Add "samplepixelcenter" option to HaltonSampler.

This forces all image samples to be at the center of the pixel. (Not so good
for anti-aliasing, but useful for some applications.)

src/samplers/halton.cpp

@@ -62,8 +62,9 @@ static void extendedGCD(uint64_t a, uint64_t b, int64_t *x, int64_t *y) {
 }
 
 // HaltonSampler Method Definitions
-HaltonSampler::HaltonSampler(int samplesPerPixel, const Bounds2i &sampleBounds)
-    : GlobalSampler(samplesPerPixel) {
+HaltonSampler::HaltonSampler(int samplesPerPixel, const Bounds2i &sampleBounds,
+                             bool sampleAtPixelCenter)
+    : GlobalSampler(samplesPerPixel), sampleAtPixelCenter(sampleAtPixelCenter) {
     // Generate random digit permutations for Halton sampler
     if (radicalInversePermutations.empty()) {
         RNG rng;
@@ -115,6 +116,7 @@ int64_t HaltonSampler::GetIndexForSample(int64_t sampleNum) const {
 }
 
 Float HaltonSampler::SampleDimension(int64_t index, int dim) const {
+    if (sampleAtPixelCenter && (dim == 0 || dim == 1)) return 0.5f;
     if (dim == 0)
         return RadicalInverse(dim, index >> baseExponents[0]);
     else if (dim == 1)
@@ -132,7 +134,8 @@ HaltonSampler *CreateHaltonSampler(const ParamSet &params,
                                    const Bounds2i &sampleBounds) {
     int nsamp = params.FindOneInt("pixelsamples", 16);
     if (PbrtOptions.quickRender) nsamp = 1;
-    return new HaltonSampler(nsamp, sampleBounds);
+    bool sampleAtCenter = params.FindOneBool("samplepixelcenter", false);
+    return new HaltonSampler(nsamp, sampleBounds, sampleAtCenter);
 }
 
 }  // namespace pbrt

src/samplers/halton.h

@@ -48,7 +48,8 @@ namespace pbrt {
 class HaltonSampler : public GlobalSampler {
   public:
     // HaltonSampler Public Methods
-    HaltonSampler(int nsamp, const Bounds2i &sampleBounds);
+    HaltonSampler(int nsamp, const Bounds2i &sampleBounds,
+                  bool sampleAtCenter = false);
     int64_t GetIndexForSample(int64_t sampleNum) const;
     Float SampleDimension(int64_t index, int dimension) const;
     std::unique_ptr<Sampler> Clone(int seed);
@@ -62,6 +63,9 @@ class HaltonSampler : public GlobalSampler {
     mutable Point2i pixelForOffset = Point2i(std::numeric_limits<int>::max(),
                                              std::numeric_limits<int>::max());
     mutable int64_t offsetForCurrentPixel;
+    // Added after book publication: force all image samples to be at the
+    // center of the pixel area.
+    bool sampleAtPixelCenter;
 
     // HaltonSampler Private Methods
     const uint16_t *PermutationForDimension(int dim) const {