diff --git a/src/mol-geo/geometry/direct-volume/direct-volume.ts b/src/mol-geo/geometry/direct-volume/direct-volume.ts
index 443b4c8b32019a121bf2e22b76738d3fbb93461a..0467e61cb09eee299e265e47ea7524bc1260804e 100644
--- a/src/mol-geo/geometry/direct-volume/direct-volume.ts
+++ b/src/mol-geo/geometry/direct-volume/direct-volume.ts
@@ -86,17 +86,12 @@ export namespace DirectVolume {
 
         const counts = { drawCount: VolumeBox.indices.length, groupCount, instanceCount }
 
-        const vertices = new Float32Array(VolumeBox.vertices)
-        transformPositionArray(gridTransform.ref.value, vertices, 0, vertices.length / 3)
-        const boundingSphere = calculateBoundingSphere(
-            vertices, vertices.length / 3,
-            transform.aTransform.ref.value, transform.instanceCount.ref.value
-        )
+        const boundingSphere = getBoundingSphere(gridDimension.ref.value, gridTransform.ref.value, transform.aTransform.ref.value, transform.instanceCount.ref.value)
 
         const controlPoints = getControlPointsFromVec2Array(props.controlPoints)
         const transferTex = createTransferFunctionTexture(controlPoints)
 
-        const maxSteps = Math.ceil(Vec3.magnitude(gridDimension.ref.value)) * 2
+        const maxSteps = Math.ceil(Vec3.magnitude(gridDimension.ref.value)) * 2 * 5
 
         return {
             ...color,
@@ -135,12 +130,7 @@ export namespace DirectVolume {
     }
 
     export function updateBoundingSphere(values: DirectVolumeValues, directVolume: DirectVolume) {
-        const vertices = new Float32Array(values.aPosition.ref.value)
-        transformPositionArray(values.uTransform.ref.value, vertices, 0, vertices.length / 3)
-        const boundingSphere = calculateBoundingSphere(
-            vertices, Math.floor(vertices.length / 3),
-            values.aTransform.ref.value, values.instanceCount.ref.value
-        )
+        const boundingSphere = getBoundingSphere(values.uGridDim.ref.value, values.uTransform.ref.value, values.aTransform.ref.value, values.instanceCount.ref.value)
         if (!Sphere3D.equals(boundingSphere, values.boundingSphere.ref.value)) {
             ValueCell.update(values.boundingSphere, boundingSphere)
         }
@@ -156,4 +146,19 @@ export namespace DirectVolume {
         Geometry.updateRenderableState(state, props)
         state.opaque = false
     }
+}
+
+//
+
+const mTmp = Mat4.identity()
+const mTmp2 = Mat4.identity()
+const vHalfUnit = Vec3.create(0.5, 0.5, 0.5)
+const tmpVertices = new Float32Array(VolumeBox.vertices.length)
+function getBoundingSphere(gridDimension: Vec3, gridTransform: Mat4, transform: Float32Array, transformCount: number) {
+    tmpVertices.set(VolumeBox.vertices)
+    Mat4.fromTranslation(mTmp, vHalfUnit)
+    Mat4.mul(mTmp, Mat4.fromScaling(mTmp2, gridDimension), mTmp)
+    Mat4.mul(mTmp, gridTransform, mTmp)
+    transformPositionArray(mTmp, tmpVertices, 0, tmpVertices.length / 3)
+    return calculateBoundingSphere(tmpVertices, tmpVertices.length / 3, transform, transformCount)
 }
\ No newline at end of file
diff --git a/src/mol-gl/shader/direct-volume.frag b/src/mol-gl/shader/direct-volume.frag
index 47ccb7dd689daca83a62ebf63c36e2b8670f78f1..7eabed7ff24c447579f6aaee2c5505cb71e62cf7 100644
--- a/src/mol-gl/shader/direct-volume.frag
+++ b/src/mol-gl/shader/direct-volume.frag
@@ -209,7 +209,7 @@ void main () {
 
     vec3 rayDir = normalize(origPos - cameraPos);
     vec3 startLoc = unitCoord;
-    vec3 step = rayDir * (1.0 / uGridDim) * 0.5;
+    vec3 step = rayDir * (1.0 / uGridDim) * 0.1;
 
     gl_FragColor = raymarch(startLoc, step, normalize(cameraPos));
     if (length(gl_FragColor.rgb) < 0.00001) discard;
diff --git a/src/mol-gl/shader/direct-volume.vert b/src/mol-gl/shader/direct-volume.vert
index 14cd4822c16de0e596a6df0e4b8fbc3fc3c86d74..13d51f77120a1927041756c887ba32eb3093199b 100644
--- a/src/mol-gl/shader/direct-volume.vert
+++ b/src/mol-gl/shader/direct-volume.vert
@@ -30,4 +30,11 @@ void main() {
     origPos = unitCoord * uBboxSize + uBboxMin;
     instance = aInstance;
     gl_Position = uProjection * mvPosition;
+
+    // clamp z position to clip space
+    if(gl_Position.z > gl_Position.w) {
+        gl_Position.z = gl_Position.w - 0.0001;
+    } else if(gl_Position.z < -gl_Position.w) {
+        gl_Position.z = -gl_Position.w + 0.0001;
+    }
 }
\ No newline at end of file