Instructions: Open your image file that includes a photographic reference scale, then drag the red markers to the corners of the reference scale in the image, and press 'Rectify'. If desired, refine the corner positions and 'Rectify' again. Detailed instructions are on the bottom half of this page. Works only with a mouse/trackpad.
Distortion due to photographing at an oblique angle can be rectified using geometric transformations, as illustrated in fig 1 below. After rectification, the photographic reference scale, and anything in the same plane, appears as if it had been photographed from the direction perpendicular to that plane.
Fig 1. Image (close-up with wide angle lens) before and after use of rectification tool.
This tool is specialized to work with images containing this photographic reference scale design. You locate the corners of the reference scale in the image by dragging the red markers. Then the tool's browser-based software uses homology to map those points to the known shape of the reference scale, as outlined by the yellow dashed rectangle superimposed on your image -- and in the process transforms the entire image similarly.
This works well to rectify the reference scale. Whether the same transformation rectifies the rest of your image depends on what is in your photo. The transformation works best when everything in your photo is in the same plane as the reference scale. If the subject was not planar, or not co-planar with the reference scale, off-plane points in the image will likely still look distorted. Also, of course the image transformation cannot create information that isn't already in your photo; if something isn't visible in the photo, it won't be visible in the rectified image either, even though that 'something' might have been visible to the camera if the photo had been taken perpendicular to the subject.
The transformation can be based upon either three or four corners, using either orthographic or perspective transformation respectively. Orthographic transformation is a special case of perspective transformation, where the camera was an infinite distance from the subject. There is no harm in always using four corners -- it's just more slightly work, having to position the fourth marker.
Images are processed in your browser; they don't leave your browser. Your browser's local storage is used if you save marker locations (the file name and marker locations are saved).
Step 1: Open an image containing the reference scale
Step 2: Drag the corner markers to the corners of the reference scale
Step 3: Press 'Rectify'. The image will be transformed such that the area within your markers is mapped to the shape of the reference scale, as indicated by the dashed yellow lines. If necessary, reposition the markers and press 'Rectify' again.
To see the entire image, press 'Fit image to window'. The Firefox built-in screenshot tool is a convenient way to save the image to a file.
To view without the markers and yellow dashed lines, unclick 'Show guide and corner markers'.
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