From 9f607b858b6117e58f89ef6dfadf859f06f0ecb6 Mon Sep 17 00:00:00 2001
From: Eric Wasylishen <ewasylishen@gmail.com>
Date: Wed, 10 Aug 2016 12:22:41 -0600
Subject: [PATCH] light: Add GetLightDist function

---
 light/ltface.cc | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/light/ltface.cc b/light/ltface.cc
index 579821be..ba9b8645 100644
--- a/light/ltface.cc
+++ b/light/ltface.cc
@@ -927,6 +927,43 @@ GetLightValue(const light_t *entity, vec_t dist)
     }
 }
 
+#define SQR(x) ((x)*(x))
+
+// this is the inverse of GetLightValue
+float
+GetLightDist(const light_t *entity, vec_t desiredLight)
+{
+    float fadedist;
+    if (entity->formula.intValue() == LF_LINEAR) {
+        /* Linear formula always has a falloff point */
+        fadedist = fabs(entity->light.floatValue()) - desiredLight;
+        fadedist = fadedist / entity->atten.floatValue() / scaledist.floatValue();
+        fadedist = qmax(0.0f, fadedist);
+    } else {
+        /* Calculate the distance at which brightness falls to desiredLight */
+        switch (entity->formula.intValue()) {
+            case LF_INFINITE:
+            case LF_LOCALMIN:
+                fadedist = VECT_MAX;
+                break;
+            case LF_INVERSE:
+                fadedist = (LF_SCALE * fabs(entity->light.floatValue())) / (scaledist.floatValue() * entity->atten.floatValue() * desiredLight);
+                break;
+            case LF_INVERSE2:
+            case LF_INVERSE2A:
+                fadedist = sqrt(fabs(entity->light.floatValue() * SQR(LF_SCALE) / (SQR(scaledist.floatValue()) * SQR(entity->atten.floatValue()) * desiredLight)));
+                if (entity->formula.intValue() == LF_INVERSE2A) {
+                    fadedist -= (LF_SCALE / (scaledist.floatValue() * entity->atten.floatValue()));
+                }
+                fadedist = qmax(0.0f, fadedist);
+                break;
+            default:
+                Error("Internal error: formula not handled in %s", __func__);
+        }
+    }
+    return fadedist;
+}
+
 static inline void
 Light_Add(lightsample_t *sample, const vec_t light, const vec3_t color, const vec3_t direction)
 {