Dynamically Drawing Lines in Blender Game Engine

Mesh painting is the ability for players to paint on objects in-game. Examples of mesh painting are the goop in Super Mario Sunshine, the gels in Portal 2 and the ink in Splatoon. It can be used as a gameplay element or it can be purely for visuals. Either way, mesh painting opens up a lot of possibilities for gameplay designers and artists.

Although the previous examples are all pretty much the same effect, you can use mesh painting for many other effects too. For example, you can use it to spray paint objects, render wounds on characters or even let players draw their own character's face!

In this tutorial, you will learn how to paint onto a skeletal mesh. To do this you will:

• Unwrap a mesh into its UV form
• Use the hit location from a line trace to sphere mask the mesh
• Render the unwrapped mesh and sphere mask to a render target using a scene capture
• Use the mask to blend between textures in the character material

Note that this is not a tutorial on vertex painting. Vertex painting is dependent on mesh resolution and can not be changed in-game. On the other hand, the method in this tutorial works independently from mesh resolution and also works in-game.

Note: This tutorial assumes you already know the basics of using Unreal Engine. If you are new to Unreal Engine, check out our 10-part Unreal Engine for Beginners tutorial series.

Getting Started

Start by downloading the materials for this tutorial (you can find a link at the top or bottom of this tutorial). Unzip it and navigate to MeshPainterStarter and open MeshPainter.uproject. If you press Play, you will see the character that you will be painting on.

Just like the snow and grass trails tutorials, this method also requires a scene capture. To save time, I have already created a scene capture Blueprint for you. If you'd like to learn more about the capture settings, check out our tutorial on creating snow trails.

First, let's look at how you can paint on a mesh.

Mesh Painting

In most cases, the mesh you're working with is already UV mapped. So the obvious thing to do would be to create a mask using a render target and then apply it to the mesh. However, generating a mask directly on the render target (using Draw Material to Render Target) will usually result in discontinuities across UV shells.

Here's an example of a cube's UV map and a sphere mask texture:

And here is the mask applied to the cube:

As you can see, a 2D sphere mask does not wrap around corners since it doesn't take the geometry into account. To generate a correct mask, the sphere mask needs to sample the world positions of the mesh instead. But how do you access world positions when using the Draw Material to Render Target node?

If you've been looking at methods on mesh painting, you may have come across Ryan Bruck's video on character damage using render targets (the method in this tutorial is based on his method). In the video, he successfully generates 3D sphere masks and accumulates them into a render target. He is able to do this because he creates a render target to store the mesh's world positions which he can then sample using a sphere mask. Let's take a closer look.

Ryan's Method

There are four steps to this method. The first step is to "unwrap" the mesh in question. This is simply moving all of the vertices so that you get the mesh in its UV form. For example, here are the character's UVs:

Here is the character after unwrapping in Unreal:

You can unwrap a mesh by applying some simple World Position Offset math (which you'll see later on).

After unwrapping, the second step is to encode the world positions of the mesh into a render target. You can do this by setting the unwrap material's color to Absolute World Position and then using a scene capture to capture the unwrap. Here is what the render target would look like:

Note: The render target changes color because the character is animated. This causes the world positions to constantly change.

The third step is to create the sphere masks. Now that you can access the mesh's world positions, you can sample them in a sphere mask. You can then draw the sphere mask directly to a second render target.

The final step is to use the mask in the character's material to blend between colors, textures or materials. Here are steps three and four visualized:

Now, let's look at the proposed method.

Proposed Method

While Ryan's method works, it requires:

• Two render target draws. The first is capturing the unwrapped mesh. The second is accumulating the sphere masks.
• One render target to store the world positions
• A render target to accumulate the sphere masks. You will need a separate render target for each actor you want to paint on.

The method in this tutorial discards the second draw and world position render target. It does this by combining the unwrap and sphere masks into one material (the unwrap material). It then captures the unwrap using an additive composite mode to accumulate the sphere masks.

One thing to note is that both methods work best when the mesh does not have overlapping UVs. Overlapping UVs will cause pixels sharing the same UV space to also share the same mask information. For example, you have both hands UV mapped to the same space. If one hand becomes masked, the other hand will become masked too.

Now that you know the method, let's start with creating the unwrap material.

Creating the Unwrap Material

Navigate to the Materials folder and create a new material. Name it M_Unwrap and then open it.

Next, change the following settings:

• Shading Model: Unlit. This is to make sure the scene capture doesn't capture any lighting information.
• Two Sided: Enabled. Sometimes unwrapped faces can be facing the other way depending on how the mesh is UV mapped. Two Sided will make sure you can still see any flipped faces.
• Usage\Used with Skeletal Mesh: Enabled. This will compile the shaders necessary for the material to work on skeletal meshes.

Up next is to unwrap the mesh. To do this, create the setup below. Note that I have already created the CaptureSize and UnwrapLocation parameters within the MPC_Global asset.

This will UV unwrap the mesh to the specified location at the specified size. Note that if your mesh's unique UV map is on a separate channel, you will need to change the Coordinate Index of the TextureCoordinate node. For example, if the unique UVs is on channel 1, you would set Coordinate Index to 1.

The next step is to create the sphere mask. For this, you will need two parameters: the hit location and sphere radius. Create the highlighted nodes:

This will return white for pixels within the sphere mask and black for pixels outside. Don't worry about setting a value for the parameters since you will do that later in Blueprints.

It is important that you set the Absolute World Position node to Absolute World Position (Excluding Material Shader Offsets). This is because the pixel's world position will change due to the unwrapping. Excluding material shader offsets will give you the original world position before unwrapping.

That's all you need for the unwrap material. Click Apply and then close the material. Up next is to apply the material to the character to unwrap it.

Unwrapping the Character

For this tutorial, the capture Blueprint will handle the unwrapping and capturing. First, you will need a dynamic instance of the unwrap material. Navigate to the Blueprints folder and open BP_Capture. Afterwards, add the highlighted nodes to Event BeginPlay. Make sure you set Parent to M_Unwrap.

Next, you need a function to perform the unwrap and capture. Create a new function named PaintActor. Afterwards, create the following inputs:

• ActorToPaint: Type is Actor. The actor to unwrap and capture.
• HitLocation: Type is Vector. This will be the center point of the sphere mask.
• BrushRadius: Type is Float. The radius of the sphere mask in world units.

Although this painting method can work with any actor, we will only check if the incoming actor inherits from the Character class. And for code simplicity, we'll store the character's skeletal mesh component into a variable since you will need to reference it a few times. To do this, add the highlighted nodes:

Now it's time to do the unwrapping and sphere masking. To do this, add the higlighted nodes to the end of the node chain:

Here's what each line does:

1. First, this will save the mesh's original material so you can reapply it later. Afterwards, it will apply the unwrap material.
2. This line will pass in the hit location and brush radius to the unwrap material for sphere masking

Before you can test the unwrapping out, you will need to do a line trace from the player to get a hit location.

Getting the Hit Location

Click Compile and then go back to the main editor. Afterwards, open BP_Player. Open the Shoot function and add the highlighted nodes. For this tutorial, set Brush Radius to 10.

Click Compile and then close BP_Player. Press Play and then left-click on the character to do an unwrap and sphere mask.

If you're wondering why the mask keeps moving, it is because parts are moving in and out of the sphere mask. This is not a problem though because you will only capture the unwrap at the time of the hit.

Now that you have unwrapped the mesh, you need to capture it.

Capturing the Unwrap

First, it's a good idea to add an unlit black plane behind the unwrapped mesh. This will help prevent seams at the edge of UV shells. Open BP_Capture and then add a Plane component named BackgroundPlane. To save time, I have already created the black material for you. Set the material to M_Background.

For this tutorial, the unwrap and capture size are 500×500 units so the background plane will need to be at least that large. Set the Scale to (5.0, 5.0, 1.0).

Since the plane's location and unwrap location are the same, it is also a good idea to offset the plane down to prevent any z-fighting. To do this, set Location to (0.0, 0.0, -1.0).

Next, you need to perform the capture. Go back to the PaintActor function and add the highlighted nodes:

This will capture the unwrapped mesh. Afterwards, it will reapply the mesh's original material.

Currently, the scene capture will overwrite the previous contents of the render target. To make sure the sphere masks accumulate, you need to make sure the scene capture adds to the previous contents instead. To do this, select the SceneCapture component and set Scene Capture\Composite Mode to Additive.

Click Compile and then close the Blueprint. Next, you need to use the render target in the character's material.

Using the Mask

Navigate to Characters\Mannequin\Materials and open M_Mannequin. Afterwards, add the highlighted nodes. Make sure to set the Texture Sample to RT_Capture.

This will display red wherever the mask is white and orange wherever the mask is black. If you wanted to though, you could blend between textures or material layers instead.

Click Apply and then close the material. Press Play and left-click on the character to start painting.

Where to Go From Here?

You can download the completed project using the link at the top or bottom of this tutorial.

While you've only used sphere masks in this tutorial, they are not the only shapes you can use. There are boxes, cylinders, cones and more! To learn more about these shapes and how to implement them, check out these two posts:

• Modelling with Distance Functions by Inigo Quilez
• Volumetric Rendering: Signed Distance Functions by Alan Zucconi

If you'd like to check out another method of mesh painting, check out Tom Looman's Rendering Wounds on Characters. Instead of accumulating sphere masks, he sets a fixed number of sphere masks. The advantage of this method is that it's cheap (depending on how many sphere masks you have) and it is relatively easy to animate the masks. The disadvantage is that there is a hard limit on how many sphere masks you can do.

If there are any effects you'd like to me cover, let me know in the comments below!

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Dynamically Drawing Lines in Blender Game Engine

Source: https://www.raywenderlich.com/6817-dynamic-mesh-painting-in-unreal-engine-4

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