detect corners of transparent rectangles in a bigger picture

Hi, I need a script that detects the coordinates of the transparent rectangles within an image. example image: [login to view URL]\_museum\_4\_3\[login to view URL] on the right side you see transparent spaces, and I need of every image the x and y coordinate of each corner. the script shall be executable with linux command line or within php, and return the coordinates machinable readable. The script shall also work with big images like here [login to view URL]\_museum\_4\_3\[login to view URL] I know that it is hard or even impossible to detect the areas on the left side, but let me know if you have an idea. The detected areas must always be rectangles (areas with 4 corners) Best, Kai