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@LouisMarshall There's a few different topology strategies for sharpening those corners while also minimizing the visibility of any smoothing artifacts. Which approach makes the most sense will depend on how accurate the shapes need to be, how visible the area is and how much time was spent creating the rest of the model.

If the subtracted shape is relatively small or won't spend a lot of time in-front of the player then the easiest solution would be to use the existing geometry and adjust the topology routing so the adjacent edges create a support loop that flows around the outside of the shape intersection. After the topology flow is resolved it should be possible to select the edges of the shapes and bevel to add support loops. The bevels in the corner can be resolved to flattened diamond shaped quads that connect to the geometry of the underlying curve.

Vertices at the top of the inside corners may need to be moved downward to compensate for the smoothing stress when the subdivision is applied. Using the existing geometry of the curved surface as the outer support loop and using the transitional area between the loops to make up the difference between the shapes should prevent the corner support loops from running out into the adjacent shape and causing undesired deformation when subdivision is applied.

Here's an example of what this process and topology layout could look like.

If more shape accuracy is required then it may be necessary to increase the geometry density of the underlying curve before subtracting the shape. Adjust the number of segments in the curved surface until the corners of the intersecting geometry lands between a shared set of edges. These radial edges in the curve can then be used as support loops that tie into the corners of the subtracted shape.

Adjust the topology so the edges that run across the curved surface act as outer support loops for the shape intersection. Add the rest of the support loops around the shapes using a bevel operation and merge down any stray geometry to the outside support loop on the curved shape. Join through or add support loops as required. Any difference between the curved surface and the subtracted shape should be taken up by the outer support loop. This will help constrain any potential smoothing deformation to a smaller area. Which will help minimize any potential smoothing artifacts.

Here's an example of what this process and topology layout could look like.

In subdivision modeling, it's generally considered best practice to use the existing geometry of the intersecting shapes as support loops and to try and resolve most of the topology flow issues at the lowest possible level by matching the segments of adjacent shapes. Both of the topology strategies shown above take advantage of the existing geometry in the curve. The only major difference between these two strategies is whether the support loops for the corners need to end on the existing geometry of the curved surfaces or whether these support loops can run out into the existing geometry of the curved surface.

Here's a close up comparison of the topology layouts and the subdivision previews. Both topology strategies should be viable at intermediate view distances. The subtle difference in surface quality really isn't all that noticeable unless the player is viewing the model in an extreme close up. So whether or not it makes sense to add in all of that extra geometry really comes down to how the model will be used and whether the tradeoff in modeling efficiency is worth the slight loss of surface quality.

Recap:

• Solve topology flow issues at the lowest level possible.
• Match the segments of intersecting shapes when possible.
• Use the existing geometry as support for shape intersections.
• Route the topology flow around the perimeter of the intersecting shapes.
• Consider the viewpoint and make tradeoffs between accuracy and efficiency when appropriate.