Hi Willem,

However, when I use this transformation matrix in my own code, I get strange results, even using the function gimp_matrix3_transform_point from gimpmatrix.c.

It looks like you are applying the transform in the wrong direction...

To compute a transformed coordinate (x', y') I simply multiply (x, y, 1) by the transformation matrix that GIMP shows

In that case, I think that the transformation converts the destination coordinate to the corresponding source coordinate.

Hi,

On Tue, 2007-05-29 at 14:50 +0200, Willem Vervuurt wrote:

I am trying to build an algorithm that transforms pictures (all taken from the same viewpoint) into their corrected-for-perspective equivalents.
In the GIMP, I can apply the desired perspective transformation to an image and retrieve the required transform matrix (it is {{0.485, -0.374, 178}, {0, 0.45, 0}, {0, 0, 1}}, reading rows). (See http://www.rodo.nl/input.jpg and http://www.rodo.nl/result.jpg for input image & desired output, respectively).

I think you are making the wrong assumption that the perspective transform tool would apply an affine transformation which can be expressed as a 3x3 matrix. You probably know these matrices for example from SVG files. The transformation performed by the Perspective transform tool is however not necessarily an affine transformation. It uses a 4x4 matrix. Only the upper left 3x3 submatrix 3x3 is displayed because the other values are always 0.0 or 1.0.

Sven