效果图
制作可交互的水体，大致分为三步
1.标记水体碰撞的位置
2.计算水波的传递 通过波动公式，3D或者2D 波动公式都行
3.水面顶点采样波动传递结果计算结果做顶点Y轴偏移
本文参考的波动相关资料
https://en.wikipedia.org/wiki/Wave_equation
https://www.amazon.com/Mathematics-Programming-Computer-Graphics-Third/dp/1435458869 流体 章节
相关公式
根据公式可知波的下次一次传递 z(i,j,k+1) 为 当前波值+上一次波值+周围波值
当前波值 *= (4-8*c^2*t^2/d^2/d^2)/(u*t)
上一次波值 *= (ut-2) / (ut + 2)
四周波值 *= (2c^2t^2/d^2) / (ut + 2)
其中各参数含义为 c 波速， u 粘度, d 波的递进距离, t 为递进时间
ok~ 我们重头开始
首先要建立水面
这里直接用Unity Wiki的轮子的创建平面
https://wiki.unity3d.com/index.php/CreatePlane
这里我们直接创建一个宽10米，长10米，间隔100的平面， 间隔越多，水体的颗粒感越小
对应本文开头描述的三大步骤
创建3个纹理
对应水体碰撞标记，传递，渲染
m_waterWaveMarkTexture = new RenderTexture(WaveTextureResolution, WaveTextureResolution, 0, RenderTextureFormat.Default);
m_waterWaveMarkTexture.name = "m_waterWaveMarkTexture";
m_waveTransmitTexture = new RenderTexture(WaveTextureResolution, WaveTextureResolution, 0, RenderTextureFormat.Default);
m_waveTransmitTexture.name = "m_waveTransmitTexture";
m_prevWaveMarkTexture = new RenderTexture(WaveTextureResolution, WaveTextureResolution, 0, RenderTextureFormat.Default);
m_prevWaveMarkTexture.name = "m_prevWaveMarkTexture";
标记水体碰撞位置
void WaterPlaneCollider()
{
hasHit = false;
if (Input.GetMouseButton(0))
{
Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
RaycastHit hitInfo = new RaycastHit();
bool ret = Physics.Raycast(ray.origin, ray.direction, out hitInfo);
if (ret)
{
Vector3 waterPlaneSpacePos = WaterPlane.transform.worldToLocalMatrix * new Vector4(hitInfo.point.x, hitInfo.point.y, hitInfo.point.z, 1);
float dx = (waterPlaneSpacePos.x / WaterPlaneWidth) + 0.5f;
float dy = (waterPlaneSpacePos.z / WaterPlaneLength) + 0.5f;
hitPos.Set(dx, dy);
m_waveMarkParams.Set(dx, dy, WaveRadius * WaveRadius, WaveHeight);
hasHit = true;
}
}
}
由于我们默认Raycast 获取的是碰撞的世界坐标，我们期望的是直接获取到 [0-1] 范围的数值用来映射到uv空间，直接在 m_waterWaveMarkTexture 进行标记， 因此我们乘以一个 world2Local 矩阵变换到本地， 又因为CreatePlane默认创建的Pivot 位于中心，再除以宽高缩放到1区间时，值域落在[-0.5,0.5]上，因此我们还要做 + 0.5偏移
标记水体碰撞Shader
float dx = i.uv.x - _WaveMarkParams.x;
float dy = i.uv.y - _WaveMarkParams.y;
float disSqr = dx * dx + dy * dy;
int hasCol = step(0, _WaveMarkParams.z - disSqr);
float waveValue = DecodeHeight(tex2D(_MainTex, i.uv));
if (hasCol == 1) {
waveValue = _WaveMarkParams.w;
}
根据传入的_WaveMarkParams.xy 跟当前uv 对比，在笔刷范围内的像素标记位默认波高度
波的传递Shader
static const float2 WAVE_DIR[4] = { float2(1, 0), float2(0, 1), float2(-1, 0), float2(0, -1) };
float dx = _WaveTransmitParams.w;
float avgWaveHeight = 0;
for (int s = 0; s < 4; s++)
{
avgWaveHeight += DecodeHeight(tex2D(_MainTex, i.uv + WAVE_DIR[s] * dx));
}
//(2 * c^2 * t^2 / d ^2) / (u * t + 2)*(z(x + dx, y, t) + z(x - dx, y, t) + z(x, y + dy, t) + z(x, y - dy, t);
float agWave = _WaveTransmitParams.z * avgWaveHeight;
// (4 - 8 * c^2 * t^2 / d^2) / (u * t + 2)
float curWave = _WaveTransmitParams.x * DecodeHeight(tex2D(_MainTex, i.uv));
// (u * t - 2) / (u * t + 2) * z(x,y,z, t - dt) 上一次波浪值 t - dt
float prevWave = _WaveTransmitParams.y * DecodeHeight(tex2D(_PrevWaveMarkTex, i.uv));
//波衰减
float waveValue = (curWave + prevWave + agWave) * _WaveAtten;
最后就是水体的呈现，因为需要做顶点纹理采样，因此需要至少ES3.0 硬体
v2f vert (appdata v)
{
v2f o;
float4 localPos = v.vertex;
float4 waveTransmit = tex2Dlod(_WaveResult, float4(v.uv, 0, 0));
float waveHeight = DecodeFloatRGBA(waveTransmit);
localPos.y += waveHeight * _WaveScale;
float3 worldPos = mul(unity_ObjectToWorld, localPos);
float3 worldSpaceNormal = mul(unity_ObjectToWorld, v.normal);
float3 worldSpaceViewDir = UnityWorldSpaceViewDir(worldPos);
o.vertex = mul(UNITY_MATRIX_VP, float4(worldPos, 1));
o.uv = v.uv;
o.worldSpaceReflect = reflect(-worldSpaceViewDir, worldSpaceNormal);
return o;
}
github地址
https://github.com/dreamfairy/interactivity-waterplane
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