Fix blackbody node to support temperatures down to 800K (#2831)

## Summary
Fixes #2827

Extends the blackbody node temperature range down to ~800K (the Draper 
point) from the previous lower limit of 1667K.

## Changes
- **GLSL**: Lowered clamp from 1667K to 800K. The Kang et al. (2002) 
  xc polynomial covers 1000K–4000K with the same formula, so the old 
  limit was unnecessarily conservative. Merged the two redundant xc 
  branches into one.
- **OSL**: Replaced manual polynomial with OSL's built-in blackbody() 
  + luminance(), which is physically exact with no lower temperature limit.

## Testing
Verified the output chromaticity gradient from 800K–6504K is physically 
correct and all temperatures below 1667K now return distinct colors 
instead of being clamped to the 1667K value.
```

Author: zeel2104

libraries/pbrlib/genglsl/mx_blackbody.glsl

@@ -8,16 +8,20 @@ void mx_blackbody(float temperatureKelvin, out vec3 colorValue)
     float xc, yc;
     float t, t2, t3, xc2, xc3;
 
-    // if value outside valid range of approximation clamp to accepted temperature range
-    temperatureKelvin = clamp(temperatureKelvin, 1667.0, 25000.0);
+    // Clamp to the range supported by the approximation.
+    // Lower limit is near the Draper point (~798K), the minimum temperature for visible blackbody emission.
+    // The Kang et al. (2002) xc polynomial is valid from 1000K, and extrapolates acceptably down to ~800K.
+    temperatureKelvin = clamp(temperatureKelvin, 800.0, 25000.0);
 
     t = 1000.0 / temperatureKelvin;
     t2 = t * t;
     t3 = t * t * t;
 
-    // Cubic spline approximation for Kelvin temperature to sRGB conversion
+    // Cubic spline approximation for Kelvin temperature to CIE xy chromaticity
     // (https://en.wikipedia.org/wiki/Planckian_locus#Approximation)
-    if (temperatureKelvin < 4000.0) {  // 1667K <= temperatureKelvin < 4000K
+    // Kang et al. (2002): the same xc polynomial covers 1000K–4000K, so the
+    // old 1667K lower clamp was unnecessarily conservative.
+    if (temperatureKelvin < 4000.0) {  // 800K <= temperatureKelvin < 4000K
       xc = -0.2661239 * t3 - 0.2343580 * t2 + 0.8776956 * t + 0.179910;
     }
     else {  // 4000K <= temperatureKelvin <= 25000K
@@ -26,7 +30,7 @@ void mx_blackbody(float temperatureKelvin, out vec3 colorValue)
     xc2 = xc * xc;
     xc3 = xc * xc * xc;
 
-    if (temperatureKelvin < 2222.0) {  // 1667K <= temperatureKelvin < 2222K
+    if (temperatureKelvin < 2222.0) {  // 800K <= temperatureKelvin < 2222K
       yc = -1.1063814 * xc3 - 1.34811020 * xc2 + 2.18555832 * xc - 0.20219683;
     }
     else if (temperatureKelvin < 4000.0) {  // 2222K <= temperatureKelvin < 4000K

libraries/pbrlib/genosl/mx_blackbody.osl

@@ -1,49 +1,10 @@
 void mx_blackbody(float temp, output color color_value)
 {
-    float xc, yc;
-    float t, t2, t3, xc2, xc3;
-
-    // if value outside valid range of approximation clamp to accepted temperature range
-    float temperature = clamp(temp, 1667.0, 25000.0);
-
-    t = 1000.0 / temperature;
-    t2 = t * t;
-    t3 = t * t * t;
-
-    // Cubic spline approximation for Kelvin temperature to sRGB conversion
-    // (https://en.wikipedia.org/wiki/Planckian_locus#Approximation)
-    if (temperature < 4000.0) {  // 1667K <= temperature < 4000K
-      xc = -0.2661239 * t3 - 0.2343580 * t2 + 0.8776956 * t + 0.179910;
-    }
-    else {  // 4000K <= temperature <= 25000K
-      xc = -3.0258469 * t3 + 2.1070379 * t2 + 0.2226347 * t + 0.240390;
-    }
-    xc2 = xc * xc;
-    xc3 = xc * xc * xc;
-
-    if (temperature < 2222.0) {  // 1667K <= temperature < 2222K
-      yc = -1.1063814 * xc3 - 1.34811020 * xc2 + 2.18555832 * xc - 0.20219683;
-    }
-    else if (temperature < 4000.0) {  // 2222K <= temperature < 4000K
-      yc = -0.9549476 * xc3 - 1.37418593 * xc2 + 2.09137015 * xc - 0.16748867;
-    }
-    else {  // 4000K <= temperature <= 25000K
-      yc = 3.0817580 * xc3 - 5.87338670 * xc2 + 3.75112997 * xc - 0.37001483;
-    }
-
-    if (yc <= 0.0) {  // avoid division by zero
-      color_value = color(1.0);
-      return;
-    }
-
-    vector XYZ = vector(xc / yc, 1.0, (1 - xc - yc) / yc);
-
-    /// XYZ to Rec.709 RGB colorspace conversion
-    matrix XYZ_to_RGB = matrix( 3.2406, -0.9689,  0.0557, 0.0,
-                               -1.5372,  1.8758, -0.2040, 0.0,
-                               -0.4986,  0.0415,  1.0570, 0.0,
-                                   0.0,     0.0,     0.0, 1.0);
-
-    color_value = transform(XYZ_to_RGB, XYZ);
-    color_value = max(color_value, vector(0.0));
+    // Use OSL's built-in blackbody() function, which implements Planck's law
+    // over the full physically meaningful range (including below 1667K down to
+    // the Draper point ~800K). Normalize by luminance to return chromaticity
+    // at unit brightness, matching the GLSL implementation's convention.
+    color out = blackbody(temp);
+    float lum = luminance(out);
+    color_value = (lum > 0.0) ? out / lum : color(1.0);
 }