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# Color - PowerPoint PPT Presentation

Color. CS 445/645 Introduction to Computer Graphics David Luebke, Spring 2003. Admin. Drop deadline imminent!. Recap: Rasterizing Triangles. Edge walking: Use DDA approach to “walk” down edges of triangle At each scanline, walk the span from one edge to another Edge equations:

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### Color

CS 445/645Introduction to Computer Graphics

David Luebke, Spring 2003

David Luebke 18/20/2014

David Luebke 28/20/2014

• Edge walking:

• Use DDA approach to “walk” down edges of triangle

• At each scanline, walk the span from one edge to another

• Edge equations:

• Compute bounding box

• Edge equations: compute from vertices (numerical issues!)

• Orientation: ensure area is positive

David Luebke 38/20/2014

findBoundingBox(&xmin, &xmax, &ymin, &ymax);

setupEdges (&a0,&b0,&c0,&a1,&b1,&c1,&a2,&b2,&c2);

/* Optimize this: */

for (int y = yMin; y <= yMax; y++) {

for (int x = xMin; x <= xMax; x++) {

float e0 = a0*x + b0*y + c0;

float e1 = a1*x + b1*y + c1;

float e2 = a2*x + b2*y + c2;

if (e0 > 0 && e1 > 0 && e2 > 0) setPixel(x,y);

}

}

David Luebke 48/20/2014

• Some speed hacks for the inner loop:

int xflag = 0;

for (int x = xMin; x <= xMax; x++) {

if (e0|e1|e2 > 0) {

setPixel(x,y);

xflag++;

} else if (xflag != 0) break;

e0 += a0; e1 += a1; e2 += a2;

}

• Incremental update of edge equation values(think DDA)

• Early termination (why does this work?)

• Faster test of equation values

David Luebke 58/20/2014

Edge Equations: Interpolating Color

• Given colors (and later, other parameters) at the vertices, how to interpolate across?

• Idea: triangles are planar in any space:

• This is the “redness” parameter space

• Note:plane follows formz = Ax + By + C

• Look familiar?

David Luebke 68/20/2014

Edge Equations: Interpolating Color

• Given redness at the 3 vertices, set up the linear system of equations:

• The solution works out to:

David Luebke 78/20/2014

Edge Equations:Interpolating Color

• Notice that the columns in the matrix are exactly the coefficients of the edge equations!

• So the setup cost per parameter is basically a matrix multiply

• Per-pixel cost (the inner loop) cost equates to tracking another edge equation value (which is?)

David Luebke 88/20/2014

• Exactly which pixels should be lit?

• A: Those pixels inside the triangle edges

• What about pixels exactly on the edge?(Ex.)

• Draw them: order of triangles matters (it shouldn’t)

• Don’t draw them: gaps possible between triangles

• We need a consistent (if arbitrary) rule

• Example: draw pixels on left or top edge, but not on right or bottom edge

David Luebke 98/20/2014

• Now that we can rasterize triangles, what about general polygons?

• We’ll not take an edge-equations approach (why?)

David Luebke 108/20/2014

• Consider the following polygon:

• How do we know whether a given pixel on the scanline is inside or outside the polygon?

D

B

C

A

E

F

David Luebke 118/20/2014

• Does it still work?

D

B

H

C

A

G

I

E

F

David Luebke 128/20/2014

• Basic idea: use a parity test

for each scanline

edgeCnt = 0;

for each pixel on scanline (l to r)

if (oldpixel->newpixel crosses edge)

edgeCnt ++;

// draw the pixel if edgeCnt odd

if (edgeCnt % 2)

setPixel(pixel);

• Why does this work?

• What assumptions are we making?

David Luebke 138/20/2014

• How can we optimize the code?

for each scanline

edgeCnt = 0;

for each pixel on scanline (l to r)

if (oldpixel->newpixel crosses edge)

edgeCnt ++;

// draw the pixel if edgeCnt odd

if (edgeCnt % 2)

setPixel(pixel);

• Big cost: testing pixels against each edge

• Solution: active edge table (AET)

David Luebke 148/20/2014

• Idea:

• Edges intersecting a given scanline are likely to intersect the next scanline

• Within a scanline, the order of edge intersections doesn’t change much from scanline to scanline

David Luebke 158/20/2014

• Algorithm:

• Sort all edges by their minimum y coord

• Starting at bottom, add edges with Ymin= 0 to AET

• For each scanline:

• Sort edges in AET by x intersection

• Walk from left to right, setting pixels by parity rule

• Increment scanline

• Retire edges with Ymax < Y

• Add edges with Ymin > Y

• Recalculate edge intersections and resort (how?)

• Stop when Y > Ymax for last edges

David Luebke 168/20/2014

• Next topic: Color

To understand how to make realistic images, we need a basic understanding of the physics and physiology of vision. Here we step away from the code and math for a bit to talk about basic principles.

David Luebke 178/20/2014