how to straighten lines in an image?

hello I'm trying to detect lanes in an image and for that I want a way to choose only the lanes part of the image and straighten it to a rectangle, I think there's some function in OpenCV so I was wondering if there is one in Matlab as well or some other way to straighten the image at certain points.

thanks :).

Answer 1

[Image Analyst]

Try imwarp().

[noam Y]

thanks ill give it a try

UPDATE:

it's a nice function but it doesnt serve my purpose all the way.

is there any other way that could virtually transform the image on a certain axel? changing shape from an upright trapezoid to a rectangle. thanks.

[Image Analyst]

You can use imtransform(). Here's a snippet from my program. Go ahead and use/adapt it it you want:

  while strcmpi(reply, 'Redraw') > 0
      hold off;
      imshow(original_RGB_Image);
    %   hold on;
      title('Original Image', 'FontSize', 18);
      set(gcf, 'units','normalized','outerposition',[0 0 1 1]); % Maximize figure.
      [rows, columns, numberOfColors] = size(original_RGB_Image);

      userPrompt = sprintf('Left click at the centers of the corner spots on the 96 well plate\ngoing counter-clockwise from the upper left spot.\nFor the 4th spot (upper right) right-click to finish the quadrilateral.');
      uiwait(msgbox(userPrompt));
      numberOfVertices = 0;
      while numberOfVertices ~= 5
        [~, xCorners, yCorners] = roipolyold();
        numberOfVertices = length(xCorners);
        % Take the first 5 points.  If it's less than 5, let them know.
        if numberOfVertices > 5
          % Take the first 5.
          xCorners = xCorners(1:5);
          yCorners = yCorners(1:5);
          numberOfVertices = length(xCorners);  % Recalculate it.
        elseif numberOfVertices < 5
          userPrompt = sprintf('You clicked %d points.\nPlease click exactly 4 points only.\n\nRemember LEFT-CLICK the first 3 corners,\nthen RIGHT-CLICK the last (4th) point.', numberOfVertices-1);
          uiwait(msgbox(userPrompt));
        end
      end
      if printToCommandWindow
        fprintf(1, 'x=%.1f ', xCorners);
        fprintf(1, '\n');
        fprintf(1, 'y=%.1f ', yCorners);
      end

      % Sort the corners into the first one being the upper left, and then going counter-clockwise after that.
      % It won't work otherwise.
      [xCorners, yCorners] = sortCorners(xCorners, yCorners, rows, columns);

      % Draw the quadrilateral.
      hold on;
      plot([xCorners(1) xCorners(2) xCorners(3) xCorners(4) xCorners(1)], [yCorners(1) yCorners(2) yCorners(3) yCorners(4) yCorners(1)], 'linewidth', 3, 'color', [1 0 1]); % Top line
      caption = sprintf('(%.1f, %.1f)', xCorners(1), yCorners(1));
      text(xCorners(1)-25, yCorners(1)+9, caption, 'fontsize', 8, 'Color', [1 1 0]);
      caption = sprintf('(%.1f, %.1f)', xCorners(2), yCorners(2));
      text(xCorners(2)-25, yCorners(2)+9, caption, 'fontsize', 8, 'Color', [1 1 0]);
      caption = sprintf('(%.1f, %.1f)', xCorners(3), yCorners(3));
      text(xCorners(3)-25, yCorners(3)+9, caption, 'fontsize', 8, 'Color', [1 1 0]);
      caption = sprintf('(%.1f, %.1f)', xCorners(4), yCorners(4));
      text(xCorners(4)-25, yCorners(4)+9, caption, 'fontsize', 8, 'Color', [1 1 0]);

      % Create the desired coordinates;
      x1 = mean([xCorners(1) xCorners(2)]);
      x2 = mean([xCorners(3) xCorners(4)]);
      y1 = mean([yCorners(1) yCorners(4)]);
      y2 = mean([yCorners(2) yCorners(3)]);
      % T = maketform('projective',U,X) builds a TFORM struct T for a two-dimensional projective transformation 
      % that maps each row of U to the corresponding row of X. 
      % The U and X arguments are each 4-by-2 and define the corners of input and output quadrilaterals. 
      % No three corners can be collinear.
      U = [xCorners(1), yCorners(1);...
        xCorners(2), yCorners(2);...
        xCorners(3), yCorners(3);...
        xCorners(4), yCorners(4)];
      X = [x1 y1; x1 y2; x2 y2; x2 y1];
      T = maketform('projective', U, X);
      % tformfwd([10 20],T)
      % Let's see where the corners of the image get moved to.
      imageCornersX = [1 1 columns columns];
      imageCornersY = [1 rows rows 1];
      [mappedCornerLocationsX, mappedCornerLocationsY] = tformfwd(T, imageCornersX, imageCornersY);
      if printToCommandWindow
        fprintf(1, '%6.3f\n', mappedCornerLocationsX);
        fprintf(1, '%6.3f\n', mappedCornerLocationsY);
      end
      newUpperLeftX = min(mappedCornerLocationsX);
      newUpperLeftY = min(mappedCornerLocationsY)  ;

      % Transform (warp) the image.
      transformedImage = imtransform(original_RGB_Image, T);
      clear('original_RGB_Image'); % Free up memory for wimpy 32 bit computers.
      axes(handles.axesImage);
      imshow(transformedImage);
      % subplot(1,2,2);
      figure(handleToOriginal);
      imshow(transformedImage, []);
    %   set(gcf, 'units','normalized','outerposition',[0 0 1 1]); % Maximize figure.
      title('Transformed image', 'FontSize', 18);
      drawnow;
      hold on;

      diameter = UserSettings.circleDiameter; % Diameter of spot or width of rectangle.

      % Draw the shapes and bounding box.
      % The line was drawn in the centers of the circles, but the circles are drawn with the rectangle function which actually has
      % the (x,y) location as the upper left of the bounding box of the circle.
      xSpacing = abs(x2 - x1) / (UserSettings.columns - 1);
      ySpacing = abs(y2 - y1) / (UserSettings.rows - 1);
      % Draw the bounding box.
      line([x1 - newUpperLeftX, x2 - newUpperLeftX], [y1 - newUpperLeftY, y1 - newUpperLeftY], 'linewidth', 3, 'color', [1 0 1]); % Top line
      line([x1 - newUpperLeftX, x1 - newUpperLeftX], [y1 - newUpperLeftY, y2 - newUpperLeftY], 'linewidth', 3, 'color', [1 0 1]); % Left line
      line([x1 - newUpperLeftX, x2 - newUpperLeftX], [y2 - newUpperLeftY, y2 - newUpperLeftY], 'linewidth', 3, 'color', [1 0 1]); % Bottom line
      line([x2 - newUpperLeftX, x2 - newUpperLeftX], [y1 - newUpperLeftY, y2 - newUpperLeftY], 'linewidth', 3, 'color', [1 0 1]); % Right line

      % Make up arrays of the dpecified shape.
      if UserSettings.isCircle
        % Initialize an image to hold one single small circle. 
        smallCircleRadius = diameter / 2.0;
        smallCircleImage = zeros(2*smallCircleRadius, 2*smallCircleRadius); 
        [x, y] = meshgrid(1:smallCircleRadius*2, 1:smallCircleRadius*2); 
        singleShapeImage = false(2*smallCircleRadius, 2*smallCircleRadius); 
        singleShapeImage((x - smallCircleRadius).^2 + (y - smallCircleRadius).^2 <= smallCircleRadius.^2) = true; 
      else
        % The shapes are rectangles.
        % Initialize an image to hold one single small rectangle. 
        singleShapeImage = true(diameter); 
      end
      hold off;
%       imshow(singleShapeImage, []);
%       drawnow;
      
      [transformedRows, transformedCols, transformedNumberOfColors] = size(transformedImage);
      shapeMaskArray = false([transformedRows, transformedCols]);
      [singleHeight, singleWidth] = size(singleShapeImage); 

      % x1, x2, y1, and y2 all refer to the center of the circle, not the upper left of the bounding box.
      for row = 1 : UserSettings.rows
    %     yctr0 = (row - 1) * ySpacing + y1;
        yctr = (row - 1) * ySpacing + y1 - newUpperLeftY;
        boxY = yctr - diameter/2;
        for col = 1 : UserSettings.columns
    %       xctr0 = (col - 1) * xSpacing + x1;
    %       plot(xctr0, yctr0, '+');
          xctr = (col - 1) * xSpacing + x1 - newUpperLeftX;
          boxX = xctr - diameter/2;
          rectangle('Position',[boxX, boxY, diameter, diameter], 'Curvature',[1,1], 'EdgeColor', [0 .5 0], 'LineWidth', 2);
  %         caption = sprintf('(%d, %d)', round(xctr), round(yctr));
  %         text(xctr-25, yctr+9, caption, 'fontsize', 8, 'Color', [1 1 0]);
          centers(row, col, 1) = xctr;
          centers(row, col, 2) = yctr;

          % Find the square in the big image where we're going to add a small circle. 
          x1b = ClipToRange(boxX, 1, transformedCols);
          x2b = ClipToRange((x1b + singleWidth - 1), 1, transformedCols);
          y1b = ClipToRange(boxY, 1, transformedRows);
          y2b = ClipToRange((y1b + singleHeight - 1), 1, transformedRows);
          % Add in one small circle to the existing big image. 
          shapeMaskArray(y1b:y2b, x1b:x2b) = shapeMaskArray(y1b:y2b, x1b:x2b) + singleShapeImage; 
        end
      end
      % It might be 2 if shapes overlap.  Make sure the max is 1
      shapeMaskArray = min(shapeMaskArray, 1);
%       imshow(shapeMaskArray);
%       drawnow;
    %   handleToMaskFigure = figure;
    %   imshow(circleMaskArray, []);
    %   set(gcf, 'units','normalized','outerposition',[0 0 1 1]); % Maximize figure.
    
      % Make a visualization where the masked array regions are full brightness but the out of mask regions are half brightness.
      % Mask the image using bsxfun() function
      maskedRgbImage = bsxfun(@times, transformedImage, cast(shapeMaskArray, class(transformedImage)));
      outsideMaskRgbImage = uint8(bsxfun(@times, transformedImage, cast(~shapeMaskArray, class(transformedImage)))/2); % Half brightness.
      imshow(maskedRgbImage + outsideMaskRgbImage);
      drawnow;
      clear('maskedRgbImage', 'outsideMaskRgbImage'); % Free up memory
      [labeledImage, numberOfBlobs] = bwlabel(shapeMaskArray, 8);     % Label each blob so we can make measurements of it
      clear('shapeMaskArray'); % Free up memory.
      
      % Get each plane and its measurements.  Then free up the image to reduce memory usage as much as possible for 32 bit computers.
      redPlane = transformedImage(:, :, 1);
      blobMeasurementsR = regionprops(labeledImage, redPlane, 'all');   
      clear('redPlane');
      greenPlane = transformedImage(:, :, 2);
      blobMeasurementsG = regionprops(labeledImage, greenPlane, 'all');   
      clear('greenPlane');
      bluePlane = transformedImage(:, :, 3);
      blobMeasurementsBlue = regionprops(labeledImage, bluePlane, 'all');   
      clear('bluePlane');

      if printToCommandWindow
        fprintf(1,'Blob #      Mean Red, Mean Green, Mean Blue,   Centroid-x Centroid-y\n');
      end
      for k = 1 : numberOfBlobs           % Loop through all blobs.
        % Find the mean of each blob.  (R2008a has a nice way where you can pass the original image
        % directly into regionprops.)
        thisBlobsPixelsR = blobMeasurementsR(k).PixelIdxList;  % Get list of pixels in current blob.
        thisBlobsPixelsG = blobMeasurementsG(k).PixelIdxList;  % Get list of pixels in current blob.
        thisBlobsPixelsBlue = blobMeasurementsBlue(k).PixelIdxList;  % Get list of pixels in current blob.
        meanGLR = blobMeasurementsR(k).MeanIntensity; % Mean again, but only for version >= R2008a
        meanGLG = blobMeasurementsG(k).MeanIntensity; % Mean again, but only for version >= R2008a
        meanGLBlue = blobMeasurementsBlue(k).MeanIntensity; % Mean again, but only for version >= R2008a

        blobCentroid = blobMeasurementsR(k).Centroid;    % Get centroid.
        if printToCommandWindow
          fprintf(1,'     #%d:   %8.2f %8.2f    %8.2f,     %8.1f %8.1f\n', k, meanGLR, meanGLG, meanGLBlue, blobCentroid);
        end
        text(blobCentroid(1), blobCentroid(2), num2str(k), 'Color', 'g');
      end
      drawnow;

      % Ask user if it's okay.
      userPrompt = sprintf('Please verify the mask placement.\nDo you want to accept it, or redraw it?');
      reply = questdlg(userPrompt, ...
        'Mask OK?', 'Accept', 'Redraw', 'Accept');
      drawnow;
      % reply = '' for Upper right X, otherwise it's the exact wording.
    end