From 4a154d9c4ee5f60dacafa7107494ea89282382ea Mon Sep 17 00:00:00 2001
From: Luke Benstead <kazade@gmail.com>
Date: Thu, 31 May 2018 09:38:34 +0100
Subject: [PATCH] Add a near-z line clipping function in preparation for
 implementing near-z clipping

---
 GL/clip.c    | 34 ++++++++++++++++++++++++++++++++++
 GL/clip.h    | 22 ++++++++++++++++++++++
 GL/private.h |  2 +-
 Makefile     |  2 +-
 4 files changed, 58 insertions(+), 2 deletions(-)
 create mode 100644 GL/clip.c
 create mode 100644 GL/clip.h

diff --git a/GL/clip.c b/GL/clip.c
new file mode 100644
index 0000000..a5c101d
--- /dev/null
+++ b/GL/clip.c
@@ -0,0 +1,34 @@
+#include <float.h>
+#include "clip.h"
+
+ClipResult clipLineToNearZ(const float* v1, const float* v2, const float dist, float* vout, float* t) {
+    if(v1[2] < dist && v2[2] < dist) {
+        // Both behind, no clipping
+        return CLIP_RESULT_ALL_BEHIND;
+    }
+
+    if(v1[2] > dist && v2[2] > dist) {
+        return CLIP_RESULT_ALL_IN_FRONT;
+    }
+
+    float vec [] = {v2[0] - v1[0], v2[1] - v1[1], v2[2] - v1[2]};
+
+    /*
+     * The plane normal will always be pointing down the negative Z so we can simplify the dot products as x and y will always be zero
+     * the resulting calculation will result in simply -z of the vector
+    */
+    float vecDotP = -vec[2];
+
+    /* If the dot product is zero there is no intersection */
+    if(vecDotP > FLT_MIN || vecDotP < -FLT_MIN) {
+        *t = (-(dist - v1[2])) / vecDotP;
+
+        vout[0] = v1[0] + (vec[0] * (*t));
+        vout[1] = v1[1] + (vec[1] * (*t));
+        vout[2] = v1[2] + (vec[2] * (*t));
+
+        return (v1[2] >= dist) ? CLIP_RESULT_FRONT_TO_BACK : CLIP_RESULT_BACK_TO_FRONT;
+    } else {
+        return CLIP_RESULT_ALL_ON_PLANE;
+    }
+}
diff --git a/GL/clip.h b/GL/clip.h
new file mode 100644
index 0000000..c6a496b
--- /dev/null
+++ b/GL/clip.h
@@ -0,0 +1,22 @@
+#ifndef CLIP_H
+#define CLIP_H
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef enum {
+    CLIP_RESULT_ALL_IN_FRONT,
+    CLIP_RESULT_ALL_BEHIND,
+    CLIP_RESULT_ALL_ON_PLANE,
+    CLIP_RESULT_FRONT_TO_BACK,
+    CLIP_RESULT_BACK_TO_FRONT
+} ClipResult;
+
+ClipResult clipLineToNearZ(const float* v1, const float* v2, const float dist, float* vout, float* t);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // CLIP_H
diff --git a/GL/private.h b/GL/private.h
index 5f837c1..eac62de 100644
--- a/GL/private.h
+++ b/GL/private.h
@@ -4,7 +4,7 @@
 #include "../include/gl.h"
 #include "../containers/aligned_vector.h"
 #include "../containers/named_array.h"
-
+#include "./clip.h"
 
 #define TRACE_ENABLED 0
 #define TRACE() if(TRACE_ENABLED) {fprintf(stderr, "%s\n", __func__);}
diff --git a/Makefile b/Makefile
index 5dc4723..4f25a13 100644
--- a/Makefile
+++ b/Makefile
@@ -7,7 +7,7 @@
 
 TARGET = libGLdc.a
 OBJS = GL/draw.o GL/flush.o GL/framebuffer.o GL/immediate.o GL/lighting.o GL/state.o GL/texture.o GL/glu.o
-OBJS += GL/matrix.o GL/fog.o GL/error.o containers/stack.o containers/named_array.o containers/aligned_vector.o
+OBJS += GL/matrix.o GL/fog.o GL/error.o GL/clip.o containers/stack.o containers/named_array.o containers/aligned_vector.o
 
 SUBDIRS =