Illustration of the different steps in the extraction of a spectrogram (right) of an extended source along its photometry thumbnail (left). Top: Original orientation in the FP. Middle: Rotation to bring the dispersion direction to horizontal. Bottom: Shear to bring the virtual slit along the cross-dispersion direction and minimise self-contamination. This illustration is a simplified case with no initial tilt or curvature in the spectral trace. In practice, rotation and shear are performed in a single step to minimise correlations between resampled pixels.