Trapezoidal Shadow Maps (TSM) - Recipe

TSM Idea

Trapezoidal Approximation

Trapezoidal Transformation

(-1, -1, 1, 1)^T = N_T * t_0 (+1, -1, 1, 1)^T = N_T * t_1 (+1, +1, 1, 1)^T = N_T * t_2 (-1, +1, 1, 1)^T = N_T * t_3

1.	As a first step, T_1 transforms the center of the top edge to the origin (the vectors u = (x_u, y_u, z_u, w_u) and v = (x_v, y_v, z_v, w_v) hold intermediate results): u = (t_2 + t_3) / 2, |1 0 0 -x_u| T_1 = |0 1 0 -y_u| |0 0 1 0 | |0 0 0 1 |
2.	Then, the trapezoid T is rotated by applying R around the origin in such a way, that the top edge is colliniear with the x-axis: u = (t_2 - t_3) / |t_2 - t_3|, |x_u y_u 0 0| R = |y_u -x_u 0 0| | 0 0 1 0| | 0 0 0 1|
3.	After step 2, the intersection i of the two side lines containing the two side edges (t_0, t_3) and (t_1, t_2) is transformed, by applying T_2, to the origin: u = R * T_1 * i, |1 0 0 -x_u| T_2 = |0 1 0 -y_u| |0 0 1 0 | |0 0 0 1 |
4.	As a next step, the trapezoid has to be sheared with H, so that it is symmetrical to the y-axis, i.e. that the line passing through the center of the bottom edge and center of the top edge is collinear with the y-axis: u = (T_2 * R * T_1 * (t_2 + t_3)) / 2, |1 -x_u/y_u 0 0| H = |0 1 0 0| |0 0 1 0| |0 0 0 1|
5.	Now, the trapezoid is scaled by applying S_1, so that the angle between the two side lines containing the two side edges (t_0, t_3) and (t_1, t_2) is 90 degrees, and so that the distance between the top edge and the x-axis is 1: u = H * T_2 * R * T_1 * t_2, |1/x_u 0 0 0| S_1 = | 0 1/y_u 0 0| | 0 0 1 0| | 0 0 0 1|
6.	The following transformation N transforms the trapezoid to a rectangle: |1 0 0 0| N = |0 1 0 1| |0 0 1 0| |0 1 0 0|
7.	Then, the rectangle is translated along the y-axis until its center is coincident with the origin. This is done by applying T_3. After this transformation the rectangle is symmetrical to the x-axis as well: u = N * S_1 * H * T_2 * R * T_1 * t_0, v = N * S_1 * H * T_2 * R * T_1 * t_2, |1 0 0 0 | T_3 = |0 1 0 -(y_u/w_u+y_v/w_v)/2| |0 0 1 0 | |0 0 0 1 |
8.	As a last step the rectangle has to be scaled with S_2 along the y-axis so that it covers the front side ot the unit cube: u = T_3 * N * S_1 * H * T_2 * R * T_1 * t_0, |1 0 0 0| S_2 = |0 -w_u/y_u 0 0| |0 0 1 0| |0 0 0 1|
	Now the trapezoidal transformation N_T can be computed as follows: N_T = S_2 * T_3 * N * S_1 * H * T_2 * R * T_1 .

#define ASSIGN_MAT(M, u0, u3, u6, u1, u4, u7, u2, u5, u8) { \
	M[0][0] = u0; M[0][1] = u3; M[0][2] = u6; \
	M[1][0] = u1; M[1][1] = u4; M[1][2] = u7; \
	M[2][0] = u2; M[2][1] = u5; M[2][2] = u8; \
	}

#define DET2(a, b, c, d) ((a) * (d) - (b) * (c))

#define DOT2(u, v) (u[0] * v[0] + u[1] * v[1])

void intersect(float i[2], float g0[3], float g1[3], float h0[3], float h1[3]) {
	float a, b;

	i[0] = i[1] = 1.0f / DET2(g0[0] - g1[0], g0[1] - g1[1], h0[0] - h1[0], h0[1] - h1[1]);

	a = DET2(g0[0], g0[1], g1[0], g1[1]);
	b = DET2(h0[0], h0[1], h1[0], h1[1]);

	i[0] *=	DET2(a, g0[0] - g1[0], b, h0[0] - h1[0]);
	i[1] *=	DET2(a, g0[1] - g1[1], b, h0[1] - h1[1]);
}

void map_Trapezoid_To_Square(float TR[3][3], float t0[3], float t1[3], float t2[3], float t3[3]) {
	float i[2], a, b, c, d;

	//M1 = R * T1
	a = 0.5f * (t2[0] - t3[0]);
	b = 0.5f * (t2[1] - t3[1]);

	ASSIGN_MAT(TR, a  ,  b  , a * a + b * b,
	               b  , -a  , a * b - b * a,
	              0.0f, 0.0f, 1.0f);

	//M2 = T2 * M1 = T2 * R * T1
	intersect(i, t0, t3, t1, t2);

	TR[0][2] = -DOT2(TR[0], i);
	TR[1][2] = -DOT2(TR[1], i);

	//M1 = H * M2 = H * T2 * R * T1
	a = DOT2(TR[0], t2) + TR[0][2];
	b = DOT2(TR[1], t2) + TR[1][2];
	c = DOT2(TR[0], t3) + TR[0][2];
	d = DOT2(TR[1], t3) + TR[1][2];

	a = -(a + c) / (b + d);

	TR[0][0] += TR[1][0] * a;
	TR[0][1] += TR[1][1] * a;
	TR[0][2] += TR[1][2] * a;

	//M2 = S1 * M1 = S1 * H * T2 * R * T1
	a = 1.0f / (DOT2(TR[0], t2) + TR[0][2]);
	b = 1.0f / (DOT2(TR[1], t2) + TR[1][2]);

	TR[0][0] *= a; TR[0][1] *= a; TR[0][2] *= a;
	TR[1][0] *= b; TR[1][1] *= b; TR[1][2] *= b;

	//M1 = N * M2 = N * S1 * H * T2 * R * T1
	TR[2][0] = TR[1][0]; TR[2][1] = TR[1][1]; TR[2][2] = TR[1][2];
	TR[1][2] += 1.0f;

	//M2 = T3 * M1 = T3 * N * S1 * H * T2 * R * T1
	a = DOT2(TR[1], t0) + TR[1][2];
	b = DOT2(TR[2], t0) + TR[2][2];
	c = DOT2(TR[1], t2) + TR[1][2];
	d = DOT2(TR[2], t2) + TR[2][2];

	a = -0.5f * (a / b + c / d);

	TR[1][0] += TR[2][0] * a;
	TR[1][1] += TR[2][1] * a;
	TR[1][2] += TR[2][2] * a;

	//M1 = S2 * M2 = S2 * T3 * N * S1 * H * T2 * R * T1
	a = DOT2(TR[1], t0) + TR[1][2];
	b = DOT2(TR[2], t0) + TR[2][2];

	c = -b / a;

	TR[1][0] *= c; TR[1][1] *= c; TR[1][2] *= c;
}

...
float TR[3][3];

map_Trapezoid_To_Square(TR, t0, t1, t2, t3);

GLdouble N_T[16];

N_T[0] = TR[0][0]; N_T[4] = TR[0][1]; N_T[ 8] =  0.0f; N_T[12] = TR[0][2];
N_T[1] = TR[1][0]; N_T[5] = TR[1][1]; N_T[ 9] =  0.0f; N_T[13] = TR[1][2];
N_T[2] =     0.0f; N_T[6] =     0.0f; N_T[10] =  1.0f; N_T[14] =     0.0f;
N_T[3] = TR[2][0]; N_T[7] = TR[2][1]; N_T[11] =  0.0f; N_T[15] = TR[2][2];
...

Another more general approach is to calculate the quadrilateral to quad mapping which can be found in Paul Heckbert's Master Thesis: Fundamentals of Texture Mapping and Image Warping at page 17-21, and source code in Appendix A.2.2, page 70-72.

The trapezoidal approximation and the computation of N_T is the only part of the algorithm where the CPU is used.

Addressing the Polygon Offset Problem

Figure: N_T affects the distribution of the z-values. The specified polygon offset is too big for areas nearby which results in shadow disappearing. By reducing the polygon offset, wrong self-shadowing would be introduced in areas further away.

Figure: By maintaining the depth value in the post-perspective space of the light, a constant polygon offset can be specified like in the standard shadow map approach. The distribution remains uniform.

In the following we show how the trapezoidal transformation can be implemented (using the NV_vertex_program and NV_fragment_program extension) so that severe polygon offset problems can be completely avoided. We only show here the TSM generation step, i.e. the first pass of the algorithm, because the second pass of the algorithm works in the same way (Transform the first texture coordinate to trapezoidal space and the second to the post-perspective space of the light. Then, on the fragment stage replace z_T with z_L).

light's vertex program:

!!VP1.0

# c[0-3]  : N_T
# c[8-11] : light's projection and modelview matrix
# c[12-15]: world matrix
# c[16-19]: inverse world matrix
# v[OPOS] : object position
# o[HPOS] : result vertex

# transform v[OPOS] into world space and store result in R4:
# R4 = W * v[OPOS]
DP4 R4.x, c[12], v[OPOS];
DP4 R4.y, c[13], v[OPOS];
DP4 R4.z, c[14], v[OPOS];
DP4 R4.w, c[15], v[OPOS];

# transform R4 into light's post-perspective space and store result in R1:
# R1 = P_L * C_L * (R4) = P_L * C_L * W * v[OPOS]
DP4 R1.x, c[8], R4;
DP4 R1.y, c[9], R4;
DP4 R1.z, c[10], R4;
DP4 R1.w, c[11], R4;

# store this R1 in the first texture coordinate:
# o[TEX0] = P_L * C_L * W * v[OPOS]
MOV o[TEX0], R1;

# transform R4 into trapezoidal space:
# o[HPOS] = N_T * P_L * C_L * W * v[OPOS]
DP4 o[HPOS].x, c[0], R1;
DP4 o[HPOS].y, c[1], R1;
DP4 o[HPOS].z, c[2], R1;
DP4 o[HPOS].w, c[3], R1;

MOV o[COL0], v[COL0];

END

light's fragment program:

!!FP1.0

# f[WPOS] : fragment in trapezoidal space (window position)
# f[TEX0] : fragment's position in post-perspective space of the light

RCP R0.x, f[TEX0].w;        # R0.x = 1 / w_L
MUL R1, f[TEX0], R0.x;      # R1 = (x_L/w_L, y_L/w_L, z_L/w_L, 1)

MAD R2.z, R1.z, 0.5, 0.5;   # R2.z = R1.z * 0.5 + 0.5; depth is now in the range [0;1]

MOV o[DEPR], R2.z;          # replace "z_T" with "z_L"

MOV o[COLR], f[COL0];

END

A wrong impression could be, that TSM works only on hardware which supports "xxx"_fragment_program. By using those simple shaders we address the polygon offset problem which is worsened because of the projective nature of N_T. (We note that other approaches use 2D/3D perspective transformations as well to increase shadow map resolution (like PSM). Therefore, addressing the polygon offset problem in other approaches fragment/vertex programs might be necessary as well. However, in our case the problem can be addressed very easily.)

If graphics hardware does not support "xxx"_fragment_program another approach can be used to "address" the polygon offset problem. Instead of a fragment program one can use a vertex program to maintain the z-value of the vertex in the post-perspecive space of the light. This is achieved by output v_T = (x_T, y_T, (z_L*w_T)/w_L, w_T), i.e. v_T = (x_T/w_T, y_T/w_T, z_L/w_L, 1) after homogeneous division. Note, this is not a general solution because this leads to a wrong interpolation in the interior of the triangles: It is correct on the vertex level but not on the pixel level. However, if the scene is sufficiently tesselated, no visible artifacts occur.

Using these shaders and the trapezoidal transformation N_T as calculated above, the display routine could roughly look like:

void display() {
	// 1st path: TSM generation


	// as described in Section 6 in the TSM paper,
	// as a first step we have to calculate a trapezoid based on the
	// eye frustum E.
	// The four vertices are stored in t_0, t_1, t_2, t_3
	calculateTrapezoid(&t_0, &t_1, &t_2, &t_3, P_L, C_L, E);

	// ...after that N_T is calculated as described above
	calculateTrapezoidalTransformation(&N_T, t_0, t_1, t_2, t_3);

	glViewport(0, 0, shadowmapWidth, shadowmapHeight);

	// Bind the above vertex program...
	glBindProgramNV(GL_VERTEX_PROGRAM_NV, vpLight);

	// ...and bind the above fragment program
	glBindProgramNV(GL_FRAGMENT_PROGRAM_NV, fpLight);

	glMatrixMode(GL_PROJECTION); // store N_T in GL_PROJECTION
	glLoadMatrixf(N_T);
	glTrackMatrixNV(GL_VERTEX_PROGRAM_NV, 0, GL_PROJECTION, GL_IDENTITY_NV);

	glMatrixMode(GL_MATRIX1_NV); // store in P_L * C_L and track in GL_MATRIX1_NV
	glLoadMatrixf(P_L);          // light's projection matrix, e.g. achieved with gluPerspective()
	glMultMatrixf(C_L);          // light's camera matrix, e.g. achieved with gluLookAt()
	glTrackMatrixNV(GL_VERTEX_PROGRAM_NV, 8, GL_MATRIX1_NV, GL_IDENTITY_NV);

	glMatrixMode(GL_MODELVIEW);  // store the modelview matrix in GL_MODELVIEW
	glLoadIdentity();
	glTrackMatrixNV(GL_VERTEX_PROGRAM_NV, 12, GL_MODELVIEW, GL_IDENTITY_NV);

	renderScene();

	// Like in the standard shadow map approach we copy the depth buffer into a texture:
	CopyDepthBufferToTexture();
	...

	// 2nd path: render scene from eye's position and project TSM texture over the scene
	...
	SwapBuffers();
}

Tips and Tricks

• The basic algorithm maps the focus region to the 80% line. The paper also mentions an iteractive approach to search for a good percentage line in general. Such searching should not be expensive for current CPUs. On the other hand, as mentioned in the paper, one may pre-compute the proper percentage for each eye/light configuration and store it in a table (this works because it is scene independent). Alternatively, one may save some computation by stopping the searching (decreasing gradually from 80%) once a (local) maximal area is identified. In our test scenes (the fantasy and the urban scene), we did not pre-compute the proper percentage, but we adopt the mentioned stopping criterion and the performance of TSM is comparable to that of SSM.
• A non-quadratic texture (a texture with different height and width) could be used to reduce aliasing in z-direction.
• Figure 6 in the paper shows the curve over the area of the focus region in the shadow map. This area decreases when the eye and light approach the dueling frusta case. This is in general a difficult situation in shadow map approaches. The TSM-curve (Figure 6 in the paper) could be used to "adaptively" change the texture size gradually (to maintain continuity still), to get a bigger texture for dueling frusta cases and a smaller one for the others (support of non-power of two texture is needed).
• Use only a vertex program to address the polygon offset problem. However, this has some limitations (as discussed above and in the paper).

For further enquiries/suggestions please contact tobiasmartin@t-online.de or tants@comp.nus.edu.sg. Here is a part of the source code that may help in developing the above.

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