How to draw a perpendicular line using a point and slope

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Hessa Alali
Hessa Alali 2019-3-24
评论： Image Analyst 2022-2-20
I have a line between two point, P1=[374 448] and P2=[385 562]. i want to find a perpendicular line to the original line passing throw the middle point of the original line.

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回答（3 个）

Image Analyst
Image Analyst 2019-3-24
Try this:
P1=[374 448]
P2=[385 562]
p1x = P1(1);
p1y = P1(2);
p2x = P2(1);
p2y = P2(2);
plot([p1x, p2x], [p1y, p2y], 'ro-', 'LineWidth', 2);
grid on;
xlabel('x', 'FontSize', 15);
ylabel('y', 'FontSize', 15);
% Find midpoint
midX = mean([p1x, p2x])
midY = mean([p1y, p2y])
hold on;
plot(midX, midY, 'r*', 'LineWidth', 2, 'MarkerSize', 25);
% Get the slope
slope = (p2y-p1y) / (p2x-p1x)
% For perpendicular line, slope = -1/slope
slope = -1/slope
% Point slope formula (y-yp) = slope * (x-xp)
% y = slope * (x - midX) + midY
% Compute y at some x, for example at x=300
x = 300
y = slope * (x - midX) + midY
plot([x, midX], [y, midY], 'bo-', 'LineWidth', 2);
axis equal
0001 Screenshot.png
  1 个评论
Hessa alali
Hessa alali 2019-4-2
I used this method and it works fine.
But when i applied it in an image, it doesn't gave me a perpendicular line.
does the image have a difference in their axis ? x and y?

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John D'Errico
John D'Errico 2019-3-24
编辑：John D'Errico 2019-3-24
You want to "find" it? Or you want to draw it?
The midpoint of the line segment is at
(P1 + P2)/2
ans =
379.5 505
Y is irrelevant there, since you are asking about a VERTICAL line, thus a line with infinite slope. The line is described by the simple equation X = 379.5. Y takes on any value you wish.
How to draw that line? This will suffice.
xline((P1(1) + P2(1))/2)
xline was introduced in R2018b.
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Rik
Rik 2019-3-24
Do you mean you get an error using xline (possibly due to using an older release than R2018b), or do you mean you don't get the expected output? In the former case, provide details about the error and about what Matlab version you're using. In the latter case, explain how the result is different from what you expect.

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michael scheinfeild
why if we dont use axis equal seems line is not
perpendicular
  1 个评论
Image Analyst
Image Analyst 2022-2-20
Because the scaling for each direction would be different. Imagine if you had equal scaling for x and y and had a 45 degree line going from (0,0) to (1,1) and it took up 4 inches on your screen in each direction.. Now let's say that instead you cut the distance along the x direction from 4 inches to 1 inch. It still goes from (0,0) to (1,1) but now the x=1 point is not at 4 inches, but it's at 1 inch. Would that change the apparent slope of the line? Sure it would. It's stil 45 degrees mathematically but on screen it's not - it's at a steeper angle now.

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