it in OpenCV. This is slightly better, but it’s again a roundabout way of doing it because there is an intermediate step of saving the image on disk. How do we circumvent this and directly load an image from a URL into OpenCV-Python? Continue reading “How To Read An Image From A URL In OpenCV-Python”
Category: Computer Vision
Installing OpenCV 3 With Python On Mac OS X
OpenCV is the world’s most popular computer vision library and it’s used extensively by researchers and developers around the world. OpenCV has been around for a while now and they add something new and interesting with every new release. One of the main additions of OpenCV 3 is “opencv_contrib” which contains a lot of cutting edge algorithms for feature descriptors, text detection, object tracking, shape matching, and so on. They have greatly improved Python support in this release as well. Since OpenCV is available on almost all the popular platforms, this version looks very promising. Let’s see how to install OpenCV 3 with Python support on Mac OS X. Continue reading “Installing OpenCV 3 With Python On Mac OS X”
Image Classification Using Fisher Vectors
This is a continuation of my previous blog post on image classification and the bag of words (BoW) model. If you already know how BoW works, then you will feel right at home. If you need a refresher, you can read the blog post here. In our previous post, we discussed how BoW works, and how we construct the codebook. An interesting thing to note is that we don’t consider how things are ordered as such. A given image is treated as a combination of codewords regardless of where they are located with respect to each other. If we want to improve the performance of BoW, we can definitely increase the size of the vocabulary. If we have more codewords, we can describe a given image better. But what if we don’t want to do that? Is there a more efficient method that can be used? Continue reading “Image Classification Using Fisher Vectors”
Image Classification Using Bag-Of-Words Model
Image classification is one of the classical problems in computer vision. Basically, the goal is to determine whether or not the given image contains a particular thing like an object or a person. Humans tend to classify images effortlessly, but machines seem to have a hard time doing this. Computer Vision has achieved some success in the case of specific problems, like detecting faces in an image. But it has still not satisfactorily solved the problem for the general case where we have random objects in different positions and orientations. Bag-of-words (BoW) model is one of the most popular approaches in this field, and many modern approaches are based on top of this. So what exactly is it? Continue reading “Image Classification Using Bag-Of-Words Model”
What Is A Markov Chain?
If you have studied probability theory, then you must have heard Markov’s name. When we study probability and statistics, we tend to deal with independent trials. What this means is that if you conduct an experiment a lot of times, we assume that the outcome of one trial doesn’t influence the outcome of the next trial. For example, let’s say you are tossing a coin. If you toss the coin 5 times, you are bound to get either heads or tails with equal probability. If the outcome of the first toss is heads, it doesn’t tell us anything about the next trial. But what if we are dealing with a situation where this assumption is not true? If we are dealing with something like estimating the weather, we cannot assume that today’s weather is not affected by what happened yesterday. If we go ahead with the independence assumption here, we are bound to get wrong results. How do we formulate this kind of model? Continue reading “What Is A Markov Chain?”
The Concept Of Homogeneous Coordinates
People in computer vision and graphics deal with homogeneous coordinates on a very regular basis. They are actually a nice extension of standard three dimensional vectors and allow us to simplify various transforms and their computations. When I say “transformations”, I am talking about all those special effects on the screen, and the corresponding movements and scaling of various objects. But why do we need homogeneous coordinates to do all that? Why can’t we just move the objects around? Well, we can’t directly do that, not easily anyway! This will become clear soon. The concept of homogeneous coordinates is fundamental when we talk about cameras. In order to design our algorithms, we need to understand how the cameras are looking at the real world. This is in fact utilized heavily by game programmers as well. So what is it all about? Why is it so important? Continue reading “The Concept Of Homogeneous Coordinates”
Understanding Camera Calibration
Cameras have been around for a long time now. When cameras were first introduced, they were expensive and you needed a good amount of money to own one. However, people then came up with pinhole cameras in the late 20th century. These cameras were inexpensive and they became a common occurrence in our everyday life. Unfortunately, as is the case with any trade off, this convenience comes at a price. These pinhole cameras have significant distortion! The good thing is that these distortions are constant and they can be corrected. This is where camera calibration comes into picture. So what is this all about? How can we deal with this distortion? Continue reading “Understanding Camera Calibration”
What Is Gamma Correction?
Gamma correction is an integral part of all the digital imaging systems, but a lot of people don’t know about it! It is an essential part of all the imaging devices like cameras, camcorders, monitors, video players, etc. It basically defines the relationship between a pixel’s numerical value and its actual luminance. Now wait a minute, why would they be different? Isn’t a pixel’s numerical value supposed to be exactly the same as its luminance? Well, not really! Without gamma, shades captured by digital cameras wouldn’t appear as they did to our eyes. If we fully understand how gamma works, we can improve our exposure technique, along with making the most of image editing. So what is it all about? Why do we need gamma correction at all? Continue reading “What Is Gamma Correction?”
Recognizing Shapes Using Point Distribution Models
In the field of computer vision, we often come across situations where we need to recognize the shapes of different objects. Not only that, we also need our machines to understand the shapes so that we can identify them even if we encounter them in different forms. Humans are really good at these things. We somehow make a mental note about these shapes and create a mapping in our brain. But if somebody asks you to write a formula or a function to identify it, we cannot come up with a precise set of rules. In fact, the whole field of computer vision is based on chasing this hold grail. In this blog post, we will discuss a particular model which is used to identify different shapes. Continue reading “Recognizing Shapes Using Point Distribution Models”
Understanding Gabor Filters
In the field of image processing, filters play an extremely important role. If you don’t know what a filter is, you may quickly want to read the wiki article and come back. Otherwise, this post will not make much sense to you. All image processing operations can be viewed as applying a series of filters to an image and transforming it in some way. We do this for a variety of reasons, like understanding the content of images, transforming the images into another domain, detecting the presence of something in the images, and so on. Gabor filter is a particular type of filter, and it happens to be an important one. If you google “Gabor filter”, you will get a lot of articles. So in this post, rather than looking at the mathematics behind it, we will try to understand the underlying concept. Let’s go ahead, shall we? Continue reading “Understanding Gabor Filters”
Perpetual Enigma 