How to accurately acquire line segments from the projection plot?

Question

So this is basically something very simple, as in just get the horizontal projection plot and from that get the location of the lines on the image. But the problem is that the threshold that is applied is very variable. If I stay at a safe level, the correct number of lines are extracted whereas on the other hand unwanted results are extracted.

For example here is the image:

And its horizontal projection:

And here is the code I am using to extract the text lines:

%complementing as text must be non zero and background should be 0
img_comp = imcomplement(img);

%calculate the horizontal projections and plot it to verify the threshold
horizontal_projections = sum(img_comp, 2);
plot(horizontal_projections)

%A very crude method of automatically detecting the threshold

proj_mean = mean(horizontal_projections);
lines = horizontal_projections > floor(proj_mean); 

% Find Rising and falling edges
d = diff(lines);
startingColumns = find(d>0);
endingColumns = find(d<0);

% Extract each line and save it in a cell
for lines_k = 1 : length(startingColumns)
  lines_extracted{lines_k} = img(startingColumns(lines_k):endingColumns(lines_k), :);
end

I want to automate the threshold selection but am having trouble, if I use the threshold shown in my code that is the mean of the projections, it does extract 9 lines which are correct but the lines lose a lot of data as in:

This is the second line, the extenders and descenders of the letters have been cut off. Using the half of mean or third of it works but its different for every image and does not automate it at all.

Answer 1

What about converting to YCbCr color space? Using the conversion formula from Wikipedia.

img = im2double(imread('StackOverflow-Example.jpg'));
rp = img(:, :, 1) / 255 ;
bp = img(:, :, 2) / 255 ;
gp = img(:, :, 3) / 255 ;
kb = 0.114;
kr = 0.299;
y = kr * rp + (1 - kr - kb) * gp + kb * bp;
y = max(max(y))-y;
y = y ./ y;
surf(y,'EdgeColor','none','LineStyle','none')
view(0, -90)

It looks like a good job of maintaining the information.

Edit:

I think you want each line

%% Load image and find intensity %%
img = im2double(imread('test.jpg')); % load image and convert to doubles to allow for calculations
rp = img(:, :, 1) / 255 ; % normalized red portion
bp = img(:, :, 2) / 255 ; % normalized blue portion
gp = img(:, :, 3) / 255 ; % normalized green portion
kb = 0.114; % blue constant from Wikipedia
kr = 0.299; % red constant from Wikipedia
x = kr * rp + (1 - kr - kb) * gp + kb * bp; % normalized intensity in image
x = max(max(x))-x; % removed background

y = x ./ x; % everything left is high

z = y;
z(isnan(y)) = 0; % turn nan's to zero
divisions = find(sum(z,2) > 5); % find all lines that have less than 5 pixels
divisions = [divisions(1); divisions(diff(divisions) > 10); size(z, 1)]; % find the line breaks

rows = cell(length(divisions), 1);

for i = 1:numel(rows)-1
    line = z(divisions(i):divisions(i+1), :); % grab line
    j = divisions(i) + find(sum(line,2) > 5) - 1; % remove the white space
    line = y(j, :);
    rows{i} = line; %store the line
end

rows(numel(rows)) = [];

%% plot each line %%
for i = 1:numel(rows) ; 
    figure(i) ; 
    surf(rows{i},'EdgeColor','none','LineStyle','none');
    view(0, -90) ;
end

%% plot entire page %%
figure(numel(rows) + 1)
surf(y,'EdgeColor','none','LineStyle','none') % plot of entire image
view(0, -90)

Edit: 2015/05/18 15:45 GMT

This has the values for the intensity left in:

img = im2double(imread('test.jpg'));
rp = img(:, :, 1) / 255 ;
bp = img(:, :, 2) / 255 ;
gp = img(:, :, 3) / 255 ;
kb = 0.114;
kr = 0.299;
x = kr * rp + (1 - kr - kb) * gp + kb * bp;
x = max(max(x))-x;
xp = x;
xp(xp == min(min(xp))) = nan;

y = x ./ x;

z = y;
z(isnan(y)) = 0;
divisions = find(sum(z,2) > 5);
divisions = [divisions(1); divisions(diff(divisions) > 10); size(z, 1)];

rows = cell(length(divisions) - 1, 1);

for i = 1:numel(rows)
    line = z(divisions(i):divisions(i+1), :);
    j = divisions(i) + find(sum(line,2) > 5) - 1;
    line = xp(j, :);
    rows{i} = line;

    figure(i) ; 
    surf(rows{i},'EdgeColor','none','LineStyle','none');
    axis('equal')
    view(0, -90) ;
end

figure(numel(rows) + 1)
surf(xp,'EdgeColor','none','LineStyle','none')
axis('equal')
view(0, -90)

---

Edit 2015-05-22 13:21 GMT

%Turn warning message off
warning('off', 'Images:initSize:adjustingMag');

%Read in image in int8
originalImg = imread('test.jpg');

%Convert to double
img = im2double(originalImg);

%Take R, G, & B components
rp = img(:, :, 1) ;
gp = img(:, :, 2) ;
bp = img(:, :, 3) ;

%Get intensity
kb = 0.114;
kr = 0.299;
yp = kr * rp + (1 - kr - kb) * gp + kb * bp;

%Flip to opposite of intensity
ypp = max(max(yp))-yp;

%Normalize flipped intensity
z = ypp ./ ypp;
z(isnan(z)) = 0;

%Find lines, this may need to be tuned
MaxPixelsPerLine = 5;
MinRowsPerLine = 10;
divisions = find(sum(z,2) > MaxPixelsPerLine);
divisions = [divisions(1); divisions(diff(divisions) > MinRowsPerLine); size(z, 1)];

%Preallocate for number of lines
colorRows = cell(length(divisions) - 1, 1);

for i = 1:numel(rows)
    %Extract the lines in RGB
    line = z(divisions(i):divisions(i+1), :);
    j = divisions(i) + find(sum(line,2) > 5) - 1;
    colorRows{i} = originalImg(j, :, :);

    %Print out the line
    figure(i) ;
    imshow(colorRows{i})
end

%Print out the oringinal image
figure(numel(rows) + 1)
imshow(originalImg)

%Turn the warning back on
warning('on', 'Images:initSize:adjustingMag');