I was doing a fun project: Solving a Sudoku from an input image using OpenCV (as in Google goggles etc). And I have completed the task, but at the end I found a little problem for which I came here.
I did the programming using Python API of OpenCV 2.3.1.
Below is what I did :
 Read the image
 Find the contours
 Select the one with maximum area, ( and also somewhat equivalent to square).

Find the corner points.
e.g. given below:
(Notice here that the green line correctly coincides with the true boundary of the Sudoku, so the Sudoku can be correctly warped. Check next image)

warp the image to a perfect square
eg image:

Perform OCR ( for which I used the method I have given in Simple Digit Recognition OCR in OpenCVPython )
And the method worked well.
Problem:
Check out this image.
Performing the step 4 on this image gives the result below:
The red line drawn is the original contour which is the true outline of sudoku boundary.
The green line drawn is approximated contour which will be the outline of warped image.
Which of course, there is difference between green line and red line at the top edge of sudoku. So while warping, I am not getting the original boundary of the Sudoku.
My Question :
How can I warp the image on the correct boundary of the Sudoku, i.e. the red line OR how can I remove the difference between red line and green line? Is there any method for this in OpenCV?
Best Solution
I have a solution that works, but you'll have to translate it to OpenCV yourself. It's written in Mathematica.
The first step is to adjust the brightness in the image, by dividing each pixel with the result of a closing operation:
The next step is to find the sudoku area, so I can ignore (mask out) the background. For that, I use connected component analysis, and select the component that's got the largest convex area:
By filling this image, I get a mask for the sudoku grid:
Now, I can use a 2nd order derivative filter to find the vertical and horizontal lines in two separate images:
I use connected component analysis again to extract the grid lines from these images. The grid lines are much longer than the digits, so I can use caliper length to select only the grid linesconnected components. Sorting them by position, I get 2x10 mask images for each of the vertical/horizontal grid lines in the image:
Next I take each pair of vertical/horizontal grid lines, dilate them, calculate the pixelbypixel intersection, and calculate the center of the result. These points are the grid line intersections:
The last step is to define two interpolation functions for X/Y mapping through these points, and transform the image using these functions:
All of the operations are basic image processing function, so this should be possible in OpenCV, too. The splinebased image transformation might be harder, but I don't think you really need it. Probably using the perspective transformation you use now on each individual cell will give good enough results.