From 2dab8f427efc98b054a1d0b4498080918c6545a5 Mon Sep 17 00:00:00 2001
From: UnknownShadow200 <unknownshadow200@gmail.com>
Date: Tue, 20 Jul 2021 08:59:26 +1000
Subject: [PATCH] Make OpenGL 1.5/1.2+VBO backend almost work with 1.1 too,
 just slower

---
 src/Graphics_GL.c | 63 ++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 51 insertions(+), 12 deletions(-)

diff --git a/src/Graphics_GL.c b/src/Graphics_GL.c
index f4a936b5e..263dfb6a7 100644
--- a/src/Graphics_GL.c
+++ b/src/Graphics_GL.c
@@ -1103,13 +1103,54 @@ void Gfx_DrawVb_IndexedTris(int verticesCount) {
 	glDrawElements(GL_TRIANGLES, ICOUNT(verticesCount), GL_UNSIGNED_SHORT, IB_PTR);
 }
 
-#ifndef CC_BUILD_GL11
+#ifdef CC_BUILD_GL11
+void Gfx_DrawIndexedTris_T2fC4b(int list, int ignored) { glCallList(list); }
+
+static void GL_CheckSupport(void) { MakeIndices(gl_indices, GFX_MAX_INDICES); }
+#else
 void Gfx_DrawIndexedTris_T2fC4b(int verticesCount, int startVertex) {
 	cc_uint32 offset = startVertex * SIZEOF_VERTEX_TEXTURED;
-	glVertexPointer(3, GL_FLOAT,        SIZEOF_VERTEX_TEXTURED, (void*)(offset));
-	glColorPointer(4, GL_UNSIGNED_BYTE, SIZEOF_VERTEX_TEXTURED, (void*)(offset + 12));
-	glTexCoordPointer(2, GL_FLOAT,      SIZEOF_VERTEX_TEXTURED, (void*)(offset + 16));
-	glDrawElements(GL_TRIANGLES,        ICOUNT(verticesCount),   GL_UNSIGNED_SHORT, NULL);
+	glVertexPointer(3, GL_FLOAT,        SIZEOF_VERTEX_TEXTURED, (void*)(VB_PTR + offset));
+	glColorPointer(4, GL_UNSIGNED_BYTE, SIZEOF_VERTEX_TEXTURED, (void*)(VB_PTR + offset + 12));
+	glTexCoordPointer(2, GL_FLOAT,      SIZEOF_VERTEX_TEXTURED, (void*)(VB_PTR + offset + 16));
+	glDrawElements(GL_TRIANGLES,        ICOUNT(verticesCount),   GL_UNSIGNED_SHORT, IB_PTR);
+}
+
+
+/*########################################################################################################################*
+*-------------------------------------------------------Compatibility-----------------------------------------------------*
+*#########################################################################################################################*/
+/* fake vertex buffer objects with client side pointers */
+typedef struct fake_buffer { cc_uint8* data; } fake_buffer;
+static fake_buffer* cur_ib;
+static fake_buffer* cur_vb;
+#define fake_GetBuffer(target) (target == _GL_ELEMENT_ARRAY_BUFFER ? &cur_ib : &cur_vb);
+
+static void APIENTRY fake_glBindBuffer(GLenum target, GLuint src) {
+	fake_buffer** buffer = fake_GetBuffer(target);
+	*buffer = (fake_buffer*)src;
+}
+
+static void APIENTRY fake_glDeleteBuffers(GLsizei n, const GLuint *buffers) {
+	Mem_Free((void*)buffers[0]);
+}
+
+static void APIENTRY fake_glGenBuffers(GLsizei n, GLuint *buffers) {
+	fake_buffer* buffer = (fake_buffer*)Mem_TryAlloc(1, sizeof(fake_buffer));
+	buffer->data = NULL;
+	buffers[0]   = (GLuint)buffer;
+}
+
+static void APIENTRY fake_glBufferData(GLenum target, cc_uintptr size, const GLvoid* data, GLenum usage) {
+	fake_buffer* buffer = *fake_GetBuffer(target);
+	Mem_Free(buffer->data);
+
+	buffer->data = Mem_TryAlloc(size, 1);
+	if (data) Mem_Copy(buffer->data, data, size);
+}
+static void APIENTRY fake_glBufferSubData(GLenum target, cc_uintptr offset, cc_uintptr size, const GLvoid* data) {
+	fake_buffer* buffer = *fake_GetBuffer(target);
+	Mem_Copy(buffer->data, data, size);
 }
 
 static void GL_CheckSupport(void) {
@@ -1138,15 +1179,13 @@ static void GL_CheckSupport(void) {
 	} else {
 		Logger_Abort("Only OpenGL 1.1 supported.\n\n" \
 			"Compile the game with CC_BUILD_GL11, or ask on the ClassiCube forums for it");
+
+		_glBindBuffer = fake_glBindBuffer; _glDeleteBuffers = fake_glDeleteBuffers;
+		_glGenBuffers = fake_glGenBuffers; _glBufferData    = fake_glBufferData;
+		_glBufferSubData = fake_glBufferSubData;
 	}
 	customMipmapsLevels = true;
 }
-#else
-void Gfx_DrawIndexedTris_T2fC4b(int list, int ignored) { glCallList(list); }
-
-static void GL_CheckSupport(void) {
-	MakeIndices(gl_indices, GFX_MAX_INDICES);
-}
-#endif /* CC_BUILD_GL11 */
+#endif /* !CC_BUILD_GL11 */
 #endif /* !CC_BUILD_GLMODERN */
 #endif