Unity HDRP中如何用Polybrush绘制Decal透明区域以生成独特Decal？

Great question! I’ve tackled a similar workflow before when trying to reuse decal textures for varied "Roman fantasy" style assets in HDRP, so let me walk you through the two main approaches depending on whether you’re using a flat mesh as a decal or the HDRP Decal Projector.

Approach 1: Flat Mesh as Decal (Direct Vertex Painting) #

If you’re using a plane mesh directly as your decal, you can leverage Polybrush’s vertex painting to control transparency directly on the mesh:

• Enable Vertex Colors: First, make sure your plane mesh has vertex colors enabled. In the Mesh Inspector, check the "Vertex Colors" box (if it’s grayed out, you may need to reimport the mesh with vertex colors enabled).
• Paint with Polybrush: Open Polybrush’s Vertex Paint tool, set your brush to target the Alpha channel (or a specific color channel like red/green/blue—just pick one you won’t use for tinting). Paint over the areas you want to make transparent; lower alpha values will translate to more transparency.
• Modify HDRP Decal Material: Open your HDRP Decal material, and adjust its shader to sample the vertex color:
  • If using the built-in HDRP Decal Shader, look for the "Opacity" section. Add a Vertex Color node (in the shader graph or via the material’s custom inputs) and connect its alpha channel to the opacity mask input. This will override or blend with your decal texture’s alpha, letting your painted areas show through as transparent.
  • For smoother transitions, you can add a threshold or gradient node to control how the vertex color translates to transparency.

Approach 2: HDRP Decal Projector (Baked Vertex Paint Mask) #

Decal Projectors don’t have their own mesh vertices, so you’ll need to bake your Polybrush-painted mask into a texture to use as a decal mask:

• Create a Mask Canvas: Make a plane that matches the size and position of your Decal Projector’s projection area. Place it so it’s fully within the projector’s frustum.
• Paint the Mask: Use Polybrush’s Vertex Paint tool on this plane to define your transparent areas (same as Approach 1—target an alpha or color channel).
• Bake Vertex Colors to Texture: You’ll need to convert the plane’s vertex colors into a texture:
  • One simple way is to use a Render Texture: Set up a camera pointing directly at the plane, set its render target to a new Render Texture, and use a shader that outputs the vertex color. Render the camera once to capture the painted mask.
  • Alternatively, use a custom script to iterate over the mesh’s vertex colors and write them to a Texture2D asset.
• Apply Mask to Decal Projector: In your HDRP Decal Projector material, add the baked mask texture to the Opacity Mask slot. Adjust the mask’s channel (red/green/blue/alpha) to match the one you painted with Polybrush, and the projector will apply transparency only where you painted.

Pro Tips for Maximizing Reusability #

• Layered Painting: Use Polybrush’s layered painting feature to paint multiple color channels on the same mesh/canvas. You can then switch between channels in your material to create different decal variations from a single base texture.
• Custom Shader Graph: Build a custom HDRP Decal Shader Graph to combine vertex colors, base texture, and additional controls (like tint, roughness) for even more flexibility. This lets you tweak how the painted transparency blends with your decal’s original details.
• Alpha Clipping vs. Blending: Depending on your needs, use HDRP’s Alpha Clipping for sharp transparent edges (great for hard-edged decals) or Alpha Blending for soft, faded edges (perfect for weathered "Roman fantasy" effects).

内容的提问来源于stack exchange，提问作者Tertium