change the way we store the LightsBuffer data to better fit openGL

previously, we trying to store the 'header' for LightsBuffer as flat
data types in cpp. This gave an incorrect offset because the pointers
for the array data were also included in the 'buffer'. Now we've got a
new pointer, 'buffer', and all the fields in the header are pointers
into the buffer address space, so we can align everything properly

src/tangerine.cpp

@@ -314,19 +314,31 @@ initLights(MemoryArena* arena, GLContext* gl_ctx, u32 max_lights)
 	// FIXME: revisit for 'Scene' abstraction
 
 	// TODO: we need a way to do this automatically somehow
-	u32 buf_size = 8 * sizeof(u32) // NOTE: 'header'
+	LightsBuffer* lb = ARENA_ALLOC(arena, LightsBuffer, 1);
+	lb->buf_size = 8 * sizeof(u32) // NOTE: 'header'
 				+ 6 * max_lights * sizeof(glm::vec4); // NOTE: vector arrays
+	LOGF(Debug, "buf_size: %d\n", lb->buf_size);
+	lb->buffer = ARENA_ALLOC(arena, u8, lb->buf_size);
 
-	LOGF(Debug, "buf_size: %d\n", buf_size);
+	lb->max_p_lights = (u32*) lb->buffer;
+	lb->active_p_lights =
+		(u32*) arenaGetAddressOffset(lb->max_p_lights, sizeof(u32));
+	lb->max_d_lights =
+		(u32*) arenaGetAddressOffset(lb->active_p_lights, sizeof(u32));
+	lb->active_d_lights =
+		(u32*) arenaGetAddressOffset(lb->max_d_lights, sizeof(u32));
 
-	LightsBuffer* lb = (LightsBuffer*) ARENA_ALLOC(arena, u8, buf_size);
+	*lb->max_p_lights = max_lights;
-	lb->buf_size = buf_size;
+	*lb->max_d_lights = max_lights;
-	lb->max_p_lights = max_lights;
+
-	lb->max_d_lights = max_lights;
+	// NOTE: add padding, we're not actually using  this since 4 * u32 is on a
+	// 	16 byte boundary, but will be helpful if we need to add more 'headers'
+	// 	in the future
+	void* arr_start = arenaGetAddressOffset(lb->buffer, 8 * sizeof(u32));
 
 	// NOTE: set offsets for array pointers
-	lb->pl_positions = (glm::vec4*) arenaGetAddressOffset(lb, sizeof(*lb));
 	u32 arr_size = max_lights * sizeof(glm::vec4);
+	lb->pl_positions = (glm::vec4*) arr_start;
 	lb->pl_colors =
 		(glm::vec4*) arenaGetAddressOffset(lb->pl_positions, arr_size);
 	lb->pl_intensities =
@@ -350,8 +362,8 @@ initLights(MemoryArena* arena, GLContext* gl_ctx, u32 max_lights)
 	glGenBuffers(1, &lights_ubo->id);
 	glBindBuffer(lights_ubo->target, lights_ubo->id);
 	glBufferData(lights_ubo->target,
-				buf_size,
+				lb->buf_size,
-				lb,
+				lb->buffer,
 				GL_DYNAMIC_DRAW);
 	glBindBufferBase(lights_ubo->target,
 					lights_ubo->binding_idx,

src/tangerine.h

@@ -107,16 +107,17 @@ struct GLClearColor
 };
 
 // NOTE: structure to match the layout of the 'lights' uniform in a shader
+// 	all the pointers are pointers into the address space of 'buffer'. the vec4
+// 	pointers are required to start on 16 byte boundaries as per layout std140,
+// 	so there may be some padding added between the 'header', and the start of
+// 	the arrays
 struct LightsBuffer
 {
 	u32 buf_size;
-	u32 max_p_lights;
+	u32* max_p_lights;
-	u32 active_p_lights;
+	u32* active_p_lights;
-	u32 max_d_lights;
+	u32* max_d_lights;
-	u32 active_d_lights;
+	u32* active_d_lights;
-
-	// NOTE: padding for std140, vec4 need to be aligned at 16 bytes
-	u8 padding[12];
 
 	glm::vec4* pl_positions;
 	glm::vec4* pl_colors;
@@ -125,6 +126,8 @@ struct LightsBuffer
 	glm::vec4* dl_directions;
 	glm::vec4* dl_colors;
 	glm::uvec4* dl_intensities;
+
+	void* buffer;
 };
 
 struct RenderState
@@ -146,11 +149,6 @@ struct RenderState
 	// 	could match up with gltf scene graph where everyting is part of a node
 	// TODO: will also need other types of lights: directional, spotlight
 	LightsBuffer* lights_buf;
-#if 0
-	u32 num_lights;
-	u32 max_lights;
-	PointLight* lights;
-#endif
 };