Object Mode Engine: Fixing the 3D grid 1/2

Do not use the inverse perspective matrix inside the shader to recover world positions.
That leads to severe float imprecision leading to nasty artifacts.

Instead we compute the world view vector for each pixels and do a ray to plane intersection.
We are still getting low precision derivatives when going far away from the origin, and thus artifacts.

This commit also fixes the non-appearing negative Z axis in 3D view.

1 files changed, 30 insertions, 7 deletions

diff --git a/source/blender/draw/modes/object_mode.c b/source/blender/draw/modes/object_mode.c
index f308021a3c8..735c1173353 100644
--- a/source/blender/draw/modes/object_mode.c
+++ b/source/blender/draw/modes/object_mode.c
@@ -208,6 +208,7 @@ static struct {
 	GPUShader *part_prim_sh;
 	GPUShader *part_axis_sh;
 	float camera_pos[3];
+	float screenvecs[3][4];
 	float grid_settings[5];
 	float grid_mat[4][4];
 	int grid_flag;
@@ -306,7 +307,8 @@ static void OBJECT_engine_init(void *vedata)
 
 	{
 		/* Grid precompute */
-		float viewinvmat[4][4], winmat[4][4], invwinmat[4][4], viewmat[4][4];
+		float invviewmat[4][4], invwinmat[4][4];
+		float viewmat[4][4], winmat[4][4];
 		const DRWContextState *draw_ctx = DRW_context_state_get();
 		View3D *v3d = draw_ctx->v3d;
 		Scene *scene = draw_ctx->scene;
@@ -321,10 +323,11 @@ static void OBJECT_engine_init(void *vedata)
 
 		DRW_viewport_matrix_get(winmat, DRW_MAT_WIN);
 		DRW_viewport_matrix_get(viewmat, DRW_MAT_VIEW);
-		DRW_viewport_matrix_get(viewinvmat, DRW_MAT_VIEWINV);
+		DRW_viewport_matrix_get(invwinmat, DRW_MAT_WININV);
+		DRW_viewport_matrix_get(invviewmat, DRW_MAT_VIEWINV);
 
 		/* Setup camera pos */
-		copy_v3_v3(e_data.camera_pos, viewinvmat[3]);
+		copy_v3_v3(e_data.camera_pos, invviewmat[3]);
 
 		/* if perps */
 		if (winmat[3][3] == 0.0f) {
@@ -333,13 +336,11 @@ static void OBJECT_engine_init(void *vedata)
 			    {1.0f, -1.0f, -1.0f, 1.0f},
 			    {-1.0f, 1.0f, -1.0f, 1.0f}
 			};
-			/* invert the proj matrix */
-			invert_m4_m4(invwinmat, winmat);
 
 			/* convert the view vectors to view space */
 			for (int i = 0; i < 2; i++) {
 				mul_m4_v4(invwinmat, viewvecs[i]);
-				mul_v3_fl(viewvecs[i], 1.0f / viewvecs[i][2]); /* normalize */
+				mul_v3_fl(viewvecs[i], 1.0f / viewvecs[i][2]); /* perspective divide */
 			}
 
 			fov = angle_v3v3(viewvecs[0], viewvecs[1]) / 2.0f;
@@ -356,6 +357,7 @@ static void OBJECT_engine_init(void *vedata)
 				e_data.grid_flag |= SHOW_AXIS_Y;
 			if (show_floor)
 				e_data.grid_flag |= SHOW_GRID;
+
 		}
 		else {
 			float viewdist = 1.0f / max_ff(fabsf(winmat[0][0]), fabsf(winmat[1][1]));
@@ -393,12 +395,31 @@ static void OBJECT_engine_init(void *vedata)
 			}
 		}
 
+		/* Vectors to recover pixel world position. Fix grid precision issue. */
+		copy_v4_fl4(e_data.screenvecs[0],  1.0f, -1.0f, 0.0f, 1.0f);
+		copy_v4_fl4(e_data.screenvecs[1], -1.0f,  1.0f, 0.0f, 1.0f);
+		copy_v4_fl4(e_data.screenvecs[2], -1.0f, -1.0f, 0.0f, 1.0f);
+
+		for (int i = 0; i < 3; i++) {
+			/* Doing 2 steps to recover world position of the corners of the frustum.
+			 * Using the inverse perspective matrix is giving very low precision output. */
+			mul_m4_v4(invwinmat, e_data.screenvecs[i]);
+			e_data.screenvecs[i][0] /= e_data.screenvecs[i][3]; /* perspective divide */
+			e_data.screenvecs[i][1] /= e_data.screenvecs[i][3]; /* perspective divide */
+			e_data.screenvecs[i][2] /= e_data.screenvecs[i][3]; /* perspective divide */
+			e_data.screenvecs[i][3] = 1.0f;
+			mul_m4_v4(invviewmat, e_data.screenvecs[i]);
+		}
+
+		sub_v3_v3(e_data.screenvecs[0], e_data.screenvecs[2]);
+		sub_v3_v3(e_data.screenvecs[1], e_data.screenvecs[2]);
+
 		/* Z axis if needed */
 		if (((rv3d->view == RV3D_VIEW_USER) || (rv3d->persp != RV3D_ORTHO)) && show_axis_z) {
 			e_data.zpos_flag = SHOW_AXIS_Z;
 
 			float zvec[4] = {0.0f, 0.0f, -1.0f, 0.0f};
-			mul_m4_v4(viewinvmat, zvec);
+			mul_m4_v4(invviewmat, zvec);
 
 			/* z axis : chose the most facing plane */
 			if (fabsf(zvec[0]) < fabsf(zvec[1])) {
@@ -754,9 +775,11 @@ static void OBJECT_cache_init(void *vedata)
 		DRW_shgroup_uniform_int(grp, "gridFlag", &e_data.zneg_flag, 1);
 		DRW_shgroup_uniform_mat4(grp, "ViewProjectionOffsetMatrix", (float *)e_data.grid_mat);
 		DRW_shgroup_uniform_vec3(grp, "cameraPos", e_data.camera_pos, 1);
+		DRW_shgroup_uniform_vec4(grp, "screenvecs[0]", e_data.screenvecs[0], 3);
 		DRW_shgroup_uniform_vec4(grp, "gridSettings", e_data.grid_settings, 1);
 		DRW_shgroup_uniform_float(grp, "gridOneOverLogSubdiv", &e_data.grid_settings[4], 1);
 		DRW_shgroup_uniform_block(grp, "globalsBlock", globals_ubo);
+		DRW_shgroup_uniform_vec2(grp, "viewportSize", DRW_viewport_size_get(), 1);
 		DRW_shgroup_call_add(grp, quad, mat);
 
 		grp = DRW_shgroup_create(e_data.grid_sh, psl->grid);