This document is intended for engineers and describes in detail the algorithms used inside Air Sticker.
Air Sticker dynamically generates a decal mesh to which the decal texture is applied to create a decal representation.
When a sticker is applied to a character as a decal, as shown in the following figure, a decal mesh is dynamically generated so that the sticker will nicely adhere to the model.
Generated decal mesh
The steps of the algorithm for creating a decal mesh are as follows.
- Get the decal mesh to be edited from the decal mesh pool.
- Get the triangle polygons in model space of the receiver object to which the decal will be applied.
- Conduct an early branch cut (broad phase) on triangle polygons before applying decal textures.
- Defines the clipping plane using the information in the decal box.
- Split the triangle polygon intersecting the clipping plane defined in Step-4.
- Decal mesh is generated based on the polygonal polygon information created in step-5.
Also, since polygon division in step-4 to 6 is based on "9.2 Decal Application" in "Mathematics for 3D Game Programming & Computer Graphics, 3rd Edition" only the outline of the algorithm and the related source code are described. For details of polygon division, please refer to the references.
Section 3 will go into more detail on the various steps.
Get the decal mesh to be edited from the decal mesh pool held by AirDecalSystem.
The decal mesh is registered in the pool using the hash values of the receiver object, renderer, and material as keys, and if these values are identical, the registration is reused. If this hash value is not registered in the pool, a new decal mesh is created.
Therefore, the following decals are treated as a single decal mesh.
Single Decal Mesh
Two Decal Mesh
Number of decal meshes is equal to number of draw calls. Therefore, one guideline for optimization is to reduce the number of decal mesh types.
[Code that retrieves the decal mesh from the pool]
// We want to collect only the renderer of receiver objects,
// But the renderer of decal mesh hanging from receiver object.
// Therefore, temporarily disable to the renderer of decal mesh.
Instance._decalMeshPool.DisableDecalMeshRenderers();
var renderers = receiverObject.GetComponentsInChildren<Renderer>();
foreach (var renderer in renderers)
{
if (!renderer) return;
var pool = Instance._decalMeshPool;
var hash = AirStickerMeshPool.CalculateHash(receiverObject, renderer, decalMaterial);
if (pool.Contains(hash))
{
results.Add(pool.GetDecalMesh(hash));
}
else
{
var newMesh = new DecalMesh(receiverObject, decalMaterial, renderer);
results.Add(newMesh);
pool.RegisterDecalMesh(hash, newMesh);
}
}
// Restore the renderer of decal mesh was disabled.
Instance._decal3.EnableDecalMeshRenderers();Related Source Code
Assets/AirSticker/Runtime/Scripts/Core/DecalMeshPool.cs
Assets/AirSticker/Runtime/Scripts/Core/DecalMesh.cs
3.2 Get the triangle polygons in model space of the receiver object to which the decal will be applied
Gets the triangle polygons of the receiver object from the triangle polygons pool held by AirStickerSystem.
This pool is keyed to the receiver object, and the triangle polygons are registered and used if it has already been registered. If it is a new receiver object, the triangle polygons are created from the renderer's information.
[Code that collects triangle polygon information from mesh filters]
private IEnumerator BuildFromMeshFilter(MeshFilter[] meshFilters, MeshRenderer[] meshRenderers,
List<ConvexPolygonInfo> convexPolygonInfos)
{
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foreach (var meshFilter in meshFilters)
{
if (!meshFilter || meshFilter.sharedMesh == null)
// Mesh filter is deleted, so process is terminated.
yield break;
var mesh = meshFilter.sharedMesh;
using var meshDataArray = Mesh.AcquireReadOnlyMeshData(mesh);
var meshData = meshDataArray[0];
meshData.GetVertices(_workingVertexPositions);
meshData.GetNormals(_workingVertexNormals);
var subMeshCount = meshData.subMeshCount;
for (var meshNo = 0; meshNo < subMeshCount; meshNo++)
{
meshData.GetIndices(_workingTriangles, meshNo);
var numPoly = polygonCounts[indexOfPolygonCounts++];
for (var i = 0; i < numPoly; i++)
{
if ((newConvexPolygonNo + 1) % MaxGeneratedPolygonPerFrame == 0)
// Maximum number of polygons processed per frame is MaxGeneratedPolygonPerFrame.
yield return null;
if (!meshFilter || meshFilter.sharedMesh == null)
// Mesh filter is deleted, so process is terminated.
yield break;
// Collect triangle data.
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// New triangle infomation.
newConvexPolygonInfos[newConvexPolygonNo] = new ConvexPolygonInfo
{
ConvexPolygon = new ConvexPolygon(
positionBuffer,
normalBuffer,
boneWeightBuffer,
lineBuffer,
localPositionBuffer,
localNormalBuffer,
meshRenderers[rendererNo],
startOffsetOfBuffer,
VertexCountOfTrianglePolygon,
rendererNo,
VertexCountOfTrianglePolygon)
};
newConvexPolygonNo++;
startOffsetOfBuffer += VertexCountOfTrianglePolygon;
}
}
rendererNo++;
}
convexPolygonInfos.AddRange(newConvexPolygonInfos);
}Related Source Code
Assets/AirSticker/Runtime/Scripts/Core/ReceiverObjectTrianglePolygonsPool.cs
Assets/AirSticker/Runtime/Scripts/Core/TrianglePolygonsFactory.cs
Assets/AirSticker/Runtime/Scripts/Core/ConvexPolygon.cs
In this step, the distance between the starting coordinates of the decal box and the vertex of each polygon is used to remove the triangle polygons.
This inexpensive computational branch cutting lowers the computational complexity of the subsequent steps, resulting in a significant speedup.
Also, a decal box is a box that represents a space for attaching decals.
Decal Box
[Codes with early branch cutting]
// Loop by triangle polygons data.
foreach (var convexPolygonInfo in convexPolygonInfos)
{
if (Vector3.Dot(decalSpaceNormalWs, convexPolygonInfo.ConvexPolygon.FaceNormal) < 0)
{
// Set the flag of outside the clip space.
convexPolygonInfo.IsOutsideClipSpace = true;
continue;
}
var vertNo_0 = convexPolygonInfo.ConvexPolygon.GetRealVertexNo(0);
var v0 = convexPolygonInfo.ConvexPolygon.GetVertexPositionInWorldSpace(vertNo_0);
v0 -= centerPosInDecalBox;
if (v0.sqrMagnitude > threshold)
{
var vertNo_1 = convexPolygonInfo.ConvexPolygon.GetRealVertexNo(1);
var v1 = convexPolygonInfo.ConvexPolygon.GetVertexPositionInWorldSpace(vertNo_1);
v1 -= centerPosInDecalBox;
if (v1.sqrMagnitude > threshold)
{
var vertNo_2 = convexPolygonInfo.ConvexPolygon.GetRealVertexNo(2);
var v2 = convexPolygonInfo.ConvexPolygon.GetVertexPositionInWorldSpace(vertNo_2);
v2 -= centerPosInDecalBox;
if (v2.sqrMagnitude > threshold)
{
// Set the flag of outside the clip space.
convexPolygonInfo.IsOutsideClipSpace = true;
continue;
}
}
}
broadPhaseConvexPolygonCount++;
}Next, define a clipping plane to divide the triangle polygon based on the center coordinates, width, and height of the decal box. See "9.2.1 Construction of Decal Mesh" in "Mathematics for 3D Game Programming & Computer Graphics, 3rd Edition" for more information on defining a clipping plane.
[Code defining the clipping plane]
private void BuildClipPlanes(Vector3 basePoint)
{
var trans = transform;
var decalSpaceTangentWS = _decalSpace.Ex;
var decalSpaceBiNormalWS = _decalSpace.Ey;
var decalSpaceNormalWS = _decalSpace.Ez;
// Build left plane.
_clipPlanes[(int)ClipPlane.Left] = new Vector4
{
x = decalSpaceTangentWS.x,
y = decalSpaceTangentWS.y,
z = decalSpaceTangentWS.z,
w = width / 2.0f - Vector3.Dot(decalSpaceTangentWS, basePoint)
};
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// Build back plane.
_clipPlanes[(int)ClipPlane.Back] = new Vector4
{
x = decalSpaceNormalWS.x,
y = decalSpaceNormalWS.y,
z = decalSpaceNormalWS.z,
w = _basePointToFarClipDistance - Vector3.Dot(decalSpaceNormalWS, basePoint)
};
}Related Source Code
Assets/AirSticker/Runtime/Scripts/AirStickerProjector.cs
Here, the triangular polygon is divided by finding the intersection of each side of the triangle polygons and the six clipping planes. See "9.2.2 Polygon Clipping" in "Mathematics for 3D Game Programming & Computer Graphics, 3rd Edition" for more information on triangular polygon segmentation.
Related Source Code
Assets/AirSticker/Runtime/Scripts/AirStickerProjector.cs
Triangle polygons are generated based on the vertex information of the convex polygon obtained by dividing the triangle polygons, and the final decal mesh is generated. Since convex polygons can be treated as a set of triangle-fan triangles, this property is used to add new triangles to the decal mesh. See "9.2.2 Polygon Clipping" in "Mathematics for 3D Game Programming & Computer Graphics, 3rd Edition" for details on constructing triangle polygons from convex polygons.
Treat convex polygons as triangle polygons
Related Source Code
Assets/AirSticker/Runtime/Scripts/Core/DecalMesh.cs