Black columns in the disparity map when using StereoBM

Question

I am trying to use StereoBM in OpenCV to extract a disparity map from a pair of images. Ignoring the bad quality of the disparity map below, you can see that it has a number of black columns on the left which correspond to the parameter ndisparities. I thought that ndisparities only tells StereoBM how "far" away it can search for a correspondence. What could cause this behaviour? It seems to limit the width of the resulting depth map, but I don't see why. You can see the stereo pair here and my code below. Thanks in advance for any pointers! ``` Mat Limg = imread("left.jpg", CV_LOAD_IMAGE_GRAYSCALE); Mat Rimg = imread("right.jpg", CV_LOAD_IMAGE_GRAYSCALE); Mat disp(Limg.size(), CV_16SC1), disp8U; int ndisparities = 512; StereoBM SBM(StereoBM::BASIC_PRESET, ndisparities , 11); SBM(Limg, Rimg, disp, CV_16S); double minVal, maxVal; minMaxLoc( disp, &minVal, &maxVal ); disp.convertTo( disp8U, CV_8UC1, 255/(maxVal - minVal)); imshow("disparity", disp8U) ```

Accepted Answer

The disparity map quantifies the longitudinal (epipolar) offset between corresponding pairs of points in the left and right image.

In principle,

```
 disparity := x_l - x_r
```

where `x_l` and `x_r` are the projections on the focal plane of the two cameras of a given point in the space. (Keep in mind that large disparities characterize pixels which are closer to the cameras. [A])

The parameter `ndisparities` quantifies the expected maximum disparity you can have (assuming you can neglect the min disparity).

Since you are assuming that `ndisparities` is your largest disparity, it holds true

```
    x_l - x_r < ndisparities,
```

i.e.

```
    x_r > x_l - ndisparities 
```

therefore, it makes no sense looking for correspondences with the right image of any pixel in the first `ndisparities` columns of the left image: simply you can't have.

In a sense, the view cone of the right camera starts exactly `ndisparities` columns on the right of view cone of the left camera.

A FIX:

If you want less black columns on the left hand side of the disparity map, you need to be able to assume lower values of `ndisparities`.

Since `ndisparities` depends on the distance from the cameras of the closest object (from [A]), either put the cameras further away from the object or put the cameras closer one another.

In your very specific case you have a huge disparity (a lot of work for the stereoBM)!! The 'x' symbol in the foreground shows a disparity comparable with the scale of the image!! I believe you need to put your cameras further away.

Black columns in the disparity map when using StereoBM

Solution

Problem