From c3c36d8517cb8fe764c0e45df669b7de2de586c1 Mon Sep 17 00:00:00 2001
From: Henri Verbeet <hverbeet@locutus.nl>
Date: Tue, 10 Jun 2025 18:31:36 +0200
Subject: [PATCH] demos: Introduce a helper function to create a projection
 matrix.

---
 demos/demo.h  | 38 ++++++++++++++++++++++++++++++++++++++
 demos/gears.c | 46 +++++++++++++---------------------------------
 2 files changed, 51 insertions(+), 33 deletions(-)

diff --git a/demos/demo.h b/demos/demo.h
index 2d5aae33a..765a3c0e3 100644
--- a/demos/demo.h
+++ b/demos/demo.h
@@ -98,6 +98,11 @@ struct demo_vec4
     float x, y, z, w;
 };
 
+struct demo_matrix
+{
+    float m[4][4];
+};
+
 struct demo_swapchain_desc
 {
     unsigned int width;
@@ -123,6 +128,39 @@ static inline void demo_vec4_set(struct demo_vec4 *v, float x, float y, float z,
     v->w = w;
 }
 
+static inline void demo_matrix_multiply(struct demo_matrix *out,
+        const struct demo_matrix *a, const struct demo_matrix *b)
+{
+    unsigned int i, j;
+
+    for (i = 0; i < 4; ++i)
+    {
+        for (j = 0; j < 4; ++j)
+        {
+            out->m[i][j] = a->m[i][0] * b->m[0][j]
+                    + a->m[i][1] * b->m[1][j]
+                    + a->m[i][2] * b->m[2][j]
+                    + a->m[i][3] * b->m[3][j];
+        }
+    }
+}
+
+static inline void demo_matrix_perspective_rh(struct demo_matrix *m, float w, float h, float z_near, float z_far)
+{
+    float sx = 2.0 * z_near / w;
+    float sy = 2.0 * z_near / h;
+    float sz = z_far / (z_near - z_far);
+    float d = z_near * sz;
+
+    *m = (struct demo_matrix)
+    {{
+        {  sx, 0.0f, 0.0f,  0.0f},
+        {0.0f,   sy, 0.0f,  0.0f},
+        {0.0f, 0.0f,   sz, -1.0f},
+        {0.0f, 0.0f,    d,  0.0f},
+    }};
+}
+
 static inline void demo_rasterizer_desc_init_default(D3D12_RASTERIZER_DESC *desc)
 {
     desc->FillMode = D3D12_FILL_MODE_SOLID;
diff --git a/demos/gears.c b/demos/gears.c
index a372be3da..f7f82f4c9 100644
--- a/demos/gears.c
+++ b/demos/gears.c
@@ -59,7 +59,7 @@ struct cxg_fence
 
 struct cxg_cb_data
 {
-    float mvp_matrix[16];
+    struct demo_matrix mvp_matrix;
     float normal_matrix[12];
 };
 
@@ -230,44 +230,24 @@ static void cxg_wait_for_previous_frame(struct cx_gears *cxg)
 
 static void cxg_update_mvp(struct cx_gears *cxg)
 {
+    struct demo_matrix projection, world;
     float s1 = sinf(cxg->theta);
     float c1 = cosf(cxg->theta);
     float s2 = sinf(cxg->phi);
     float c2 = cosf(cxg->phi);
     float z_offset = -40.0f;
-    float z_max = 60.0f;
-    float z_min = 5.0f;
-    float sx = z_min;
-    float sy = z_min * cxg->aspect_ratio;
-    float sz = -((z_max + z_min) / (z_max - z_min));
-    float d = (-2.0f * z_max * z_min) / (z_max - z_min);
-    unsigned int i, j;
-    float world[] =
-    {
-          c1,  s2 * s1, c2 * -s1, 0.0f,
-        0.0f,       c2,       s2, 0.0f,
-          s1, -s2 * c1,  c2 * c1, 0.0f,
-        0.0f,     0.0f, z_offset, 1.0f,
-    };
-    float projection[] =
-    {
-          sx, 0.0f, 0.0f,  0.0f,
-        0.0f,   sy, 0.0f,  0.0f,
-        0.0f, 0.0f,   sz, -1.0f,
-        0.0f, 0.0f,    d,  0.0f,
-    };
 
-    for (i = 0; i < 4; ++i)
-    {
-        for (j = 0; j < 4; ++j)
-        {
-            cxg->cb_data->mvp_matrix[i * 4 + j] = projection[j] * world[i * 4]
-                    + projection[j +  4] * world[i * 4 + 1]
-                    + projection[j +  8] * world[i * 4 + 2]
-                    + projection[j + 12] * world[i * 4 + 3];
-        }
-    }
-    memcpy(cxg->cb_data->normal_matrix, world, sizeof(cxg->cb_data->normal_matrix));
+    world = (struct demo_matrix)
+    {{
+        {  c1,  s2 * s1, c2 * -s1, 0.0f},
+        {0.0f,       c2,       s2, 0.0f},
+        {  s1, -s2 * c1,  c2 * c1, 0.0f},
+        {0.0f,     0.0f, z_offset, 1.0f},
+    }};
+    demo_matrix_perspective_rh(&projection, 2.0f, 2.0f / cxg->aspect_ratio, 5.0f, 60.0f);
+
+    demo_matrix_multiply(&cxg->cb_data->mvp_matrix, &world, &projection);
+    memcpy(cxg->cb_data->normal_matrix, &world, sizeof(cxg->cb_data->normal_matrix));
 }
 
 static void cxg_render_frame(struct cx_gears *cxg)