From 87249534efb636cc094d46a869c28c7269031223 Mon Sep 17 00:00:00 2001
From: Joe Crayne <joe@jerkface.net>
Date: Thu, 30 May 2019 15:59:18 -0400
Subject: Compute height of pixel at distance.

---
 MeshSketch.hs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/MeshSketch.hs b/MeshSketch.hs
index 02c813d..0c1b4a2 100644
--- a/MeshSketch.hs
+++ b/MeshSketch.hs
@@ -471,6 +471,13 @@ onResize glarea realized w h = do
 unit :: (Linear t c, Fractional t, Normed (c t)) => c t -> c t
 unit v = scale (1/realToFrac (norm_2 v)) v
 
+-- | Compute the height of a pixel at the given 3d point.
+pixelDelta :: Camera -> Vector Float -> Float
+pixelDelta cam x = realToFrac $ frustumHeight eyeToPoint / realToFrac (camHeight cam)
+ where
+    eyeToPoint = norm_2 (x - camPos cam)
+    frustumHeight d = 2 * d * tan (realToFrac $ camHeightAngle cam / 2)
+
 -- This computes a point in world coordinates on the view screen if
 -- we assume the camera is located at the origin.
 computeDirection :: Camera -> Double -> Double -> Vector Float
-- 
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