详细资料：
http://developer.download.nvidia.com/SDK/10.5/direct3d/Source/ScreenSpaceAO/doc/ScreenSpaceAO.pdf ； http://developer.download.nvidia.com/presentations/2008/SIGGRAPH/HBAO_SIG08b.pdf
主要计算方式：
在屏幕空间进行射线检测，比较其与XY平面的夹角Sin值，最后进行积分，获得整个半球面的AO值。
图示如下：
ao计算公式
需要补充几点：
法线是基于像素的，不能插值 ，但可通过ddx/ddy微分计算
角度加个偏移值，防止采样点的sin h过小
ao衰减参数 W(r) = 1 -((s-p)/R)2
半球面ao积分计算
流程图，最后分别对X方向和Y方向进行高斯平滑即可。
具体shader代码如下（ssao.frag）：
// 参数说明： m_R，相机坐标系ao计算半径；m_NumSteps，每方向的计算次数；m_NumDir，随机方向个数；m_inv_R，半径倒数；
// m_sqr_R，半径平方；m_FocalLen，Vector2f(1f / (tanFovY * aspectRatio), 1f / tanFovY)；m_Resolution，ao
// depthTexture的宽高；m_Contrast， 3；
float M_PI = 3.14159265; varying vec2 texCoord; uniform sampler2D m_DepthTexture; vec3 uv_to_eye(vec2 uv, float z) { // texture origin is at bottom left corner in OpenGL uv = (uv * vec2(2.0, 2.0) - vec2(1.0, 1.0)); return vec3(uv * m_InvFocalLen * z, z); } vec3 fetch_eye_pos(vec2 uv) { float z = texture2D(m_DepthTexture, uv).x; z = m_invFrustumFar / (1.0 - z);// (-2.0 * 10000 / (z * 9999 - 10001)) / 9999; to be precise, but only approximation is required return uv_to_eye(uv, z); } float length2(vec3 v) { return dot(v, v); } vec3 min_diff(vec3 P, vec3 Pr, vec3 Pl) { vec3 V1 = Pr - P; vec3 V2 = P - Pl; return (length2(V1) < length2(V2)) ? V1 : V2; } vec2 rotate_direction(vec2 Dir, vec2 CosSin) { return vec2(Dir.x * CosSin.x - Dir.y * CosSin.y, Dir.x * CosSin.y + Dir.y * CosSin.x); } vec2 snap_uv_offset(vec2 uv) { return round(uv * m_Resolution) * m_InvResolution; } vec3 tangent_vector(vec2 deltaUV, vec3 dPdu, vec3 dPdv) { return deltaUV.x * dPdu + deltaUV.y * dPdv; } float tangent(vec3 T) { return -T.z / length(T.xy); } vec2 snap_uv_coord(vec2 uv) { return uv - (fract(uv * m_Resolution) - 0.5) * m_InvResolution; } float tan_to_sin(float x) { return x / sqrt(1.0 + x * x); } float falloff(float r) { return 1.0 - m_Attenuation * r * r; } float tangent(vec3 P, vec3 S) { return (P.z - S.z) / length(S.xy - P.xy); } float AccumulatedHorizonOcclusion_Quality(vec2 deltaUV, vec2 uv0, vec3 P, float numSteps, float randstep, vec3 dPdu, vec3 dPdv) { // Jitter starting point within the first sample distance vec2 uv = (uv0 + deltaUV) + randstep * deltaUV; // Snap first sample uv and initialize horizon tangent vec2 snapped_duv = snap_uv_offset(uv - uv0); vec3 T = tangent_vector(snapped_duv, dPdu, dPdv); float tanH = tangent(T) + m_TanAngleBias; float ao = 0.0; float h0 = 0.0; for(float j = 0.0; j < numSteps; ++j) { vec2 snapped_uv = snap_uv_coord(uv); vec3 S = fetch_eye_pos(snapped_uv); uv += deltaUV; // Ignore any samples outside the radius of influence float d2 = length2(S - P); if (d2 < m_sqr_R) { float tanS = tangent(P, S); if (tanS > tanH) { // Compute tangent vector associated with snapped_uv vec2 snapped_duv = snapped_uv - uv0; vec3 T = tangent_vector(snapped_duv, dPdu, dPdv); float tanT = tangent(T) + m_TanAngleBias; // Compute AO between tangent T and sample S float sinS = tan_to_sin(tanS); float sinT = tan_to_sin(tanT); float r = sqrt(d2) * m_inv_R; float h = sinS - sinT; ao += falloff(r) * (h - h0); h0 = h; // Update the current horizon angle tanH = tanS; } } } return ao; } void main() { vec3 P = fetch_eye_pos(texCoord); // Project the radius of influence m_R from eye space to texture space. // The scaling by 0.5 is to go from [-1,1] to [0,1]. vec2 step_size = 0.5 * m_R * m_FocalLen / P.z; // Early out if the projected radius is smaller than 1 pixel. float numSteps = min (m_NumSteps, min(step_size.x * m_Resolution.x, step_size.y * m_Resolution.y)); if( numSteps < 1.0 ) { gl_FragColor = vec4(1.0); return; } step_size = step_size / ( numSteps + 1.0 ); // Nearest neighbor pixels on the tangent plane vec3 Pr, Pl, Pt, Pb; Pr = fetch_eye_pos(texCoord + vec2(m_InvResolution.x, 0)); Pl = fetch_eye_pos(texCoord + vec2(-m_InvResolution.x, 0)); Pt = fetch_eye_pos(texCoord + vec2(0, m_InvResolution.y)); Pb = fetch_eye_pos(texCoord + vec2(0, -m_InvResolution.y)); // Screen-aligned basis for the tangent plane vec3 dPdu = min_diff(P, Pr, Pl); vec3 dPdv = min_diff(P, Pt, Pb) * (m_Resolution.y * m_InvResolution.x); vec2 jitter = texCoord * 1000.0; vec3 rand = normalize(vec3(sin(jitter.x) + sin(jitter.y), cos(jitter.x) + cos(jitter.y), 0)); float ao = 0.0; float d; float alpha = 2.0 * M_PI / m_NumDir; for (d = 0.0; d < m_NumDir; d++) { float angle = alpha * float(d); vec2 dir = vec2(cos(angle), sin(angle)); vec2 deltaUV = rotate_direction(dir, rand.xy) * step_size.xy; ao += AccumulatedHorizonOcclusion_Quality(deltaUV, texCoord, P, numSteps, rand.z, dPdu, dPdv); } gl_FragColor = vec4(vec3(1.0 - ao / m_NumDir * m_Contrast), 1); }
