From 9e9fb83bb1a667c12e7b11168a7c3a7284cab994 Mon Sep 17 00:00:00 2001
From: Zack Buhman <zack@buhman.org>
Date: Mon, 29 Sep 2025 19:12:39 -0500
Subject: [PATCH] vertex: rotate around x axis

---
 src/vertex.glsl | 48 +++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 47 insertions(+), 1 deletion(-)

diff --git a/src/vertex.glsl b/src/vertex.glsl
index 1ef571c..9e8995f 100644
--- a/src/vertex.glsl
+++ b/src/vertex.glsl
@@ -7,9 +7,55 @@ attribute vec4 position;
 varying float fade_factor;
 varying vec2 texcoord;
 
+
+mat4 view_frustum(float angle_of_view,
+                  float aspect_ratio,
+                  float z_near,
+                  float z_far)
+{
+  return mat4(vec4(1.0/tan(angle_of_view),           0.0, 0.0, 0.0),
+              vec4(0.0, aspect_ratio/tan(angle_of_view),  0.0, 0.0),
+              vec4(0.0, 0.0,    (z_far+z_near)/(z_far-z_near), 1.0),
+              vec4(0.0, 0.0, -2.0*z_far*z_near/(z_far-z_near), 0.0)
+              );
+}
+
+mat4 scale(float x, float y, float z)
+{
+  return mat4(
+              vec4(x,   0.0, 0.0, 0.0),
+              vec4(0.0, y,   0.0, 0.0),
+              vec4(0.0, 0.0, z,   0.0),
+              vec4(0.0, 0.0, 0.0, 1.0)
+              );
+}
+
+mat4 translate(float x, float y, float z)
+{
+  return mat4(vec4(1.0, 0.0, 0.0, 0.0),
+              vec4(0.0, 1.0, 0.0, 0.0),
+              vec4(0.0, 0.0, 1.0, 0.0),
+              vec4(x,   y,   z,   1.0)
+              );
+}
+
+mat4 rotate_x(float theta)
+{
+  return mat4(vec4(1.0,         0.0,         0.0, 0.0),
+              vec4(0.0,  cos(time),  sin(time), 0.0),
+              vec4(0.0, -sin(time),  cos(time), 0.0),
+              vec4(0.0,         0.0,         0.0, 1.0)
+              );
+}
+
 void main()
 {
-  gl_Position = position;
+
+  gl_Position = view_frustum(radians(45.0), 4.0/3.0, 0.5, 5.0)
+        * translate(cos(time), 0.0, 3.0+sin(time))
+        * rotate_x(time)
+        * scale(4.0/3.0, 1.0, 1.0)
+        * position;
 
   vec2 tex = position.xy * vec2(0.5) + vec2(0.5);