以前一直都是在Directx或者UnityShaderLab裡做raymarch，最近在研究虛幻4的Shader所以在虛幻4裡簡單實現了一下這個。

我的step數量調得很低。

剛開始其實不好下手，虛幻的渲染架構過於複雜，高度封裝，我們無法直接像unityshaderlab一樣對shader進行直接編寫，如果想直接自己上傳shader而繞過虛幻的材質系統，那麼成本將是高昂的。首先先簡單捋一下虛幻的shader生成過程。第一步我們在材質系統裡完成各種節點的編寫，這些節點是c++寫的，可以在原始碼的MaterialExpression裡找到全部實現。這些節點帶有一個Fstring的字串，裡面儲存的是HLSL程式碼。在材質編譯的時候，HLSLTranslator會把這些字串插入到usf提供的shader模板中，最後生成我們需要的shader。生成後的shader可以在材質編輯器的HLSLCode裡看到。如果我們想在CustomNode裡呼叫自己的函式，可以在CommonUSF檔案中新增。但是可能會導致引擎崩潰，直到4.17版本，虛幻將usf檔案暴露出來，在4.17版本我們就能給自己的專案新增usf從而CustomNode就能呼叫函數了。除此之外，虛幻的材質如何模擬多pass行為也是一個難題，目前4.17的方式是使用多個Render Target來模擬，但是效率低下。

下面是我ray mach的程式碼

我喜歡在VS裡寫程式碼然後再貼上到CustomNode裡

注：直接把上面的程式碼複製貼上到程式碼裡會報錯，不清楚為什麼，如果在vs裡重新敲一次卻不會。估計網頁裡有些符號和程式碼裡的不一樣，雖然他們看起來一樣。

順便總結一下常用Raymarching模型（這些公式來自國外一個朋友）

Sphere - signed - exact

float sdSphere( vec3 p, float s )
{
  return length(p)-s;
}

Box - unsigned - exact

float udBox( vec3 p, vec3 b )
{
  return length(max(abs(p)-b,0.0));
}
 

Round Box - unsigned - exact

float udRoundBox( vec3 p, vec3 b, float r )
{
  return length(max(abs(p)-b,0.0))-r;
}

Box - signed - exact

float sdBox( vec3 p, vec3 b )
{
  vec3 d = abs(p) - b;
  return min(max(d.x,max(d.y,d.z)),0.0) + length(max(d,0.0));
}

Torus - signed - exact

float sdTorus( vec3 p, vec2 t )
{
  vec2 q = vec2(length(p.xz)-t.x,p.y);
  return length(q)-t.y;
}

Cylinder - signed - exact

float sdCylinder( vec3 p, vec3 c )
{
  return length(p.xz-c.xy)-c.z;
}

Cone - signed - exact

float sdCone( vec3 p, vec2 c )
{
    // c must be normalized
    float q = length(p.xy);
    return dot(c,vec2(q,p.z));
}

Plane - signed - exact

float sdPlane( vec3 p, vec4 n )
{
  // n must be normalized
  return dot(p,n.xyz) + n.w;
}

Hexagonal Prism - signed - exact

float sdHexPrism( vec3 p, vec2 h )
{
    vec3 q = abs(p);
    return max(q.z-h.y,max((q.x*0.866025+q.y*0.5),q.y)-h.x);
}

Triangular Prism - signed - exact

float sdTriPrism( vec3 p, vec2 h )
{
    vec3 q = abs(p);
    return max(q.z-h.y,max(q.x*0.866025+p.y*0.5,-p.y)-h.x*0.5);
}

Capsule / Line - signed - exact

float sdCapsule( vec3 p, vec3 a, vec3 b, float r )
{
    vec3 pa = p - a, ba = b - a;
    float h = clamp( dot(pa,ba)/dot(ba,ba), 0.0, 1.0 );
    return length( pa - ba*h ) - r;
}

Capped cylinder - signed - exact

float sdCappedCylinder( vec3 p, vec2 h )
{
  vec2 d = abs(vec2(length(p.xz),p.y)) - h;
  return min(max(d.x,d.y),0.0) + length(max(d,0.0));
}

float sdCappedCone( in vec3 p, in vec3 c )
{
    vec2 q = vec2( length(p.xz), p.y );
    vec2 v = vec2( c.z*c.y/c.x, -c.z );
    vec2 w = v - q;
    vec2 vv = vec2( dot(v,v), v.x*v.x );
    vec2 qv = vec2( dot(v,w), v.x*w.x );
    vec2 d = max(qv,0.0)*qv/vv;
    return sqrt( dot(w,w) - max(d.x,d.y) ) * sign(max(q.y*v.x-q.x*v.y,w.y));
}

Ellipsoid - signed - bound

float sdEllipsoid( in vec3 p, in vec3 r )
{
    return (length( p/r ) - 1.0) * min(min(r.x,r.y),r.z);
}

Triangle - unsigned - exact

float dot2( in vec3 v ) { return dot(v,v); }
float udTriangle( vec3 p, vec3 a, vec3 b, vec3 c )
{
    vec3 ba = b - a; vec3 pa = p - a;
    vec3 cb = c - b; vec3 pb = p - b;
    vec3 ac = a - c; vec3 pc = p - c;
    vec3 nor = cross( ba, ac );

    return sqrt(
    (sign(dot(cross(ba,nor),pa)) +
     sign(dot(cross(cb,nor),pb)) +
     sign(dot(cross(ac,nor),pc))<2.0)
     ?
     min( min(
     dot2(ba*clamp(dot(ba,pa)/dot2(ba),0.0,1.0)-pa),
     dot2(cb*clamp(dot(cb,pb)/dot2(cb),0.0,1.0)-pb) ),
     dot2(ac*clamp(dot(ac,pc)/dot2(ac),0.0,1.0)-pc) )
     :
     dot(nor,pa)*dot(nor,pa)/dot2(nor) );
}

Quad - unsigned - exact

float dot2( in vec3 v ) { return dot(v,v); }
float udQuad( vec3 p, vec3 a, vec3 b, vec3 c, vec3 d )
{
    vec3 ba = b - a; vec3 pa = p - a;
    vec3 cb = c - b; vec3 pb = p - b;
    vec3 dc = d - c; vec3 pc = p - c;
    vec3 ad = a - d; vec3 pd = p - d;
    vec3 nor = cross( ba, ad );

    return sqrt(
    (sign(dot(cross(ba,nor),pa)) +
     sign(dot(cross(cb,nor),pb)) +
     sign(dot(cross(dc,nor),pc)) +
     sign(dot(cross(ad,nor),pd))<3.0)
     ?
     min( min( min(
     dot2(ba*clamp(dot(ba,pa)/dot2(ba),0.0,1.0)-pa),
     dot2(cb*clamp(dot(cb,pb)/dot2(cb),0.0,1.0)-pb) ),
     dot2(dc*clamp(dot(dc,pc)/dot2(dc),0.0,1.0)-pc) ),
     dot2(ad*clamp(dot(ad,pd)/dot2(ad),0.0,1.0)-pd) )
     :
     dot(nor,pa)*dot(nor,pa)/dot2(nor) );
}

Torus82 - signed

float sdTorus82( vec3 p, vec2 t )
{
  vec2 q = vec2(length2(p.xz)-t.x,p.y);
  return length8(q)-t.y;
}

Torus88 - signed

float sdTorus88( vec3 p, vec2 t )
{
  vec2 q = vec2(length8(p.xz)-t.x,p.y);
  return length8(q)-t.y;
}

距離場運算

Union

float opU( float d1, float d2 )
{
    return min(d1,d2);
}

Substraction

float opS( float d1, float d2 )
{
    return max(-d1,d2);
}

Intersection

float opI( float d1, float d2 )
{
    return max(d1,d2);
}

Repetition

float opRep( vec3 p, vec3 c )
{
    vec3 q = mod(p,c)-0.5*c;
    return primitve( q );
}

Rotation/Translation

vec3 opTx( vec3 p, mat4 m )
{
    vec3 q = invert(m)*p;
    return primitive(q);
}

Scale

float opScale( vec3 p, float s )
{
    return primitive(p/s)*s;
}

distance deformations

Displacement

float opDisplace( vec3 p )
{
    float d1 = primitive(p);
    float d2 = displacement(p);
    return d1+d2;
}

Blend

float opBlend( vec3 p )
{
    float d1 = primitiveA(p);
    float d2 = primitiveB(p);
    return smin( d1, d2 );
}

domain deformations

Twist

float opTwist( vec3 p )
{
    float c = cos(20.0*p.y);
    float s = sin(20.0*p.y);
    mat2  m = mat2(c,-s,s,c);
    vec3  q = vec3(m*p.xz,p.y);
    return primitive(q);
}

Cheap Bend

float opCheapBend( vec3 p )
{
    float c = cos(20.0*p.y);
    float s = sin(20.0*p.y);
    mat2  m = mat2(c,-s,s,c);
    vec3  q = vec3(m*p.xy,p.z);
    return primitive(q);
}