# Find pixels at given distance along line

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Mariana on 1 May 2014
Commented: Image Analyst on 3 May 2014
Hello, as you can see on image attached I´ve computed perpendicular lines to red lines. I know the slope of red line, the slope of green line and I know the coordinates of the cross-section also. So I computed x and y coordinates of the green line as follows:
slope = (r1 - r2) / (s1 - s2); %red line slope
slRec = -1 / slope; %perpendicular green line slope
ym = (y1 + y2)/2; xm = (x1 + x2)/2; %cross-section point
according to well known line equation y = ax + b I´ve counted y when x was created as:
b = ym - a*xm;
length = 5;
y = ym - length : 0.25 : ym + length;
x = (y - b) / a;
coordinates = ([y; x])';
But as you can see on my image, there is problem that when the length of the line is given by y-axis range it is different length itself. How can I find pixel value at unit distance from the red line (and along green line)?

Image Analyst on 1 May 2014
I've done this successfully before. What you need to do, so that the cross sectional line doesn't wildly waver all over the place is to take several points along the red line, not just 2. Then use polyfit() to fit a line or quadratic. (The line is easy. Using a quadratic of cubic is also easy but requires calculus.) Then compute your perpendicular line assuming some fixed length, like 30 pixels or whatever.
Image Analyst on 3 May 2014
I've started working on a general purpose demo but it's not done yet.