Add support for writing Gaussian forward and inverse transform as meta data

Author: Ole Gulbrandsen

src/libOpenImageIO/maketexture.cpp

@@ -1184,6 +1184,91 @@ make_texture_impl(ImageBufAlgo::MakeTextureMode mode, const ImageBuf* input,
         mode = ImageBufAlgo::MakeTxTexture;
         src  = bumpslopes;
     }
+
+    if (configspec.get_int_attribute("maketx:cdf")) {
+        // Writes Gaussian CDF and Inverse Gaussian CDF as per-channel metadata. We provide both
+        // the inverse transfrom and forward transfrom, so in theory we're free to change
+        // the distribution.
+        //
+        // References:
+        //
+        // Brent Burley, On Histogram-Preserving Blending for Randomized Texture Tiling,
+        // Journal of Computer Graphics Techniques (JCGT), vol. 8, no. 4, 31-53, 2019
+        //
+        // Eric Heitz and Fabrice Neyret, High-Performance By-Example Noise using a Histogram-Preserving Blending Operator,
+        // https://hal.inria.fr/hal-01824773},
+        // Proceedings of the ACM on Computer Graphics and Interactive Techniques,
+        // ACM SIGGRAPH / Eurographics Symposium on High-Performance Graphics 2018,
+        //
+        // Benedikt Bitterli
+        // https://benedikt-bitterli.me/histogram-tiling/
+
+        const float cdf_sigma = configspec.get_float_attribute(
+            "maketx:cdfsigma");
+        const int cdf_bits  = configspec.get_int_attribute("maketx:cdfbits");
+        const uint64_t bins = 1 << cdf_bits;
+
+        // Normalization coefficient for the truncated normal distribution
+        const float c_sigma_inv = fast_erf(1.0f / (2.0f * M_SQRT2 * cdf_sigma));
+
+        // If there are channels other than R,G,B,A, we probably shouldn't do anything
+        // to them.
+
+        const int channels = std::min(4, src->spec().nchannels);
+
+        std::vector<float> invCDF(bins);
+        std::vector<float> CDF(bins);
+        std::vector<imagesize_t> hist;
+
+        for (int i = 0; i < channels; i++) {
+            hist = ImageBufAlgo::histogram(*src, i, bins, 0.0f, 1.0f);
+
+            // Turn the histogram into a non-normalized CDF
+            for (int j = 1; j < bins; j++) {
+                hist[j] += hist[j - 1];
+            }
+
+            // Store the inverse CDF as a lookup-table which we'll use to transform
+            // the image data to a Guassian distribution. As mentioned in Burley [2019]
+            // we're combining two steps here when using the invCDF lookup
+            // table: we first "look up" the image value through its CDF (the normalized histogram)
+            // which gives us a uniformly distributed value, which we're then feeding in to the
+            // Gaussian inverse CDF to transfrom the unifrom distribution to Gaussian.
+
+            for (int j = 0; j < bins; j++) {
+                float u = float(hist[j]) / hist[bins - 1];
+                float g = 0.5
+                          + cdf_sigma * M_SQRT2
+                                * fast_ierf(c_sigma_inv * (2 * u - 1));
+                invCDF[j] = std::min(1.0f, std::max(0.0f, g));
+            }
+            configspec.attribute("invCDF_" + std::to_string(i),
+                                 TypeDesc(TypeDesc::FLOAT, bins),
+                                 invCDF.data());
+
+
+            // Store the forward CDF as a lookup table to transform back
+            // to the original image distribution from a Gaussian distribution.
+
+            std::vector<float>::iterator upper;
+            for (int j = 0; j < bins; j++) {
+                upper  = std::upper_bound(invCDF.begin(), invCDF.end(),
+                                         float(j) / (float(bins - 1)));
+                CDF[j] = std::min(1.0f,
+                                  std::max(0.0f, float(upper - invCDF.begin())
+                                                     / float(bins - 1)));
+            }
+
+
+            configspec.attribute("CDF_" + std::to_string(i),
+                                 TypeDesc(TypeDesc::FLOAT, bins), CDF.data());
+        }
+
+        configspec["CDF_bits"] = cdf_bits;
+
+        mode = ImageBufAlgo::MakeTxTexture;
+    }
+
     double misc_time_2 = alltime.lap();
     STATUS("misc2", misc_time_2);
 

src/maketx/maketx.cpp

@@ -177,6 +177,9 @@ getargs(int argc, char* argv[], ImageSpec& configspec)
     bool sansattrib            = false;
     float sharpen              = 0.0f;
     float uvslopes_scale       = 0.0f;
+    bool cdf                   = false;
+    float cdfsigma             = 1.0f / 6;
+    int cdfbits                = 8;
     std::string incolorspace;
     std::string outcolorspace;
     std::string colorconfigname;
@@ -285,6 +288,13 @@ getargs(int argc, char* argv[], ImageSpec& configspec)
       .hidden(); // DEPRECATED 1.6
     ap.arg("--mipimage %L:FILENAME", &mipimages)
       .help("Specify an individual MIP level");
+    ap.arg("--cdf %d:N", &cdf)
+      .help("Store the forward and inverse Gaussian CDF as a lookup-table. The variance is set by cdfsigma (1/6 by default), and the number of buckets \
+              in the lookup table is determined by cdfbits (8 bit - 256 buckets by default)");
+    ap.arg("--cdfsigma %f:N", &cdfsigma)
+      .help("Specify the Gaussian sigma parameter when writing the forward and inverse Gaussian CDF data. The default vale is 1/6 (0.1667)");
+    ap.arg("--cdfbits %d:N", &cdfbits)
+      .help("Specify the number of bits used to store the forward and inverse Guassian CDF. The default value is 8 bits");
 
     ap.separator("Basic modes (default is plain texture):");
     ap.arg("--shadow", &shadowmode)
@@ -426,6 +436,9 @@ getargs(int argc, char* argv[], ImageSpec& configspec)
         configspec.attribute("maketx:mipimages", Strutil::join(mipimages, ";"));
     if (bumpslopesmode)
         configspec.attribute("maketx:bumpformat", bumpformat);
+    configspec.attribute("maketx:cdf", cdf);
+    configspec.attribute("maketx:cdfsigma", cdfsigma);
+    configspec.attribute("maketx:cdfbits", cdfbits);
 
     std::string cmdline
         = Strutil::sprintf("OpenImageIO %s : %s", OIIO_VERSION_STRING,

src/oiiotool/oiiotool.cpp

@@ -4692,6 +4692,14 @@ prep_texture_config(ImageSpec& configspec, ParamValueList& fileoptions)
                          fileoptions.get_string("bumpformat", "auto"));
     configspec.attribute("maketx:uvslopes_scale",
                          fileoptions.get_float("uvslopes_scale", 0.0f));
+
+    // The default values here should match the initialized values
+    // in src/maketx/maketx.cpp
+    configspec.attribute("maketx:cdf", fileoptions.get_int("cdf"));
+    configspec.attribute("maketx:cdfbits", fileoptions.get_int("cdfbits", 8));
+    configspec.attribute("maketx:cdfsigma",
+                         fileoptions.get_float("cdfsigma", 1.0f / 6));
+
     // if (mipimages.size())
     //     configspec.attribute ("maketx:mipimages", Strutil::join(mipimages,";"));