Cross process a photo
Cross processing bends red, green, and blue along separate curves instead of shifting the whole image toward one color. Each channel gets its own contrast, its own shadow toe, and its own highlight shoulder, which is why the result reads as crossed rather than tinted: the cast in the shadows runs opposite to the cast in the highlights.
Strength blends the graded result back against the original, so it works as a single dial once the other controls are set.
The two directions
Slide film in C-41 is the dramatic one. Contrast climbs, shadows go cyan, highlights go yellow-green, and saturated areas clip early. This is the look most people mean by cross processing.
Negative film in E-6 goes the other way. Contrast drops, every channel’s toe lifts, and the frame turns milky and blue with weaker color separation. Pick it when you want the washed, flat version rather than the punchy one.
Shadow and highlight crossing
The two crossing sliders control opposite ends of the same curve. Shadow crossing moves the toe, which sets how much color builds up in the dark end. Highlight crossing moves the shoulder, which sets how much color drains out of the bright end.
Raising one without the other gives a single cast rather than a cross. The look depends on both ends moving in opposite directions, so keep them within roughly 20 points of each other unless you want a plain color grade.
Example: flat daylight portrait
Start on Slide film in C-41 at the defaults. Drop Saturation to about 110 if skin turns orange, then lower Highlight crossing toward 45 so the yellow does not settle in the brightest part of the face. Leave Shadow crossing high: the cyan in the shadows is doing most of the work, and pulling it back removes the effect before it removes the problem.
Where the look breaks down
Curve contrast pushes tones toward both ends, so detail already near white or black is the first thing lost. A photo with blown highlights has nothing left for the shoulder to shape, and the yellow cast lands on flat white instead of on gradation.
Low-contrast scenes such as fog or overcast snow have the opposite problem. There is little tonal separation for the curves to act on, so the result reads as a color cast rather than a processed frame.