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How To Do Retopology In Blender

January 07, 2023

Ben Shinn
Ben Shinn
The finished engine.

Retopology is one of the most fun and enjoyable parts of modelling. Not. However, this painfully tedious process is often necessary especially when using a Boolean modifier workflow.

The Problem

I’m working on a model of the XS Light Freighter that I am creating for a Star Wars fan film I’m working on. Modelling spaceships should be fun right? This section of the model is one of the rear engines and I need to add some more detail.

XS Freighter Engine

First there are a series of vents that need to be added (I assume they’re for cooling or exhaust or some other technical purpose). There are also these square cut outs that need to be made too.

Boolean Cut Guides

However, once the Boolean cuts have been made the shading doesn’t look quite right.

Boolean Shading Issues

Tabbing into edit modes shows how the geometry has been messed up. It might not look too bad at first, but I’ve created several Ngons (faces that have more than 4 edges) and completely broken the flow of the edge loops. Trying to add a sub-surface modifier doesn’t go well either.

Subdivide Surface Modifier Issues

Retopology Time!

Time to fix the model. As both the left and right sides of the model are identical, likewise so are the top and bottom, I can save time by cutting the model into one quarter and adding a mirror modifier on the x and y axes.

The Engine Has Been Cut Into A Quarter

I’ll start with the square cut-out sections. I can add 2 edge cuts either side of the cut out, although the loops won’t extend into the Ngon faces.

Adding an Edge Loop

Next, I’ll delete the 2 Ngon faces and join the edge loops with the corresponding edge from the cut out.

Joining The Edge Loops

Now it’s time for 2 more edge loops at the end of the engine. I’ve added them halfway between the already existing edge loops instead of trying to line them up with the corners of the cut out. If I did line up the new loops with the cut out corners, this would mess up the curvature of the engine and the sub surface modifier wouldn’t be as smooth in the areas with more geometry.

The Top Edge Loops

Then I connect the new edge loops with the cut out using the bridge edge loops option.

Bridging The Edge Loops

Now to fix the cut out, the first step is to select the 2 corner vertices and subdivide the selected edge.

Subdivide The Edge

Then I can use the knife tool to add 2 cuts in the centre of the cut out.

Making A Cut With The Knife Tool

Then I can fill in the 2 missing faces.

Filling In The Gaps

After that I repeat all the steps on the other side of the cut out.

Repeating The Steps Part 1

Repeating The Steps Part 2

I could use the knife tool to add the 2 holding edges in the cut out, but because I’ve fixed all the other Ngons, I can first dissolve the edges.

Dissolving The Edges

Then re-add them. Because the cut out is now made of 4 sided faces, the edge loops will extend all the way around.

Adding The Edge Loops Back In

Now the geometry looks much better, however when I re-enable the subsurface modifier, it’s smoothing out the cut out far too much.

The Model Is Too Smooth

To fix this I added some more edge loops to act as holding edges and reduce the amount of smoothing.

Adding Holding Edges

However, the model is now starting to look too messy and in some places the model is starting to lose its curvature.

Bad Topology

Too Many Edge Loops

To fix this I’m going to have to simplify the geometry using edge loop reduction. I’ll try and explain the process as I’m fixing the model but for a more detailed overview, I recommend this guide one.

My Favorite Topology Guide

As I’m going to have fix the model either way, I’ve added a few more loop cuts to make the cut out edges even more crisp.

Even More Edge Loops

Crisp Edges

I’m going to need a temporary loop cut for the edge loop reduction process.

Adding A Temporary Edge Loop

The plan is to reduce each side from 4 edges down to 2. Reducing to 1 edge loop would result in a triangular face each side.

First, I create a Ngon by dissolving 2 edges.

Creating Evil Ngons

Then I can use the knife tool to create edges from the outside edge loops to the inside ones

Starting the Edge Loop Reduction Process

Then delete the excess loop cuts.

Deleting The Temporary Loop Cuts

Unfortunately, I have to re-add the centre line, so the model does have one extra edge loop, but it’s still a big improvement. And finally, I repeat the process on the other side, and straighten out the loop cuts to reveal the simplified geometry.

The Geometry After The Edge Loop Reduction, How Lovely

Retopology Part 2

It’s a very similar process for the vents. One side is overlapping an edge loop so I can merge the vertices from the vents with the ones from the edge loop. On the other side of the vent, I can add an edge loop and then connect the end of the vents to the edge loop using the knife tool.

The Vents In Question, Difficult E-Vents

Merging Vertices With Edge Loop

One of the horizontal edge loops is close to the side of a vent so that can be merged too.

Merging With Another Loop Cut

Next, I’ve got to add a bunch of edge loops so that the rest of the vents have something to connect to. I’ll simplify the geometry later.

Filling In The Geometry With Everyone's Favorite Edge Loop

Then I connect the edges and vents with the knife tool.

Using The Knife Tool To Connect The Edge Loops

The right side geometry can be simplified with a 4 to 2 reduction.

Edge Loop Reduction On The Right Side Of The Vents

The left side can be reduced using the 5 to 3 reduction method that I used with the cut out.

Edge Loop Reduction On The Left Side Of The Vents

Then all I’ve got to do is repeat the process on the other side.

The Vents Before Being Connected

The Vents After Being Connected

Finally, the models need a few more edge loop reductions and some addition holding edges round the vents.

Holding Edges On The Vents To Keep The Corners Crisp

The Finished Product

At last, the retopology has been finished and we can admire the fished model!

The Finished Engine

The Engine With All The Greebles

Getting Started

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