matlab通过fopen实现一个函数读取和写入PPM和PGM图像的功能

可以使用以下函数读取和写入PPM和PGM图像：

function img = readImage(filename)
% Read PPM or PGM image file
% Usage: img = readImage(filename)
% Input:
%   filename - string, the image file name
% Output:
%   img - a 3D array, the image data

fid = fopen(filename, 'r');
if fid < 0
    error('Unable to open file');
end

% Read header
header = fgetl(fid);
if strcmp(header, 'P6')
    color = true;
elseif strcmp(header, 'P5')
    color = false;
else
    error('Invalid image format');
end

line = fgetl(fid);
while line(1) == '#'
    line = fgetl(fid);
end

[width, height] = sscanf(line, '%d %d');
maxval = fscanf(fid, '%d', 1);

% Read image data
if color
    img = uint8(zeros(height, width, 3));
    data = fread(fid, [3, width*height], 'uint8');
    img(:, :, 1) = reshape(data(1, :), [width, height])';
    img(:, :, 2) = reshape(data(2, :), [width, height])';
    img(:, :, 3) = reshape(data(3, :), [width, height])';
else
    img = uint8(zeros(height, width));
    data = fread(fid, [1, width*height], 'uint8');
    img(:, :) = reshape(data, [width, height])';
end

fclose(fid);

end

function writeImage(filename, img)
% Write PPM or PGM image file
% Usage: writeImage(filename, img)
% Input:
%   filename - string, the image file name
%   img - a 3D array, the image data

if size(img, 3) == 3
    color = true;
elseif size(img, 3) == 1
    color = false;
else
    error('Invalid image format');
end

if color
    header = 'P6';
else
    header = 'P5';
end

fid = fopen(filename, 'w');
if fid < 0
    error('Unable to open file');
end

fprintf(fid, '%s\n%d %d\n%d\n', header, size(img, 2), size(img, 1), 255);

if color
    fwrite(fid, permute(img, [3, 1, 2]), 'uint8');
else
    fwrite(fid, img', 'uint8');
end

fclose(fid);

end

使用示例：

% Read PPM image
img_ppm = readImage('image.ppm');

% Write PGM image
img_pgm = rgb2gray(img_ppm);
writeImage('image.pgm', img_pgm);