From: Jacob Lifshay <programmerjake@gmail.com>
Date: Tue, 11 Apr 2023 06:07:08 +0000 (-0700)
Subject: add notes on pow's use in 3D graphics
X-Git-Tag: opf_rfc_ls012_v1~8
X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=d5f837fe00e4f3c9efaeca49fdbb29a0c11fe0df;p=libreriscv.git

add notes on pow's use in 3D graphics
---

diff --git a/openpower/transcendentals.mdwn b/openpower/transcendentals.mdwn
index 86edb7f30..9e6b86118 100644
--- a/openpower/transcendentals.mdwn
+++ b/openpower/transcendentals.mdwn
@@ -436,6 +436,11 @@ CBRT, POW, POWN, POWR, ROOTN
 These are simply much more complex to implement in hardware, and typically
 will only be put into HPC applications.
 
+Note that `pow` is commonly used in Blinn-Phong shading (the shading model used
+by OpenGL 1.0 and commonly used by shader authors that need basic 3D graphics
+with specular highlights), however it can be sufficiently emulated using
+`pow(b, n) = exp2(n*log2(b))`.
+
 * **Zfrsqrt**: Reciprocal square-root.
 
 ## Trigonometric subsets
@@ -529,6 +534,10 @@ high-performance or correctly-rounding):
 
     ASINH( x ) = ln( x + SQRT(x**2+1))
 
+`pow` sufficient for 3D Graphics:
+
+    pow(b, x) = exp2(x * log2(b))
+
 # Evaluation and commentary
 
 Moved to [[discussion]]