diff --git a/implicit3-rt/frag-render.glsl b/implicit3-rt/frag-render.glsl
index 4887b7b..e7f28bf 100644
--- a/implicit3-rt/frag-render.glsl
+++ b/implicit3-rt/frag-render.glsl
@@ -7,7 +7,8 @@ out vec4 fragColor;
 
 uniform float iSeed;
 uniform vec2 iResolution;
-uniform float iNFrame;
+uniform float iSpp;
+uniform float sSamples;
 
 uniform mat4 transformMatrix;
 uniform float uScale;
@@ -523,6 +524,8 @@ float sampleHeight(float h, vec3 w) {
     return 0.0;
 }
 
+#define RANDOM_WALK_DEPTH 8.0
+
 vec3 ggxSampleNormal(float alpha) {
     float r1 = randf(), r2 = randf();
     float su = 2.0*PI*r2;
@@ -552,7 +555,7 @@ vec4 sampleBsdfConductor(Material m, vec3 wi) {
     float h = 5.*STDEV;
     float e = 1.;
     vec3 w0, w = -wi;
-    for (int r = 0; r < 16; r++) {
+    for (float r = ZERO; r < RANDOM_WALK_DEPTH; r++) {
         h = sampleHeight(h, w);
         if(abs(h) > 5.*STDEV)
             break;
@@ -569,7 +572,7 @@ float evalBsdfConductor(Material m, vec3 wi, vec3 wo) {
     float e = 1.;
     vec3 w = -wi;
     float total = 0.0;
-    for (int r = 0; r < 16; r++) {
+    for (float r = ZERO; r < RANDOM_WALK_DEPTH; r++) {
         h = sampleHeight(h, w);
         if(abs(h) > 5.*STDEV)
             break;
@@ -598,7 +601,7 @@ vec4 sampleBsdfDielectric(Material m, vec3 wi) {
     float h = 5.*STDEV;
     vec3 w = -wi;
     float transmit = 1.0;
-    for (int r = 0; r < 16; r++) {
+    for (float r = ZERO; r < RANDOM_WALK_DEPTH; r++) {
         h = sampleHeight(h, w*transmit);
         if(abs(h) > 5.*STDEV)
             break;
@@ -619,7 +622,7 @@ float evalBsdfDielectric(Material m, vec3 wi, vec3 wo) {
     vec3 w = -wi;
     float transmit = 1.0;
     float total = 0.0;
-    for (int r = 0; r < 16; r++) {
+    for (float r = ZERO; r < RANDOM_WALK_DEPTH; r++) {
         h = sampleHeight(h, w*transmit);
         if(abs(h) > 5.*STDEV)
             break;
@@ -1264,11 +1267,24 @@ void main(void) {
 
     vec4 totcol = vec4(0);
     totcol = texelFetch(accumBuffer, ivec2(gl_FragCoord.xy), 0);
-    for (float fi=ZERO; fi<iNFrame; fi++) {
+    for (float fi=ZERO; fi<sSamples; fi++) {
         // random number seed
-        seed0 = hash13(vec3(gl_FragCoord.xy/iResolution.xy, sin(iSeed+fi/iNFrame)));
+        seed0 = hash13(vec3(gl_FragCoord.xy/iResolution.xy, sin(iSeed+fi/sSamples)));
         seed = round(65537.*seed0);
 
+        // frame skipping
+        // to-do: some way without increasing variance
+        const float tile_size = 16.0;
+        vec2 tile_uv = mod(floor(gl_FragCoord.xy), tile_size);
+        vec2 tile_ij = floor(gl_FragCoord.xy/tile_size);
+        float tile_ij_hash = floor(hash13(vec3(tile_ij,tile_size))*tile_size*tile_size);
+        float tile_i = tile_uv.y*tile_size + tile_uv.x;
+        tile_i = mod(37.0*tile_i+tile_ij_hash, tile_size*tile_size);
+        float tile_f = tile_i / (tile_size*tile_size);
+        float tile_offset = mod(iSpp, 1.0);
+        if (mod(tile_f+tile_offset, 1.0) > sSamples)
+            continue;
+
         vec2 ro_s = vXy;
         ro_s += (-1.0+2.0*vec2(randf(), randf())) / iResolution.xy;
         vec2 ro_sc = worldToScreen(vec3(0)).xy;
@@ -1293,7 +1309,7 @@ void main(void) {
                 float u = dot(col, ru), v = dot(col, rv), w = dot(col, rd);
                 totcol = vec4(u, v, w, u*u+v*v);
             }
-            else totcol += vec4(col, 1) / iNFrame;
+            else totcol += vec4(col, 1) / sSamples;
         }
     }
     fragColor = totcol;
diff --git a/implicit3-rt/index.html b/implicit3-rt/index.html
index f1bbf6f..0d0be70 100644
--- a/implicit3-rt/index.html
+++ b/implicit3-rt/index.html
@@ -57,7 +57,7 @@
                 <option value="0.004">high</option>
                 <option value="0.001">ultra high</option>
             </select>precision</span>&ensp;
-        <span><select id="select-spp" title="Maximum samples per pixel">
+        <span title="Maximum samples per pixel"><select id="select-spp">
                 <option value="1">2⁰</option>
                 <option value="4">2²</option>
                 <option value="16">2⁴</option>
@@ -67,8 +67,8 @@
                 <option value="4096">2¹²</option>
                 <option value="16384">2¹⁴</option>
             </select>spp</span>
-        <br />
-        <span><select id="select-light-path-depth" title="Maximum light path depth">
+        <br/>
+        <span title="Maximum light path depth"><select id="select-light-path-depth">
                 <option value="8">8</option>
                 <option value="16">16</option>
                 <option value="32">32</option>
@@ -78,7 +78,16 @@
                 <option value="512">512</option>
                 <option value="1024">1024</option>
             </select>path depth</span>&ensp;
-        <span title="Enable experimental Multiple Importance Sampling (MIS) for high-variance scenes"><input type="checkbox"
+        <span title="Number of samples per frame per pixel"><select id="select-samples">
+                <option value="0.015625">2⁻⁶</option>
+                <option value="0.0625">2⁻⁴</option>
+                <option value="0.25">2⁻²</option>
+                <option value="1" selected>2⁰</option>
+                <option value="4">2²</option>
+                <option value="16">2⁴</option>
+                <option value="64">2⁶</option>
+            </select>samples</span>
+        <span style="display:none" title="Enable experimental Multiple Importance Sampling (MIS) for high-variance scenes"><input type="checkbox"
                 id="checkbox-mis" />MIS (beta)</span>&ensp;
         <br />
         <span title="Shape clipping boundary"><select id="select-clip">
diff --git a/implicit3-rt/script.js b/implicit3-rt/script.js
index 1c893b6..c22a386 100644
--- a/implicit3-rt/script.js
+++ b/implicit3-rt/script.js
@@ -186,6 +186,8 @@ document.body.onload = function (event) {
         params.sDenoise = "null";
         // params.sDenoise = "runet2gan";
         params.sSpp = state.sSpp = 256;
+        state.iSpp = 0.0;
+        params.sSamples = state.sSamples = "0.0625";
         // state.defaultScreenCenter = true;
     }
     initBuiltInStates(builtinStates);
@@ -265,6 +267,7 @@ vec3 {%funname%}Color(float x, float y, float z) {\n\
         new GraphingParameter("bLight", "checkbox-light"),
         new GraphingParameter("bYup", "checkbox-yup"),
         new GraphingParameter("sSpp", "select-spp"),
+        new GraphingParameter("sSamples", "select-samples"),
         new GraphingParameter("sLightPathDepth", "select-light-path-depth"),
         new GraphingParameter("cMis", "checkbox-mis"),
         new GraphingParameter("sClip", "select-clip"),
diff --git a/scripts/render-rt.js b/scripts/render-rt.js
index f4018b4..5b9a0e1 100644
--- a/scripts/render-rt.js
+++ b/scripts/render-rt.js
@@ -16,7 +16,6 @@ var renderer = {
     denoiseTarget: null,
     tonemapProgram: null,
     tonemapTarget: null,
-    iNFrame: 1,
     timerExt: null,
     uOutput: 0,
     requireAlbedo: false,
@@ -32,7 +31,7 @@ var trainingData = {
     width: 256,
     height: 256,
     spp: 0,
-    iNFrame: 64,
+    sSamples: 64,
     frames: [],
 };
 
@@ -110,7 +109,8 @@ async function drawScene(state, transformMatrix, lightDir) {
     gl.uniform2f(gl.getUniformLocation(renderer.renderProgram, "iResolution"),
         state.width, state.height);
     gl.uniform1i(gl.getUniformLocation(renderer.renderProgram, "uOutput"), renderer.uOutput);
-    gl.uniform1f(gl.getUniformLocation(renderer.renderProgram, "iNFrame"), renderer.iNFrame);
+    gl.uniform1f(gl.getUniformLocation(renderer.renderProgram, "iSpp"), state.iSpp);
+    gl.uniform1f(gl.getUniformLocation(renderer.renderProgram, "sSamples"), Number(state.sSamples));
     gl.uniform1f(gl.getUniformLocation(renderer.renderProgram, "iSeed"), Math.random());
     gl.uniformMatrix4fv(
         gl.getUniformLocation(renderer.renderProgram, "transformMatrix"),
@@ -163,6 +163,8 @@ async function drawScene(state, transformMatrix, lightDir) {
     gl.uniform1i(gl.getUniformLocation(renderer.postProgram, "iChannel1"), 1);
     gl.uniform1i(gl.getUniformLocation(renderer.postProgram, "denoise"),
         renderer.denoiser !== null);
+    gl.uniform1f(gl.getUniformLocation(renderer.postProgram, "iSpp"),
+        state.iSpp+state.sSamples);
     renderPass();
 
     // denoising
@@ -198,7 +200,7 @@ async function drawScene(state, transformMatrix, lightDir) {
             for (var i = 0; i < timerQueries.length; i++)
                 gl.deleteQuery(timerQueries[i]);
             fpsDisplay.innerHTML = state.iTime >= 0.0 ? state.iTime.toFixed(2) + " s" :
-                state.iFrame + " spp";
+                (Math.round(state.iSpp*100)/100) + " spp";
             return;
         }
         if (timerQueries.length == 0) return;
@@ -220,7 +222,7 @@ async function drawScene(state, transformMatrix, lightDir) {
         if (countIndividualTime) console.log(indivTime.join(' '));
         fpsDisplay.innerHTML = (
             state.iTime >= 0.0 ? state.iTime.toFixed(2) + " s - " :
-            (state.iFrame*renderer.iNFrame) + " spp - ") +
+            (Math.round(state.iSpp*100)/100) + " spp - ") +
             (1000.0 / totTime).toFixed(1) + " fps";
     }
     setTimeout(checkTime, 100);
@@ -232,6 +234,8 @@ async function drawScene(state, transformMatrix, lightDir) {
 var state = {
     name: "",
     iFrame: 0,
+    sSamples: 1,
+    iSpp: 0.0,
     width: window.innerWidth,
     height: window.innerHeight,
     screenCenter: { x: 0.5, y: 0.5 },
@@ -395,8 +399,8 @@ function initRenderer() {
             state.renderNeeded = true;
         // if (state.renderNeeded)
         if (state.renderNeeded && !trainingData.recording)
-            state.iFrame = 0;
-        if (state.iFrame < Number(state.sSpp)) {
+            state.iFrame = 0, state.iSpp = 0.0;
+        if (state.iSpp < Number(state.sSpp)) {
             if (trainingData.recording) {
                 state.width = canvas.width = trainingData.width;
                 state.height = canvas.height = canvas.style.height = trainingData.height;
@@ -428,6 +432,7 @@ function initRenderer() {
                 state.iTime = -1.0;
             }
             state.iFrame += 1;
+            state.iSpp += Number(state.sSamples);
         }
         oldScreenCenter = screenCenter;
         requestAnimationFrame(render);
@@ -611,15 +616,15 @@ function recordTrainingData(spp) {
         trainingData.frames = [];
         trainingData.spp = spp;
         state.sSpp = trainingData.spp;
-        renderer.iNFrame = Math.min(state.sSpp, trainingData.iNFrame);
-        state.sSpp /= renderer.iNFrame;
-        state.iFrame = 0;
+        state.sSamples = Math.min(state.sSpp, trainingData.sSamples);
+        state.sSpp /= state.sSamples;
+        state.iFrame = 0, state.iSpp = 0.0;
         trainingData.state = "wait";
         setTimeout(recordTrainingData, 100);
     }
 
     else if (trainingData.state == "wait") {
-        if (state.iFrame * renderer.iNFrame < trainingData.spp) {
+        if (state.iSpp < trainingData.spp) {
             setTimeout(recordTrainingData, 100);
             return;
         }
@@ -657,24 +662,24 @@ function recordTrainingData(spp) {
         }
         if (renderer.uOutput != 0) {
             renderer.uOutput += 1;
-            state.iFrame = 0;
+            state.iFrame = 0, state.iSpp = 0.0;
             setTimeout(recordTrainingData, 1);
             return;
         }
         if (renderer.uOutput == 0 && trainingData.spp == 1) {
             trainingData.spp = 1;
             state.sSpp = 1;
-            renderer.iNFrame = 1;
+            state.sSamples = 1;
             renderer.uOutput = 1;
-            state.iFrame = 0;
+            state.iFrame = 0, state.iSpp = 0.0;
             setTimeout(recordTrainingData, 1);
             return;
         }
         trainingData.spp /= 2;
         state.sSpp = trainingData.spp;
-        renderer.iNFrame = Math.min(state.sSpp, trainingData.iNFrame);
-        state.sSpp /= renderer.iNFrame;
-        state.iFrame = 0;
+        state.sSamples = Math.min(state.sSpp, trainingData.sSamples);
+        state.sSpp /= state.sSamples;
+        state.iFrame = 0, state.iSpp = 0.0;
         setTimeout(recordTrainingData, 1);
     }
 
@@ -682,7 +687,7 @@ function recordTrainingData(spp) {
         trainingData.recording = false;
         trainingData.state = "unstarted";
         updateBuffers();
-        renderer.iNFrame = 1;
+        state.sSamples = 1;
         state.renderNeeded = true;
 
         let n = trainingData.frames.length;
diff --git a/shaders/frag-rt-post.glsl b/shaders/frag-rt-post.glsl
index b035a5c..087a6d6 100644
--- a/shaders/frag-rt-post.glsl
+++ b/shaders/frag-rt-post.glsl
@@ -6,6 +6,7 @@ uniform sampler2D iChannel1;
 out vec4 fragColor;
 
 uniform bool denoise;
+uniform float iSpp;
 
 void main(void) {
     vec4 pixel0 = texelFetch(iChannel0, ivec2(gl_FragCoord.xy), 0);
@@ -13,6 +14,6 @@ void main(void) {
     vec4 pixel = pixel0 + pixel1;
     vec3 col = max(pixel.xyz/pixel.w, 0.0);
     fragColor = denoise ?
-        vec4(log(pow(col, vec3(1.0/2.2))+1.0), 0.0) :
-        vec4(col, 1.0);
+        vec4(log(pow(col/min(iSpp,1.0), vec3(1.0/2.2))+1.0), 0.0) :
+        vec4(col, 1.0) * pixel.w;
 }
diff --git a/shaders/frag-tonemap.glsl b/shaders/frag-tonemap.glsl
index 2fbfbe9..19a1b0d 100644
--- a/shaders/frag-tonemap.glsl
+++ b/shaders/frag-tonemap.glsl
@@ -30,12 +30,27 @@ vec3 tonemapUncharted2(vec3 x) {
 }
 
 void main(void) {
-    vec3 x = texelFetch(iChannel0, ivec2(gl_FragCoord.xy), 0).xyz;
 
-    x = tonemap{%TONEMAP%}(x);
+    vec4 x = vec4(0);
+    for (float s = 0.; s < 16.; s++) {
+        mat2 m = mat2(1);
+        for (float r = 0.; r < 4.; r++) {
+            for (float l = 0.; l < 2.*s; l++) {
+                vec2 p = gl_FragCoord.xy + m * vec2(s, s-l);
+                x = texelFetch(iChannel0, ivec2(p), 0);
+                if (x.w > 0.0) break;
+            }
+            if (x.w > 0.0) break;
+            m = mat2(1,1,-1,1) * m;
+        }
+        if (x.w > 0.0) break;
+    }
+    x /= x.w;
+
+    x.xyz = tonemap{%TONEMAP%}(x.xyz);
 
     if (isnan(x.x+x.y+x.z))
-        x = vec3(1,0,0);
+        x.xyz = vec3(1,0,0);
 
-    fragColor = vec4(x,1);
+    fragColor = vec4(x.xyz,1);
 }