The input of DeltaE is straightforward: a supported test chart should be present inside the image. We expect the target to be present in the upright position, as straight as possible. This restriction has two reasons. One reason is that it speeds up the feature/target detection, the other is that slanted edges have an optimal angle for SFR readout that is approximatelly 5 degrees. So if the target is near that number, your slanted-edge results can be erroneous.