Line Drawing. CMPT 361 Introduction to Computer Graphics Torsten Möller

Transcription

1 Line Drawing CMPT 361 Introduction to Computer Graphics Torsten Möller

2 Rendering Pipeline Hardware Modelling Transform Visibility Illumination + Shading Perception, Interaction Color Texture/ Realism

3 Reading Angel Chapter 6.8, 6.9 Foley et al - Chapter 3 3

4 The graphics pipeline From the modeling stage to image formation Pipelined approach helps increase system throughput Throughput: rate at which data flows through the system In a pipeline, data processing in subsequent blocks can be done in parallel Especially effective when the same sequence of operations are to be performed on large quantity of data true in graphics 4

5 Vertex processor Per-vertex operations (vertices define objects/ primitives) Two main functionalities: Coordinate transformations Color computation at each vertex (shading models) Objects/geometry first defined in their own coordinate systems, then transformed into a world space modeling transformation Then objects are transformed from world space into the camera coordinate system viewing transformation and projection transformation 5

6 Clipping and primitive assembly Model a finite field of vision Remove objects outside a finite clipping volume Need to be done on a primitive by primitive basis, not on vertices Output is a set of primitives whose projections can appear in the image 6

7 Rasterizer The rasterizer converts a primitive into a set of fragments A fragment stores per pixel information for its associated primitive, later used to determine whether the fragment contributes to pixel color and to compute the pixel color Raster/pixel location (in the frame buffer) Depth, e.g., to determine whether this fragment survives Interpolated attributes, e.g., color and texture coordinate, from the previous stages Alpha value (for blending) Window ID, etc. 7

8 Fragment processing Performs per-fragment operations Main function is to compute the color at a pixel, using information stored in the fragments, e.g., depth, alpha, texture coordinates; can also add in fog and other effects A (programmable) fragment shader is a program that performs the processing which replaces the OpenGL fixed functionality, e.g., simply using interpolated attributes Similar to a vertex shader (per-vertex operations) Shader programs typically have limited instruction set 8

9 Where are we at now? Drawing or rasterization of primitives first. Consider lines and polygons.

10 Assumptions Transformation, clipping, projection already done Primitives to rasterize are actually on the screen Work with 2D screen coordinates with square pixels (N, M) (0, 0) 10

11 Primitives Polygons explicit curves lines, quadricts parametric curves/surfaces Curves, Surfaces implicit description Misc particle systems/points, fractals 11

12 Explicit Descriptions Given - an explicit equation y = f(x) cannot get multiple values of y not rotationally invariant no infinite slope (vertical line) axis dependent (y depends on x) specify one variable and resolve for the other 12

13 Lines Given - an explicit line equation y = mx+n Which pixels to set? 13

14 Scan Converting Lines Compute the coordinates of pixels that lies on or near an ideal, infinitely thin line imposed on a 2D raster grid Assumptions line will be 1 pixel thick and will approximate an infinitely fine line pixels represented as disjoint circles, centred on a grid (unless specified otherwise) integer coordinates of endpoints pixel on or off (2 states) 14

15 Scan Converting Lines (2) Desirable properties: constant brightness (irrespective of length or orientation) 1 pixel per column (-1 <= slope <= 1), 1 pixel per row (slope > +/- 1) as close to the ideal/as straight as possible allow control of pen, line and endpoint styles drawn as smoothly as possible (anti-aliasing) drawn as rapidly as possible 15

16 Scan Converting Lines (3) Difficulties: brightness and pixel criteria conflict (for bilevel displays) 16

17 Basic Brute force Find slope m = rise/run or y = mx + b increment x by 1 (xi); calculate y i = mx i + b pixel (x i, round(y i )) turned on simple, but inefficient: floating point multiplication addition round 17

18 Incremental - DDA multiplication can be eliminated: called digital differential analyzer (DDA) - after mechanical device that solves differential equations by numerical methods Drawbacks: floating point values (m,y) round operation y i+1 = mx i+1 + b = m(x i + 1) + b = y i + m special cases could be done more quickly 18

19 Lines - Bresenham Special case of Midpoint Line Algorithm uses only integer arithmetic & no rounding idea is to provide the best-fit approximation to a true line by minimizing the error (distance) to the true line slope (rest is done with reflection) 19

20 Lines - Bresenham (2) Explicit line equation: y = mx + b = y x x + b Implicit line equation: F (x, y) =x y y x + h Main idea - call on decision variable: d i = F (x i +1,y i +1/2) =(x i + 1) y (y i +1/2) x + h 20

21 Lines - Bresenham (3) if d i > 0, pick O, and: O = d i+1 d i = F (x i +2,y i +1/2) = y if d i < 0, pick NO, and: NO = d i+1 d i = F (x i +2,y i +3/2) = y x if d i = 0, pick either one, but consistently 21

22 Lines - Bresenham (4) Additional issues: Endpoint order: we want P 0 to P 1 to look exactly the same as P1 to P0 Starting at edge of a clip rectangle: we must use the midpoint test result rather than computing intersections analytically if we are to ensure the correct sequence of pixels 22

23 Lines - Bresenham (4) 23

24 Quadrics - Circles Circles, ellipsoids, etc. divide in 8 quadrants What is special about 8-fold symmetry? y = p R 2 x 2 24

25 Quadrics - Circles (2) Midpoint Circle Algorithm trace out arc in 2nd octant and draw the rest by symmetry choose between E and SE pixel at each stage 25

26 Quadrics - Circles (3) incremental - now have higher order D(x) =(x + x) 2 x 2 =2x x + x 2 higher order difference system D(x + x) D(x) = 2 x 2 26

27 Quadrics - Ellipses Can a midpoint algorithm be developed for ellipses? What are the important differences from circles? The result is Da Silva's algorithm (the idea is illustrated below): 27

28 Lines - Anti-Aliasing A continuous (zero-width) line is invisible in a discrete grid. Give line thickness - How? Signal Processing Approach: line is high-frequency want to avoid aliasing what to do first? 28

29 Lines - Anti-Aliasing (2) Bandlimit signal - how? Convolve with smoothing filter - which? 29

30 Lines - Anti-Aliasing (3) Box filter not all that good convolve it again - why? Then resample! 30

31 Lines - Anti-Aliasing (4) Other alternatives? Can we improve this algorithm? 31