pathfinder/resources/shaders/draw.cl

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// Copyright 2017 The Servo Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
// Computes exact area coverage for lines, breaking Bézier curves down into them as necessary.
// Proceeds top to bottom for better data locality during the subsequent accumulation stage. For
// details on the algorithm, see [1].
//
// [1]: http://nothings.org/gamedev/rasterize/
#define POINTS_PER_SEGMENT 32
#define TILE_SIZE 4
#define OPERATION_MOVE 0
#define OPERATION_ON_CURVE 1
#define OPERATION_OFF_CURVE 2
struct GlyphDescriptor {
short4 rect;
ushort unitsPerEm;
ushort pointCount;
uint startPoint;
};
typedef struct GlyphDescriptor GlyphDescriptor;
struct ImageDescriptor {
uint2 atlasPosition;
float pointSize;
uint glyphIndex;
uint startPointInBatch;
uint pointCount;
};
typedef struct ImageDescriptor ImageDescriptor;
__global int *getPixel(__global int *gPixels, uint2 point, uint widthInTiles) {
uint2 tile = point / TILE_SIZE, pointInTile = point % TILE_SIZE;
return &gPixels[(tile.y * widthInTiles + tile.x) * TILE_SIZE * TILE_SIZE +
pointInTile.y * TILE_SIZE +
pointInTile.x];
}
uchar getOperation(uint globalPointIndex, __global const uchar *gOperations) {
return (gOperations[globalPointIndex / 4] >> (globalPointIndex % 4 * 2)) & 0x3;
}
void plot(__global int *gPixels,
uint2 point,
uint widthInTiles,
uint imageHeight,
float coverage) {
__global int *pixel = getPixel(gPixels,
(uint2)(point.x, imageHeight - point.y - 1),
widthInTiles);
int oldCoverage = as_int(*pixel);
while (true) {
int newCoverage = as_int(as_float(oldCoverage) + coverage);
int existingCoverage = atomic_cmpxchg(pixel, oldCoverage, newCoverage);
if (existingCoverage == oldCoverage)
break;
oldCoverage = existingCoverage;
}
}
__kernel void draw(__global const ImageDescriptor *gImages,
__global const GlyphDescriptor *gGlyphs,
__global const short2 *gCoordinates,
__global const uchar *gOperations,
__global const uint *gIndices,
__global int *gPixels,
uint atlasWidth) {
// Find the image.
int batchID = get_global_id(0);
uint imageID = gIndices[batchID / POINTS_PER_SEGMENT];
__global const ImageDescriptor *image = &gImages[imageID];
while (batchID >= image->startPointInBatch + image->pointCount) {
imageID++;
image = &gImages[imageID];
}
// Find the glyph.
uint glyphIndex = image->glyphIndex;
__global const GlyphDescriptor *glyph = &gGlyphs[glyphIndex];
// Unpack glyph and image.
uint2 atlasPosition = image->atlasPosition;
float pixelsPerUnit = image->pointSize * convert_float(glyph->unitsPerEm);
uint pointIndexInGlyph = batchID - image->startPointInBatch;
uint globalPointIndex = glyph->startPoint + pointIndexInGlyph;
// Stop here if this is a move operation.
uchar curOperation = getOperation(globalPointIndex, gOperations);
if (curOperation == OPERATION_MOVE)
return;
// Unpack the points that make up this line or curve.
short2 p0, p1, p2;
float t0, t1;
uchar prevOperation = getOperation(globalPointIndex - 1, gOperations);
short2 prevPoint = gCoordinates[globalPointIndex - 1];
short2 curPoint = gCoordinates[globalPointIndex];
if (prevOperation == OPERATION_OFF_CURVE) {
p0 = gCoordinates[globalPointIndex - 2];
p1 = prevPoint;
p2 = curPoint;
t0 = 0.0f;
t1 = 0.5f;
} else if (curOperation == OPERATION_OFF_CURVE) {
p0 = prevPoint;
p1 = curPoint;
p2 = gCoordinates[globalPointIndex + 1];
t0 = 0.5f;
t1 = 1.0f;
} else {
p0 = prevPoint;
p2 = curPoint;
}
// Convert units to pixels.
float2 pP0 = convert_float2(p0) * pixelsPerUnit;
float2 pP1 = convert_float2(p1) * pixelsPerUnit;
float2 pP2 = convert_float2(p2) * pixelsPerUnit;
// Determine the direction we're going.
float2 direction = copysign((float2)(1.0f, 1.0f), pP0 - pP2);
// Set up plotting.
uint widthInTiles = atlasWidth / TILE_SIZE;
short4 glyphRect = glyph->rect;
uint imageHeight = convert_uint(glyphRect.w - glyphRect.y);
// Loop over each line segment.
float t = t0;
while (t < t1) {
// Compute endpoints.
float2 lP0, lP1;
if (direction.x >= 0.0f) {
lP0 = pP0;
lP1 = pP2;
} else {
lP0 = pP2;
lP1 = pP0;
}
// Compute the slope.
float dXdY = fabs(lP1.x - lP0.x / lP1.y - lP0.y);
// Initialize the current point. Determine how long the segment extends across the first
// pixel column.
int2 p = (int2)((int)p0.x, 0);
float dX = min(convert_float(p.x) + 1.0f, lP1.x) - lP0.x;
// Initialize `yLeft` and `yRight`, the intercepts of Y with the current pixel.
float yLeft = lP0.y;
float yRight = yLeft + direction.y * dX / dXdY;
// Iterate over columns.
while (p.x < (int)ceil(lP1.x)) {
// Flip `yLeft` and `yRight` around if necessary so that the slope is positive.
float y0, y1;
if (yLeft <= yRight) {
y0 = yLeft;
y1 = yRight;
} else {
y0 = yRight;
y1 = yLeft;
}
// Split `y0` into fractional and whole parts, and split `y1` into remaining fractional
// and whole parts.
float y0R, y1R;
float y0F = fract(y0, &y0R), y1F = fract(y1, &y1R);
int y0I = convert_int(y0R), y1I = convert_int(y1R);
if (y1F != 0.0f)
y1I++;
// Compute area coverage for the first pixel.
float coverage;
if (y1I <= y0I + 1) {
// The line is less than one pixel. This is a trapezoid.
coverage = 1.0f - mix(y0F, y1F, 0.5f);
} else {
// Usual case: This is a triangle.
coverage = 0.5f * dXdY * (1.0f - y0F) * (1.0f - y0F);
}
// Plot the first pixel of this column.
plot(gPixels, as_uint2(p), widthInTiles, imageHeight, dX * direction.x * coverage);
// Since the coverage of this row must sum to 1, we keep track of the total coverage.
float coverageLeft = coverage;
// Plot the pixels between the first and the last.
if (p.y + 1 < y1I) {
// Compute coverage for and plot the second pixel in the column.
p.y++;
if (p.y + 1 == y1I)
coverage = 1.0f - (0.5f * dXdY * y1F * y1F) - coverage;
else
coverage = dXdY * (1.5f - y0F) - coverage;
coverageLeft += coverage;
plot(gPixels, as_uint2(p), widthInTiles, imageHeight, dX * direction.x * coverage);
// Iterate over any remaining pixels.
p.y++;
coverage = dXdY;
while (p.y < y1I) {
coverageLeft += coverage;
plot(gPixels,
as_uint2(p),
widthInTiles,
imageHeight,
dX * direction.x * coverage);
p.y++;
}
}
// Plot the remaining coverage.
coverage = 1.0f - coverageLeft;
plot(gPixels, as_uint2(p), widthInTiles, imageHeight, dX * direction.x * coverage);
// Move to the next column.
p.x++;
// Compute Y intercepts for the next column.
yLeft = yRight;
float yRight = yLeft + direction.y * dX / dXdY;
// Determine how long the segment extends across the next pixel column.
dX = min(convert_float(p.x) + 1.0f, lP1.x) - convert_float(p.x);
}
}
}