Added tests for exploring depth clipping precision

gxtest/clipping.cpp

@@ -43,7 +43,7 @@ void ClipTest()
 	ctrl.early_ztest = 0;
 	CGX_LOAD_BP_REG(ctrl.hex);
 
-	for (int step = 0; step < 16; ++step)
+	for (int step = 0; step < 22; ++step)
 	{
 		auto zmode = CGXDefault<ZMode>();
 		CGX_LOAD_BP_REG(zmode.hex);
@@ -204,6 +204,49 @@ void ClipTest()
 			test_quad.VertexBottomLeft(-1.0f, -1.0f, 65537.f);
 			break;
 
+		case 16:
+		case 17:
+		case 18:
+		case 19:
+		case 20:
+		case 21:
+		{
+			// Test depth clipping precision using values from a real game.
+			// Orthographic projection near plane at approximately 1.0+epsilon, far plane 100.000000-epsilon.
+			// Observed behavior on hardware diverges from expected result if Zc was calculated following IEEE standard.
+
+			CGX_BEGIN_LOAD_XF_REGS(0x1005, 1);
+			wgPipe->U32 = 0; // 0 = enable clipping, 1 = disable clipping
+
+			float depthClipNear = 1.00000012f; // 0x3f800001
+			float depthClipFar = 99.99999237f; // 0x42c7ffff
+			float depthScale = -1.0f * (1.0f / (depthClipFar - depthClipNear)); // projection matrix row 3 column 3, -0.01010101 (0xbc257eb6)
+			float depthBias = -depthClipFar * (1.0f / (depthClipFar - depthClipNear)); // projection matrix row 3 column 4, -1.01010108 (0xbf814afe)
+
+			struct {
+				bool expectDrawn;
+				float depth;
+			} test_cases[6] = {          // Z as hex   | IEEE Z*m33  |  IEEE Zc     | expected | observed
+				{true,  -depthClipNear}, // 0xbf800001 | 0.010101012 | -1.000000119 | clipped  | drawn
+				{true,  -1.0f},          // 0xbf800000 | 0.010101011 | -1.000000119 | clipped  | drawn
+				{true,  -0.99999797f},   // 0xbf7fffde | 0.010100991 | -1.000000119 | clipped  | drawn
+				{false, -0.99999791f},   // 0xbf7fffdd | 0.010100990 | -1.000000119 | clipped  | clipped
+				{true,  -depthClipFar},  // 0xc2c7ffff | 1.010101080 |  0.000000000 | drawn    | drawn
+				{false, -100.0f},        // 0xc2c80000 | 1.010101080 |  0.000000000 | drawn    | clipped
+			};
+
+			int id = step-16;
+
+			test_quad = GXTest::Quad()
+				.AtDepth(test_cases[id].depth)
+				.ZScaleBias(depthScale, depthBias)
+				.ColorRGBA(0xff, 0xff, 0xff, 0xff);
+
+			expect_quad_to_be_drawn = test_cases[id].expectDrawn;
+
+			break;
+		}
+
 		// TODO: One vertex with z < 0, depth clipping enabled, primitive gets properly (!) clipped
 		// TODO: One vertex with z < 0, depth clipping disabled, whole primitive gets drawn
 

gxtest/gxtest_util.cpp

@@ -191,6 +191,9 @@ Quad::Quad()
 	y[3] = -1.0;
 	z[3] =  1.0;
 
+	z_scale = -1.0f;
+	z_bias = 0.0f;
+
 	has_color = false;
 }
 
@@ -233,6 +236,14 @@ Quad& Quad::AtDepth(f32 depth)
 	return *this;
 }
 
+Quad& Quad::ZScaleBias(f32 scale, f32 bias)
+{
+	z_scale = scale;
+	z_bias = bias;
+
+	return *this;
+}
+
 Quad& Quad::ColorRGBA(u8 r, u8 g, u8 b, u8 a)
 {
 	color = ((u32)r << 24) | ((u32)g << 16) | ((u32)b << 8) | (u32)a;
@@ -287,7 +298,8 @@ void Quad::Draw()
 	memset(mtx, 0, sizeof(mtx));
 	mtx[0][0] = 1;
 	mtx[1][1] = 1;
-	mtx[2][2] = -1;
+	mtx[2][2] = z_scale;
+	mtx[2][3] = z_bias;
 	CGX_LoadProjectionMatrixOrthographic(mtx);
 
 	wgPipe->U8 = 0x80; // draw quads

gxtest/gxtest_util.h

@@ -33,6 +33,7 @@ class Quad
 	Quad& VertexBottomLeft(f32 x, f32 y, f32 z);
 
 	Quad& AtDepth(f32 depth);
+	Quad& ZScaleBias(f32 scale, f32 bias);
 
 	Quad& ColorRGBA(u8 r, u8 g, u8 b, u8 a);
 
@@ -41,6 +42,10 @@ class Quad
 private:
 	f32 x[4], y[4], z[4];
 
+	// orthographic projection parameters
+	f32 z_scale;
+	f32 z_bias;
+
 	bool has_color;
 	u32 color;
 };