Mesh Paint Reference

Overview

The Mesh Paint tool allows you to paint vertex colors on static meshes interactively in the level viewport.

You can paint multiple instances of a single mesh with unique color/alpha values, and use that data however you'd like in your materials. You can visualize vertex color data directly, and easily edit multiple meshes at the same time. Also, the tool has special features for painting "blend weights" that allow you to interpolate between multiple textures per-pixel.

Note: Along with regular static meshes, InterpActors can also be painted on directly.

Quick guide

Most of the vertex paint features are pretty easy to learn, so if you want to jump right in here are the basic steps:

• Activate the Mesh Paint Mode and select your mesh actor
• Make sure the material assigned to your mesh makes use of vertex color or alpha
• Set your paint color and adjust other brush properties appropriately
• While holding down the Ctrl key, click and drag on your mesh to apply paint!

The following sections will describe the process in more detail, including the various brush settings and how materials are configured.

Mesh Paint Mode

In order to summon and use the Mesh Paint tool you'll need to enter Mesh Paint Mode. Just click the tool bar button on the left panel in the main editor frame.

While in this mode, you can still perform most common editor actions like camera movement and selection, but certain features are disabled. Also, you'll notice that your perspective viewport will be forced to real-time mode which is a requirement of this system.

Painting on Meshes

Painting on meshes is easy!

The first thing to remember is that you can only paint on selected meshes. This helps you restrict painting to only specific objects, while still allowing you to paint across multiple selected mesh actors if necessary.

While Mesh Paint mode is active, selected actors will appear with a crosshair around them instead of the regular selection cue. After selecting one or more meshes, when you move the mouse over them you should see your paint brush!

You can think of the paint brush as a cylinder projected along the surface normal of the mesh geometry, centered at the point on the mesh under your cursor. The circular brush outline shows you the radius of the brush. The inner circle shows the brush's falloff (or inner radius.) You probably figured this out already, but the small line sticking out from center of the brush is the mesh surface normal.

To paint, hold down the Control key, then click and drag over the mesh's surface. You can also hold down the Shift key to erase instead.

Paint will be applied to the surface each time you click, and also each time the mouse position changes (dragging). Also, if brush flow is enabled, paint will be applied every time the scene is rendered.

Note that you can only paint in perspective viewport windows.

Painting Colors

The default Mesh Paint mode is to paint color data (Red, Green, Blue, Alpha) directly. You can select a paint and erase color and then apply those colors to the mesh using the brush. This mode is useful when your material is configured to combine the vertex color with your pixel shader in some interesting way.

When the painting mode is set to colors you have these options available:

	"Fills" the mesh or instance being painted using the Paint color, respecting the Channels settings.
	Copies the vertex colors from the selected instance(s) to the source mesh asset.
	Imports a .TGA image file to use to populate the vertex colors of the selected instance(s).
	The color that will be applied while painting (Ctrl + LMB + Drag). A swatch displays a preview of the current color. The color can be set using the Color Picker built-in to the tool.
	The color to use as your "eraser" color while erasing (Ctrl + Shift + LMB + Drag). A swatch displays a preview of the current color. The color can be set using the Color Picker built-in to the tool.
	Swaps the Paint color and Erase color.
	These check boxes set which color/alpha channels should be affected by the paint brush.

Brush Settings

This section describes the brush settings common between all of the tool's modes. Note that for the options controlled by sliders, you can click and drag to change the value quickly, or you can click and type in a number if needed.

Asset vs Instance

While the mesh paint tool allows you to paint on the actual mesh source asset, most of the time you'll probably want to paint on an instance of that mesh placed into a level. For example, if you had a pillar mesh that was placed in four locations in your map you could paint each instance uniquely.

Use the Vertex color data option to set whether the Mesh Paint tool edits either the instance of the actor in the map, or the actual mesh source asset.

When painting vertex colors on the actor instance, the Instance vertex colors field will display information about the selected mesh(es).

This displays the number of bytes of memory used by the vertex color data (stored in the map package.) This value reflects the total for all assets that are currently selected.

Sharing Instance Color Data

There may be cases where you want to paint a mesh and then share those vertex colors with other instances of the mesh in the same level. For example, if you had a pillar mesh you were using in an outdoor map and you wanted to paint the base of the pillar green, and have all instances in that map only appear that way.

Instanced vertex color data can be copied and pasted between instances in the level very easily using the Copy and Paste buttons in the Mesh Paint Dialog.

To Copy Instanced Color Data:

1. Select the instance that contains the color data to be copied and press the Copy button.
   
2. Select the instance(s) to copy the color data to and press the Paste button.
   
3. The color data should now be applied to all of the instances.
   

Vertex Color Matching

When a static mesh is reimported with a different vertex count than the number of vertex colors on the instance you got errors like this in map checks and during cooking:

StaticMeshActor_73 (LOD 0) has hand-painted vertex colors that no longer match the original StaticMesh

Mismatched instanced vertex colors against the mesh's vertices makes the vertex colors simply nonexistent on console and look wrong on PC. To remedy that, the Fix button in the Mesh Paint Dialog lets you fix up vertex colors if they don't match up with the static mesh.

This feature will only be available if a mesh needs fixup; otherwise, it will be disabled. The tool works well in most cases especially when you are only making small tweaks. The more drastic the change to the mesh, the more unlikely the colors will match. The feature is designed to always match some color even if the changes are drastic. We elected not to make this tool automatic because that would prevent people from easily opting out of the fixup if they didn't like the results. Additionally, adding the feature to mesh paint means you can easily touchup after the fix is applied.

Original mesh:

Lower poly mesh with Fixup applied:

Colors vs Blend Weights

Instead of painting colors directly, you can switch the tool to paint texture blend weights. This is useful as often you'll configure your material to use the vertex color channels as blend factors between two or more textures. The Mesh Paint tool will help you out here by automatically normalizing your blend weights as you paint. You simply select a texture index and start painting, and the tool will make sure the color/alpha values are set correctly.

When the paint mode is set to blend weights you have these options available.

Note that you can click the little arrow between Paint texture and Erase texture to quickly swap the two texture indices.

View Modes

You can use the view mode controls to visualize vertex color and alpha values in the perspective viewport.

Setting up Materials

The mesh paint system requires the material of the mesh to contain a VertexColor expression as this is how the vertex color data is passed to the material. Please note that the VertexColor expression does not work if the material also contains a Transform expression unless the bPerPixelCameraVector property of the material is enabled.

There are a few example materials provided in EngineMaterials.upk that demonstrate possible uses for the Mesh Paint tool. Use the Content Browser to search for TexturePaint or VertexPaint to see these.

Here's an example of how you might use the vertex alpha value to blend between two diffuse textures in the material editor.

Material Example

This example uses the VertexPaint_2Tex_Color material which can be found in EngineMaterials.upk. It's an example material that does three special things:

• Modulates diffuse color by the painted vertex color (RGB)
• Blends between two separate diffuse textures (Diffuse, Diffuse2) using the blend weight stored in the vertex alpha channel (A)
• Blends between two separate normal textures (Normal, Normal2) using the blend weight stored in the vertex alpha channel (A)

After binding the material, the artist can use the Mesh Paint tool to paint both color "tint" and the blend weights between textures.

First the vertex color (RGB) is painted in using the "Color" paint mode. This color will be multiplied with the diffuse texture color in the material. This screenshot shows the mesh in the "color only" (RGB) view mode.

Next the vertex alpha (A) is painted using the "Blend Weight" paint mode. This value will be used to interpolate between a pair of diffuse/normal textures. This screenshot shows the mesh in the "alpha only" (A) view mode.

Finally, here is the end result, fully lit and shaded.