Calculating scale, rotation and translation from Homography matrix

if you can use opencv 3.0, this decomposition method is available http://docs.opencv.org/3.0-beta/modules/calib3d/doc/camera_calibration_and_3d_reconstruction.html#decomposehomographymat

The right answer is to use homography as it is defined dst = H ⋅ src and explore what it does to small segments around a particular point.

Translation

Given a single point, for translation do

T = dst - (H ⋅ src)

Rotation

Given two points p₁ and p₂

p₁ = H ⋅ p₁

p₂ = H ⋅ p₂

Now just calculate the angle between vectors p₁ p₂ and p₁' p₂'.

Scale

You can use the same trick but now just compare the lengths: |p₁ p₂| and |p₁' p₂'|.

To be fair, use another segment orthogonal to the first and average the result. You will see that there is no constant scale factor or translation one. They will depend on the src location.