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Python PIL library allows me to map any quadrilateral in an image to rectangle using

im.transform(size, QUAD, data)

What I need is a function that does the opposite, i.e. map a rectangular image to specified quadrilateral.

I figured this might be achieved with the above mentioned function like this:

enter image description here

I.e. I would find such quad (the red one in the image) that would, using the function im.transform(size, QUAD, data) transform the image to quad I want. The problem is I don't know how to find the red quad.

I would appreciate any idea on how to find the red quad or any other way to map a rect image to quad, only with PIL if possible.

Miloslav Číž
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  • So far I have found this article: http://www.fmwconcepts.com/imagemagick/bilinearwarp/FourCornerImageWarp2.pdf. – Miloslav Číž May 30 '16 at 15:34
  • I have never done this myself. However, reading the documentation, it seems like `im.transform(size, PERSPECTIVE, data) image` is doing what you try to achieve. – physicalattraction May 30 '16 at 18:26

1 Answers1

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So I solved the issue with a simple forward mapping, rather than inverse mapping, which is usually better, but in my application I only ever map the rectangle to a quad that is smaller than the rectangle, so there are usually no holes in the transformed image. The code is as follows:

def reverse_quad_transform(image, quad_to_map_to, alpha):
  # forward mapping, for simplicity

  result = Image.new("RGBA",image.size)
  result_pixels = result.load()

  width, height = result.size

  for y in range(height):
    for x in range(width):
      result_pixels[x,y] = (0,0,0,0)

  p1 = (quad_to_map_to[0],quad_to_map_to[1])
  p2 = (quad_to_map_to[2],quad_to_map_to[3])
  p3 = (quad_to_map_to[4],quad_to_map_to[5])
  p4 = (quad_to_map_to[6],quad_to_map_to[7])

  p1_p2_vec = (p2[0] - p1[0],p2[1] - p1[1])
  p4_p3_vec = (p3[0] - p4[0],p3[1] - p4[1])

  for y in range(height):
    for x in range(width):
      pixel = image.getpixel((x,y))

      y_percentage = y / float(height)
      x_percentage = x / float(width)

      # interpolate vertically
      pa = (p1[0] + p1_p2_vec[0] * y_percentage, p1[1] + p1_p2_vec[1] * y_percentage) 
      pb = (p4[0] + p4_p3_vec[0] * y_percentage, p4[1] + p4_p3_vec[1] * y_percentage)

      pa_to_pb_vec = (pb[0] - pa[0],pb[1] - pa[1])

      # interpolate horizontally
      p = (pa[0] + pa_to_pb_vec[0] * x_percentage, pa[1] + pa_to_pb_vec[1] * x_percentage)

      try:
        result_pixels[p[0],p[1]] = (pixel[0],pixel[1],pixel[2],min(int(alpha * 255),pixel[3]))
      except Exception:
        pass

  return result
Miloslav Číž
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