How to find disparity when camera is moving forward/backward only - opencv

When camera is moving forward/backward, I think stereo rectification would not work. I guess I have to search along the epipolar line directly. Currently, my only clue is to use LineIterator to iterate through epipolar line, and use its neighbors as descriptor. This is very unintelligent method. So I want to know if there is other ways, or helpful opencv functions.

Old problem - Mark Pollefeys had a nice solution for it about 18 years ago. The idea is to rectify in polar coordinates.
See his short paper and lecture notes, and look around for tech reports from there.

You could try with Scale Invariant Feature Transform (SIFT) maybe with this descriptor you could get the key-points and stabilize the image with some transformation function per frame.
SIFT in Matlab

Related

How to detect PizzaMarker

did somebody tried to find a pizzamarker like this one with "only" OpenCV so far?
I was trying to detect this one but couldn't get good results so far. I do not know where this marker is in picture (no ROI is possible), the marker will be somewhere in the room (different ligthning effects) and not faceing orthoonal towards us. What I want - the corners and later the orientation of this marker extracted with the corners but first of all only the 5Corners. (up, down, left, right, center)
I was trying so far: threshold, noiseclearing, find contours but nothing realy helped for a good result. Chessboards or square markers are normaly found because of their (parallel) lines- i guess this can't help me here...
What is an easy way to find those markers?
How would you start?
Use other colorformat like HSV?
A step-by-step idea or tutorial would be realy helpfull. Cause i couldn't find tuts at the net. Maybe this marker isn't called pizzamarker -> does somebody knows the real name?
thx for help
First - thank you for all of your help.
It seems that several methods are usefull. Some more or less time expansive.
For me it was the easiest with a template matching but not with the same marker.
I used only a small part of it...
this can be found 5 times(4 times negative and one positive) in this new marker:
now I use only the 4 most negatives Points and the most positive and got my 5 points that I finaly wanted. To make this more sure, I check if they are close to each other and will do a cornerSubPix().
If you need something which can operate in real-time I'd go down the edge detection route and look for intersecting lines like these guys did. Seems fast and robust to lighting changes.
Read up on the Hough Line Transform in openCV to get started.
Addendum:
Black to White is the strongest edge you can have. If you create a gradient image and use the strongest edges found in the scene (via histogram or other) you will be able to limit the detection to only the black/white edges. Look for intersections. This should give you a small number of center points to apply Hough ellipse detection (or alternate) to. You could rotate in a template as a further check if you wish.
BTW.. OpenCV has Edge Detection, Hough transform and FitEllipse if you do go down this route.
actually this 'pizza' pattern is one of the building blocks of the haar featured used in the
Viola–Jones object detection framework.
So what I would do is compute the summed area table, or integral image using cv::integral(img) and then run exhaustive search for this pattern, on various scales (size dependant).
In each window you are using only 9 points (top-left, top-center, ..., bottom left).
You can train and use cvHaarDetectObjects to detect the marker using VJ.
Probably not the fastest method but it should work.
You can find more info on object detection methods using OpenCV here: http://opencv.willowgarage.com/documentation/object_detection.html

Estimate motion of monocular camera

I had read lectures and topics and I work on it since weeks, but I can't found the way to describe the motion of my camera. I don't want to reconstruct 3D world. I'm using OpenCV.
I have a monocular camera and an unknown word. I have the intrinsic and distortion parameters. I have features and correspondances. So I'm looking for the rotation and the translation between two frames. I would like to consider my first image as the origin of the XYZ axes.
I use the Fundamental matrix, and the Essential matrix to find the extrinsics parameters (R, T) but I'm not convinced. I had these results:
R
[0.040437..., 0.116076..., -0.992416...,
0.076999..., -0.99063..., -0.112731...,
-0.996211.., -0.071848.., -0.048994...]
T
[0.6924183...; 0.081694...; -716885...]
How can I check if they are good?
I calculated the euclidean distance to see the distance in 3D but I had erroneous values.
Please, can anyone give me some details, or guide me? I hope I explained myself well.
Regards
By word do you mean world? This question is also not really on topic for stackoverflow since it deals with a theory and not code.
https://stackoverflow.com/faq
To answer your question. If you and R and T then you can compute the 3D coordinate for each point. From that you can reproject each point onto the other camera and compute the residual error between the observed and predicted point. If the error is within a pixel or so it's probably valid.
Basically this way you will have unknown scale factor for each consecutive frame, so you can get strange values for R and T. But you can use some initialization like known motion in order to perform first triangulation of the scene. Next you can use solvePnP in order to calculate next [R|T].
Try to read about PTAMM which is one of the most interesting implementations of monocular SLAM
http://www.robots.ox.ac.uk/~bob/research/research_ptamm.html

Head pose estimation with Opencv

I am using OpenCV Haar Algorithm to track the Head and overlay an image over the Head.
What I am doing is saving frames generated by camera and overlaying image over each frames.
And time is not a constraint as I am not doing it Real-Time.
My code is working fine for say 45 degree of left and right rotation of Head.
But I need something which will track up to 90 degree of rotation.
Even I got many reference of OpenCV functions and link to estimate Head Pose
Please provide me some reference. Code Examples will be cool.
Thanks in advance
You can use an algorithm like SURF (you have samples in OpenCV package) and use it over a picture of the face, the over the image, and then use SURF descriptors to match the points and estimate the 3d position of the face in the image.
You can use the same code on the sample "find_obj" but replace the image by the face picture you want to track.
Hope this helps.
There is a functionc in openCV called POSIT that permit to estimate the pose of 3d object in a single image. It implements POSIT algorithm. Try to have a look there.
You could check the EHCI project at http://code.google.com/p/ehci/ as it gives a nice overview about POSIT and Lukas Kanade.

OpenCV + photogrammetry

i have a stereopair,
photo 1: http://savepic.org/1671682.jpg
photo 2: http://savepic.org/1667586.jpg
there is coordinate system in each image. How can I find coordinates of point A in this system using OpenCV library. It would be nice to see sample code.
I've looked for it at opencv.willowgarage.com/documentation/cpp/camera_calibration_and_3d_reconstruction.html but haven't found (or haven't understood :) )
Your 'stereo' images are fine. What you have already done is solve the correspondence problem: in both images you have indicated points 'A'. This means that you know which pixel corresponds to eachother labeling point 'A'.
What you want to do, is triangulate where your camera is. You can only do this by first calibrating your camera. This is inside of OpenCV already.
http://docs.opencv.org/doc/tutorials/calib3d/camera_calibration/camera_calibration.html
http://docs.opencv.org/modules/calib3d/doc/camera_calibration_and_3d_reconstruction.html
This gives you the exact vector/ray of light for each vector, and the optical center of your cameras through which the ray passes. Moreover, you need stereo calibration. This establishes the orientation and position of each camera with respect through each other.
From that point on, your triangulation is simple, knowing the pixel location in both images of point 'A'. You have
Location and orientation of camera 1 and camera 2
Otical Ray Vector (pixel location) from the cameras to label 'A'.
So you have 2 locations in space, and 2 rays from these location. The intersection of these rays is your 3D answer.
Note that in practice there rays will never exactly intersect (2 lines in 3D rarely do), so you need to approximate. Use opencv function triangulatePoints(), using the input of the stereo calibration and the pixel index relating to label A.
Firstly of all this is not truly a stereo pair. A nice stereo pair needs to have 60%-80% overlap usually small rotation differences between images. Even if this pair had the necessary BASE to be a good stereo pair due to the extremely kappa rotation the resulting epipolar image would be useless.
Secondly among others you should take a look at the camera calibration and collinearity equations both supported by OpenCV
http://en.wikipedia.org/wiki/Camera_resectioning
http://en.wikipedia.org/wiki/Collinearity_equation
You need to understand the maths.
If the page isn't enough then you should look at the opencv book - it devotes a couple of chapters to this. Then there are a lot of textbooks that cover it in more detail

Find the position of a pattern/marker inside a photograph

i need to find a marker like the ones used in Augmented Reality.
Like this:
I have a solid background on algebra and calculus, but no experience whatsoever on image processing. My thing is Php, sql and stuff.
I just want this to work, i've read the theory behind this and it's extremely hard to see in code for me.
The main idea is to do this as a batch process, so no interactivity is needed. What do you suggest?
Input : The sample image.
Output: Coordinates and normal vector in 3D of the marker.
The use for this will be linking images that have the same marker to spatialize them, a primitive version of photosync we could say. Just a caroussel of pinned images, the marker acting like the pin.
The reps given allowed me to post images, thanks.
You can always look at the open source libraries such as ARToolkit and see how it works but generally in order to get the 3D coordinates of marker you would need to:
Do the camera calibration.
Find marker in image using local features for example.
Using calibrated camera parameters and 2D coordinates of marker do the approximation the 3D coordinates.
I've never implemented sth similar by myself but I think this is a general concept you should apply on your method.
Your problem can be solved by perspective n point camera pose estimation. When you can reasonably assume that all correspondences are correct, a linear algorithm should do.
Since the marker is planar, you can also recover the displacement from the homography between the model plane and the image plane (link). As usual, best results are obtained by iterative algorithms (link).

Resources