About generating the binary image from mathematical coordinates

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Ki
Ki 2012-2-29
Hi there,
I get a set of points in two dimensional Cartesian system, that is (x, y). My purpose is to generate a two-dimensional binary image with that coordinates. But the range for y and x are different and the step is not fixed. Actually, the pair of values are generated by random number in [0, 1] and mapping to a range with some formula
y: [-2pi, 2pi]
x: [0, 4pi]
if I plot each "point" on the figure it will show some shape actually. But my problem I don't want to plot it to a figure and then save it as image. So is that any way to convert that directly to a binary image of size N x N?
Actually, I try something like this
img = zeros(N, N);
xp = int32(x*N/(4*pi));
yp = int32((y+2*pi)*N/(4*pi));
img(xp, yp) = 1;
but it takes forever to run the program.
any better way to do that? Thanks.

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回答（1 个）

Walter Roberson
Walter Roberson 2012-2-29
Use vectors of xp and yp, and use subsref() to do the assignment.
fix() or round() might be more efficient than int32()
  1 个评论
Ki
Ki 2012-2-29
Thanks for your reply. I try round, seems a bit faster but still not good enough. But the way, I try to following the doc but still can't subsref to work. By the way, if I plot(x, p), it do show me the correct image. However, if I use above relation to write it to a matrix img (I try for small matrix size) and then plot img, it doesn't show me what I expect. Is the mapping sort of incorrect?

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