UNC Pixel Planesdesigned by Prof. Henry Fuchs et al

Pixel Planes

Henry Fuchs’ Idea: build processing into the frame buffer, a processor per pixel.–UNC designs are called enhanced memories, not SIMD processors Enabler was linear expression tree…

Representation of a triangle• Oriented Triangles, three vertices A, B, C• A triangle is defined by Lines AB, BC, and CA• For example, let A = (0, 0), B=(10, 0), C = (2, 5) Line AB = (0,0) + (10, 0)*t Let X = 10t, Y = 0, so Line AB => Y =0; Line BC = (10, 0) + (-8, 5)*t = (10-8t, 5t) Let X = 10 -8t, Y = 5t, so, line BC => -5X-8Y + 50=0Similarly Line CA = (2, 5) + (-2, -5)*t So, Line CA => 5X - 2Y = 0Distance from point (U,V) to Line CA is ?

Point to line distance (2D)

• AX + BY +C = 0 ----line K• Point (U, V)• What is the distance from (U,V ) to line K?• Answer: (AU + BV +C)/M M = sqrt (A*A + B*B)

Distance with a sign: for a point within a triangle, all minus, or all positive signs for its distance, assuming each line with a normal vector.

Shading with fast evaluation of AX + BY + C = 0

• Flat shading: constant color, A = B = 0, C = Color• Gouraud shading (smooth shading): color

interpolation, for example, Triangle with three vertices (x1, y1), (x2, y2), (x3, y3), each with red components R1, R2, R3 color is represented as (Red, Green , Blue) Assuming a plane (in 3D) with vertices (x1, y1,

R1), (X2, Y2, R2), and (X3, y3, R3)

Gouraud shading

• Vector equation of the plane is (x,y) = s (x2-x1, y2 – y1) + t(x3-x1, y3-y1) + (x1, y1) solved for (s, t) , then s = A1x + B1y +C1, t = A2x + B2y +C2 So, given point (x,y) in this plane, what is its color?Answer: color = Ax + By +C, whereA = A1 (R2-R1)+ A2 (R3-R1)B = B1(R2-R1) + B2 (R3-R1)C = C1(R2-R1) + C2(R3-R1) + R1So, we then broadcast A, B, C into this Pixel-Planes, and the red

components for each pixel is done!

How is the color calculated?

Since, (x,y) = s (x2-x1, y2 – y1) + t(x3-x1, y3-y1) + (x1, y1)

Therefore, (x,y, R) = s (x2-x1, y2 – y1, R2-R1) + t(x3-x1, y3-

y1, R3-R1) + (x1, y1, R1) or, color R = s (R2-R1) + t (R3-R1) + R1

• Three vertices: (0,0) (2,4), and (6,2) with red colors 10, 50, 20 Given point (2,2) inside the triangle, the color

is 28! (since s = 0.4, t = 0.2)• Goven point (x, y), the color is ? Represented as Ax + By +C, what are the values

of (A, B, C)? Hint:A = -2, B= 11 and C= 10

Implementation

Shading

Hardware design

Communications & Multimedia Lab 23

Pixel-Planes

Communications & Multimedia Lab 24

Bit operation

Binary representation: for easy hardware architecture/realizationRight most significant bit, and shift left at each clock!

Eg.: 011 equals 6 in Decimal01100 equals 60101 equals 1001111000 equals 30 in Decimal (all zeros at the right ends: redundant, for pipeline operations)

011 equals 6 in Decimal After left shift one bit, it is 110 (one 0 added to the right), the value is 3!

01100 equals 6, left shift one bit, becomes 11000, value is 3

0101 equals 10, left shift one bit, becomes 1010, value is 5

01111000 equals 30 in Decimal, left shift one bit, becomes 11110000, value is 15.

One bit adder

For example: 01100 = A 01010= C A+ C = 00001 Carry = 0 Value of A is 6, value of B is 10, A + C = 16

Left shift one bit (one time per clock trigger)