Get rid off white border in spectrum

Question

Dominik Deml 2023-6-26

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1988043-get-rid-off-white-border-in-spectrum

编辑： DGM 2023-6-26

采纳的回答： Dyuman Joshi

在 MATLAB Online 中打开

Hi there, I want to plot the

spectrum of a grayscale image.

Unfortunately there is a white border on the right side

and at the bottom.

My code:

x = 1:1:30;     % vorher>: 0:1:30;
y = 1:1:30;     % vorher: 0:1:30;
[X, Y] = meshgrid(x, y);
% Sinusfunktion für diagonale Linien
fx = 0.1;        % Ortsfrequenz (1/Pixel) 
fy = 0.2;        % Ortsfrequenz (1/Pixel)
p = cos(2*pi*(fx*X + fy*Y)) + 1;
y1 = fft2(p);
freq_x = fftfreq(length(x));
freq_y = fftfreq(length(y));
figure;
imagesc(freq_x, freq_y, log10(1 + abs(fftshift(y1))));
line([0,0], [-0.5,0.5], 'Color', 'r', 'LineWidth',1);
line([-0.5,0.5], [0,0], 'Color', 'r', 'LineWidth',1);
axis image
colormap('gray');

The spectrum currently looks like this:

This is my fftfreq function:

function freqs = fftfreq(n, fs)
end
if nargin < 2
    fs = 1;
end
results = zeros(1, n);
N = fix((n-1)/2) + 1;
results(1:N) = 0:N-1;
results(N+1:end) = (-fix(n/2):-1);
freqs = fftshift(results * (fs / n));

How can I remove the white border?

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VBBV 2023-6-26

编辑：VBBV 2023-6-26

在 MATLAB Online 中打开

Choose min & max range values for the X & Y scales in the axis function instead of default scaling of axis image

x = 1:1:30; % vorher>: 0:1:30;

y = 1:1:30; % vorher: 0:1:30;

[X, Y] = meshgrid(x, y);

% Sinusfunktion für diagonale Linien

fx = 0.1; % Ortsfrequenz (1/Pixel)

fy = 0.2; % Ortsfrequenz (1/Pixel)

p = cos(2*pi*(fx*X + fy*Y)) + 1;

y1 = fft2(p);

freq_x = fftfreq(length(x));

freq_y = fftfreq(length(y));

figure;

h = imagesc(freq_x, freq_y, log10(1 + abs(fftshift(y1))))

h =

Image with properties: CData: [30×30 double] CDataMapping: 'scaled' Show all properties

line([0,0], [-0.5,0.5], 'Color', 'r', 'LineWidth',1);

line([-0.5,0.5], [0,0], 'Color', 'r', 'LineWidth',1);

axis([min(freq_x) max(freq_x) min(freq_y) max(freq_y) ]);

colormap('gray');

function freqs = fftfreq(n, fs)

if nargin < 2

fs = 1;

end

results = zeros(1, n);

N = fix((n-1)/2) + 1;

results(1:N) = 0:N-1;

results(N+1:end) = (-fix(n/2):-1);

freqs = fftshift(results * (fs / n));

end

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Answer 1

Dyuman Joshi 2023-6-26

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1988043-get-rid-off-white-border-in-spectrum#answer_1262463

在 MATLAB Online 中打开

It's not a border, it's the area of figure where there is no data to plot, and the white background is showing.

You can either remove/modify the axis command, and/or update the x and y limits manually

x = 1:1:30; % vorher>: 0:1:30;

y = 1:1:30; % vorher: 0:1:30;

[X, Y] = meshgrid(x, y);

% Sinusfunktion für diagonale Linien

fx = 0.1; % Ortsfrequenz (1/Pixel)

fy = 0.2; % Ortsfrequenz (1/Pixel)

p = cos(2*pi*(fx*X + fy*Y)) + 1;

y1 = fft2(p);

freq_x = fftfreq(length(x));

freq_y = fftfreq(length(y));

figure;

imagesc(freq_x, freq_y, log10(1 + abs(fftshift(y1))));

line([0,0], [-0.5,0.5], 'Color', 'r', 'LineWidth',1);

line([-0.5,0.5], [0,0], 'Color', 'r', 'LineWidth',1);

%axis command commented out

%axis image

colormap('gray');

function freqs = fftfreq(n, fs)

if nargin < 2

fs = 1;

end

results = zeros(1, n);

N = fix((n-1)/2) + 1;

results(1:N) = 0:N-1;

results(N+1:end) = (-fix(n/2):-1);

freqs = fftshift(results * (fs / n));

end

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Image Analyst 2023-6-26

在 MATLAB Online 中打开

fft2 operates on the whole image, not just what is visible.

p = cos(2*pi*(fx*X + fy*Y)) + 1;
y1 = fft2(p);

So how will changing the tick marks or axis limits after the above code change y1 at all?

DGM 2023-6-26

编辑：DGM 2023-6-26

The goal isn't to change y1 at all. This is apparently a matter of axes extents. The range of freq_x and freq_y are both [-0.5 0.4667]. That is the extent of the image object in the current axes. The two red lines span [-0.5 0.5], forcing the axes to be larger than the image object. Like @Dyuman Joshi says, there is no border to crop off.

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