Example Image
I want to remove the lines (shown in RED color) as they are out of order. Lines shown in black color are repeating at same period (approximately). Period is not known beforehand. Is there any way of deleting non-periodic lines( shown in red color) automatically?
NOTE: Image is binary ( back & while).. lines shown in red color only for illustration.
Of course there is any way. There is almost always some way to do something.
Infortunately you have not provided any particular problem. The entire thing is too broad to be answered here.
To help you getting started: (I highly recommend you start with pen, paper and your brain)
Detect the lines -> google or think, there are many standard ways to detect lines in an image. if you don't have noise in your binary image its trivial.
find any aequidistant sets -> think
delete the rest -> think ( you know what is good so everything else has to go away)
I assume, your lines are (almost) vertical.
The following should work
turn the image to a column sum histogram
try a Fourier transformation on the signal (potentially padding the image appropriately)
pick the maximum/peak from the Fourier spectrum as your base period
If you need the lines rather than the position of the lines, generate a mask with lines at appropriate intervals (as determined by your analysis before) and apply to the image.
I am currently working on an algorithm to detect the playing area of a pool table. For this purpose, I captured an image, transformed it to grayscale, and used a Sobel operator on it. Now I want to detect the playing area as a box with 4 corners located in the 4 corners of the table.
Detecting the edges of the table is quite straightforward, however, it turns out that detecting the 4 corners is not so easy, as there are pockets in the pool table. Now I just want to fit a line to each of the side edges, and from those lines, I can compute the intersects, which are the corners for my table.
I am stuck here, because I could not yet come up with a good solution to find these lines in my image. I can see it very easily when I used the Sobel operator. But what would be a good way of detecting it and computing the position of the corners?
EDIT: I added some sample Images
Basic Image:
Grayscale Image
Sobel Filter (horizontal only)
For a general solution, there will be many sources of noise: problems with cloth around the rails, wood texture (or no texture) on the rails, varying lighting, shadows, stains on the cloth, chalk on the rails, and so on.
When color and lighting aren't dependable, and when you want to find the edges of geometric objects, then it's best to think in terms of edge pixels rather than gray/color pixels.
A while back I was thinking of making a phone-based app to save ball positions for later review, including online, so I've though a bit about this problem. Although I can provide some guidance for your current question, it occurs to me you'll run into new problems each step of the way, so I'll try to provide a more complete answer.
Convert the image to grayscale. If we can't get an algorithm to work in grayscale, we'll inevitably run into problems with color. (See below)
[TBD] Do some preprocessing to reduce noise.
Find edge points using Sobel or (if you must) Canny.
Run Hough lines detection, but with a few caveats and parameterizations as described below.
Find the lines described a keystone-shaped quadrilateral. (This will likely be the inner quadrilateral of two: one inside the rail on the bed, and the other slightly larger quadrilateral at the cloth/wood rail edge at top.)
(Optional) Use the side pockets to help determine the orientation of the quadrilateral.
Use an affine transform to map the perspective-distorted table bed to a rectangle of [thankfully] known relative dimensions. We know the bed sizes in advance, so you can remap the distorted rectangle to a proper rectangle. (We'll ignore some optical effects for now.)
Remap the color image to the perspective-corrected rectangle. You'll probably need to tweak the positions of some balls.
General notes:
Filtering by color in the general sense can be difficult. It's tempting to think of the cloth as being simply green, blue, or red (or some other color), but when you look at the actual RGB values and try to separate colors you'll begin to appreciate what a nightmare working in color can be.
Optical distortion might throw off some edges.
The far short rail may be difficult to detect, BUT you do this: find the inside lines for the two long rails, then search vertically between the two rails for the first strong horizontal-ish edge at the far side of the image. That'll be the far short rail.
Although you probably want to use your phone camera for convenience, using a Kinect camera or similar (preferably smaller) device would make the problem easier. Not only would you have both color data and 3D data, but you would eliminate some problems with lighting since the depth data wouldn't depend on visible lighting.
For your app, consider limiting the search region for rail edges to a perspective-distorted rectangle. The user might be able to adjust the search region. This could greatly simplify the processing, and could help you work around problems if the table isn't lit well (as can be the case).
If color segmentation (as suggested by #Dima) works, get the outline of the blob using contour following. Then simplify the outline to a quadrilateral (or a polygon of few sides) by the Douglas-Peucker algorithm. You should find the four table edges this way.
For more accuracy, you can refine the edge location by local search of transitions across it and perform line fitting. Then intersect the lines to get the corners.
The following answer assumes you have already found the positions of the lines in the image. This however can be done "easily" by directly looking at the pixels and seeing if they are in a "line". Usually it is easier to detect this if the image has been deskewed first as well, i.e. Rotated so the rectangle (pool table) is more like this: [] than like /=/. Then it is just a case of scanning the pixels and if there are ones of similar colour alongside it assuming a line is between them.
The code works by looping over the lines found in the image. Whenever the end points of each line falls within a tolerance on within the x and y coordinates it is marked as a corner. Once the corners are found I take the average value between them to find where the corner lies. For example:
A horizontal line ending at 10, 10 and a vertical line starting at 12, 12 will be found to be a corner if there is a tolerance of 2 or more. The corner found will be at: 11, 11
NOTE: This is just to find Top Left corners but can easily be adapted to find all of them. The reason it has been done like this is because in the application where I use it, it is faster to sort each array first into an order where relevant values will be found first, see: Why is processing a sorted array faster than an unsorted array?.
Also note that my code finds the first corner for each line which might not be applicable for you, this is mainly for performance reasons. However the code can easily be adapted to find all the corners with all the lines then either select the "more likely" corner or average through them all.
Also note my answer is written in C#.
private IEnumerable<Point> FindTopLeftCorners(IEnumerable<Line> horizontalLines, IEnumerable<Line> verticalLines)
{
List<Point> TopLeftCorners = new List<Point>();
Line[] laHorizontalLines = horizontalLines.OrderBy(l => l.StartPoint.X).ThenBy(l => l.StartPoint.Y).ToArray();
Line[] laVerticalLines = verticalLines.OrderBy(l => l.StartPoint.X).ThenBy(l => l.StartPoint.Y).ToArray();
foreach (Line verticalLine in laVerticalLines)
{
foreach (Line horizontalLine in laHorizontalLines)
{
if (verticalLine.StartPoint.X <= (horizontalLine.StartPoint.X + _nCornerTolerance) && verticalLine.StartPoint.X >= (horizontalLine.StartPoint.X - _nCornerTolerance))
{
if (horizontalLine.StartPoint.Y <= (verticalLine.StartPoint.Y + _nCornerTolerance) && horizontalLine.StartPoint.Y >= (verticalLine.StartPoint.Y - _nCornerTolerance))
{
int nX = (verticalLine.StartPoint.X + horizontalLine.StartPoint.X) / 2;
int nY = (verticalLine.StartPoint.Y + horizontalLine.StartPoint.Y) / 2;
TopLeftCorners.Add(new Point(nX, nY));
break;
}
}
}
}
return TopLeftCorners;
}
Where Line is the following class:
public class Line
{
public Point StartPoint { get; private set; }
public Point EndPoint { get; private set; }
public Line(Point startPoint, Point endPoint)
{
this.StartPoint = startPoint;
this.EndPoint = endPoint;
}
}
And _nCornerTolerance is an int of a configurable amount.
A playing area of a pool table typically has a distinctive color, like green or blue. I would try a color-based segmentation approach first. The Color Thresholder app in MATLAB gives you an easy way to try different color spaces and thresholds.
I'm drawing a chart in an iOS app and I want the gridlines to start in the x axis but end in the data point instead of extending all the way up to the top.
Is this possible?
Thanks in advance.
NC
Disclaimer: I work for ShinobiControls
The shinobicharts framework doesn't currently support this as an out-of-the-box feature.
A workaround may exist though. You may be able to add your own gridline subviews by using the SChartAxis method pixelValueForDataValue: to work out where in the plot area coordinate space you should draw your vertical line up to for a given data point.
Once you have your coordinates there are various ways you could draw your gridlines:
Add a canvas view behind or in front of the chart (depending on what effect you want). Then use your coordinates to draw your gridlines using CoreGraphics or some other drawing technique.
Create individual UIViews that each represent a gridline using your coordinates and add these behind or in front of the chart.
One thing to be aware of with this technique is that the gridlines will not automatically update when you pan and zoom. In order to do this you will need to override one of the chart's delegate methods that notify you of range changes and update your drawn gridlines to match the new data point positions.
Another potential workaround could be to use a column series to mimic gridlines. If you create a column series and feed it the same data points as your original series this will result in columns that go up to the y-value of each data point. You could then use the property interSeriesSetPadding on SChartAxisStyle to cause the columns to appear very thin.
I hope that information is useful!
We are currently using HighCharts for our typical bar, pie, line charts. But, we have been asked to plot some medical data that would end up resembling an irregular loop.
If I have an array of x,y points, would it be possible for me to draw this sort of graph with HighCharts? And, if so, can you point me to the right section of the documentation. I see that drawing things like rectangles and circles is possible, but I haven't found any info on drawing lines between arbitrary points.
I can see only three ways to achieve that, and none of them is easy, and have some limitations:
1) Use spline series with unsorted data - looks almost good, only tooltip doesn't work for unsorted data
2) Use scatter series with unsorted data - issue with tooltip is gone, but curves aren't smooth
3) Use renderer.path - doesn't have tooltip at all, and requires to add path manually.
And example of all three: http://jsfiddle.net/kEu3w/
I want to make a multiple plots with the x-axis aligned so that we can compare two set of data. I want it to look like this
https://stackoverflow.com/a/11796553/229075
In my case there are some addition requirement that some of the plot are lines and some are bar. So the default scale does not align even if the data range are the same.
I found some good suggestion about "stacking" the charts. It is a little bit complicated but a promising way to try.
https://stackoverflow.com/a/12179023/229075
However I also have other use case that may push the envelop of simple stacking. For example, I'd like to do scatter plot matrix sometime when both x-axis and y-axis have to align like this
http://www.statmethods.net/graphs/images/spmatrix3.png
Rather than putting many chart in one Highchart, this may requires have many separate charts but have better control in how the axis is layout.
If you want to use one one chart, you can use linkedTo option to connect axis.
If you want more simple charts, you can manage to connect axis using event afterSetExtremes, where you can call setExtremes() for another chart.