It doesn’t seem like OpenCV handles alpha the way you want it to. You have two options: Write your own cross-correlation method that will use the alpha channel Transform your images so your alpha channel becomes irrelevant Since the first option is straightforward, I will explore the second option here. I’m going to re-use the … Read more
How to calculate distance given an object of known size You need to know one of 2 things up front Focal-length (in mm and pixels per mm) Physical size of the image sensor (to calculate pixels per mm) I’m going to use focal-length since I don’t want to google for the sensor datasheet. Calibrate the … Read more
As mentioned, the problem is very hard and is often also referred to as multi-view object reconstruction. It is usually approached by solving the stereo-view reconstruction problem for each pair of consecutive images. Performing stereo reconstruction requires that pairs of images are taken that have a good amount of visible overlap of physical points. You … Read more
A feature detector is an algorithm which takes an image and outputs locations (i.e. pixel coordinates) of significant areas in your image. An example of this is a corner detector, which outputs the locations of corners in your image but does not tell you any other information about the features detected. A feature descriptor is … Read more
Here’s a very basic, visual explanation of how a Hough Transform works for detecting lines in an image:
Disparity map refers to the apparent pixel difference or motion between a pair of stereo images. To experience this, try closing one of your eyes and then rapidly close it while opening the other. Objects that are close to you will appear to jump a significant distance while objects further away will move very little. … Read more
Here is my attempt at answering my question after reading the paper Zhengyou Zhang , Li-Wei He, “Whiteboard scanning and image enhancement” http://research.microsoft.com/en-us/um/people/zhang/papers/tr03-39.pdf I manipulated the equations for some time in SAGE, and came up with this pseudo-code in c-style: // in case it matters: licensed under GPLv2 or later // legend: // sqr(x) = … Read more
First, check out /samples/c/squares.c in your OpenCV distribution. This example provides a square detector, and it should be a pretty good start on how to detect corner-like features. Then, take a look at OpenCV’s feature-oriented functions like cvCornerHarris() and cvGoodFeaturesToTrack(). The above methods can return many corner-like features – most will not be the “true … Read more
Complete solution for arbitrary Sobel kernel sizes and angles tl;dr: skip down to section ‘Examples’ To add another solution, expanding on this document (it’s not particularly high quality, but it shows some usable graphics and matrices starting at the bottom of page 2). Goal What we’re trying to do is estimate the local gradient of … Read more
Feature detection In computer vision and image processing the concept of feature detection refers to methods that aim at computing abstractions of image information and making local decisions at every image point whether there is an image feature of a given type at that point or not. The resulting features will be subsets of the … Read more