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Draft shader code for the new tessellation-based algorithm

This commit is contained in:
Patrick Walton 2017-01-23 15:33:12 -08:00
parent bdf6ebab24
commit f1ec3385de
6 changed files with 358 additions and 268 deletions
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41
resources/shaders/accum.cl
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@ -12,39 +12,26 @@
//
// This proceeds top to bottom for better data locality. For details on the algorithm, see [1].
//
// [1]: http://nothings.org/gamedev/rasterize/
// [1]: https://medium.com/@raphlinus/inside-the-fastest-font-renderer-in-the-world-75ae5270c445
#define TILE_SIZE 4
const sampler_t SAMPLER = CLK_NORMALIZED_COORDS_FALSE | CLK_ADDRESS_NONE | CLK_FILTER_NEAREST;
__global const int *getPixel(__global const 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];
}
__kernel void accum(__global const int *gPixels,
__write_only image2d_t gTexture,
uint4 kAtlasRect,
__kernel void accum(__write_only image2d_t gTexture,
__read_only image2d_t gCoverage,
uint kAtlasWidth,
uint kAtlasShelfHeight) {
// Compute our column.
int globalID = get_global_id(0);
uint atlasWidth = kAtlasRect.z - kAtlasRect.x;
uint x = globalID % atlasWidth;
uint widthInTiles = atlasWidth / TILE_SIZE;
// Compute the row range we'll traverse.
uint shelf = globalID % atlasWidth;
uint yStart = shelf * kAtlasShelfHeight;
uint yEnd = yStart + kAtlasShelfHeight;
// Determine the boundaries of the column we'll be traversing.
uint column = get_global_id(0) % kAtlasWidth, shelfIndex = get_global_id(0) / kAtlasWidth;
uint firstRow = shelfIndex * kAtlasShelfHeight, lastRow = (shelfIndex + 1) * kAtlasShelfHeight;
// Sweep down the column, accumulating coverage as we go.
float coverage = 0;
for (uint y = yStart; y < yEnd; y++) {
coverage += as_float(*getPixel(gPixels, (uint2)(x, y), widthInTiles));
float coverage = 0.0f;
for (uint row = firstRow; row < lastRow; row++) {
int2 coord = (int2)((int)column, (int)row);
coverage += read_imagef(gCoverage, SAMPLER, coord).r;
uint grayscaleValue = 255 - convert_uint(clamp(coverage, 0.0f, 255.0f));
write_imageui(gTexture, (int2)((int)x, (int)y), (uint4)(grayscaleValue, 255, 255, 255));
uint gray = 255 - convert_uint(clamp(coverage, 0.0f, 1.0f) * 255.0f);
write_imageui(gTexture, coord, (uint4)(gray, 255, 255, 255));
}
}

241
resources/shaders/draw.cl
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@ -1,241 +0,0 @@
// 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);
}
}
}

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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.
#version 410
// The size of the atlas in pixels.
uniform uvec2 uAtlasSize;
// The starting point of the segment.
flat in vec2 vP0;
// The endpoint of this segment.
flat in vec2 vP1;
// 1.0 if this segment runs left to right; -1.0 otherwise.
flat in float vDirection;
// The slope of this line.
flat in float vSlope;
// Minimum and maximum vertical extents, unrounded.
flat in vec2 vYMinMax;
out vec4 oFragColor;
void main() {
// Compute the X boundaries of this pixel.
float xMin = floor(gl_FragCoord.x), xMax = ceil(gl_FragCoord.x);
// Compute the horizontal span that the line segment covers across this pixel.
float dX = min(xMax, vP1.x) - max(xMin, vP0.x);
// Compute the Y-intercepts of the portion of the line crossing this pixel.
float yMin = clamp(vP0.y + (xMin - vP0.x) * vSlope, vYMinMax.x, vYMinMax.y);
float yMax = clamp(yMin + vSlope, vYMinMax.x, vYMinMax.y);
if (yMin > yMax) {
float tmp = yMin;
yMin = yMax;
yMax = tmp;
}
// Round the Y-intercepts out to the nearest pixel.
int yMinI = int(floor(yMin)), yMaxI = int(ceil(yMax));
// Determine which vertical pixel we're looking at.
int yI = int(floor(gl_FragCoord.y));
// Compute trapezoidal area coverage.
//
// It may be helpful to follow along with this explanation, keeping in mind that we compute
// downward coverage rather than rightward coverage:
//
// http://nothings.org/gamedev/rasterize/
//
// Note that the algorithm above computes total area coverage for each pixel, while here we
// compute *delta* coverage: that is, the *difference* in the area covered between this pixel
// and the pixel above it. In general, this means that, in contrast to the stb_truetype
// algorithm, we have to specially handle the first fully covered pixel, in order to account
// for the residual area difference between that pixel and the one above it.
float coverage = 0.0f;
if (yMaxI <= yMinI + 1) {
// The line touches only one pixel (case 1). Compute the area of that trapezoid (or the
// residual area for the pixel right after that trapezoid).
float trapArea = 0.5f * (yMin + yMax) - float(yMinI);
if (yI == yMinI)
coverage = 1.0f - trapArea;
else if (yI == yMinI + 1)
coverage = trapArea;
} else {
// The line touches multiple pixels (case 2). There are several subcases to handle here.
// Compute the area of the topmost triangle.
float yMinF = fract(yMin);
float dXdY = 1.0f / (yMax - yMin);
float triAreaMin = 0.5f * dXdY * (1.0f - yMinF) * (1.0f - yMinF);
if (yI == yMinI) {
// We're looking at the pixel that triangle covers, so we're done.
coverage = triAreaMin;
} else {
// Compute the area of the bottommost triangle.
float yMaxF = yMax - ceil(yMax) + 1.0f;
float triAreaMax = 0.5f * dXdY * yMaxF * yMaxF;
bool lineTouchesThreePixels = yMaxI == yMinI + 2;
if (lineTouchesThreePixels && yI == yMinI + 1) {
// The line touches exactly 3 pixels, and we're looking at the middle one.
coverage = 1.0f - triAreaMin - triAreaMax;
} else if (!lineTouchesThreePixels && yI < yMaxI) {
// The line touches more than 3 pixels. Compute the area of the first trapezoid.
float trapAreaMin = dXdY * (1.5f - yMinF);
if (yI == yMinI + 1) {
// We're looking at that trapezoid, so we're done.
coverage = trapAreaMin - triAreaMin;
} else if (yI == yMaxI - 1) {
// We're looking at the last trapezoid. Compute its area.
float trapAreaMax = trapAreaMin + float(yMaxI - yMinI - 3) * dXdY;
coverage = 1.0f - trapAreaMax - triAreaMax;
} else if (yI > yMinI + 1 && yI < yMaxI - 1) {
// We're looking at one of the pixels in between the two trapezoids. The delta
// coverage in this case is simply the inverse slope.
coverage = dXdY;
}
} else if (yI == yMaxI) {
// We're looking at the final pixel in the column.
coverage = triAreaMax;
}
}
}
oFragColor = vec4(dX * vDirection * coverage, 1.0f, 1.0f, 1.0f);
}

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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.
#version 410
#define OPERATION_LINE 1
#define OPERATION_QUAD_CURVE 2
#define CURVE_THRESHOLD 0.333f
#define CURVE_TOLERANCE 3.0f
layout(vertices = 1) out;
// The size of the atlas in pixels.
uniform uvec2 uAtlasSize;
// The vertex ID, passed into this shader.
flat in uint vVertexID[];
// The starting point of the segment.
patch out vec2 vpP0;
// The control point, if this is a curve. If this is a line, this value must be ignored.
patch out vec2 vpP1;
// The endpoint of this segment.
patch out vec2 vpP2;
// 1.0 if this segment runs left to right; -1.0 otherwise.
patch out float vpDirection;
void main() {
vpP0 = gl_in[0].gl_Position.xy;
vpP1 = gl_in[1].gl_Position.xy;
vpP2 = gl_in[2].gl_Position.xy;
// Compute direction. Flip around if necessary so that p0 is to the left of p2.
if (vpP0.x < vpP2.x) {
vpDirection = 1.0f;
} else {
vpDirection = -1.0f;
vec2 tmp = vpP0;
vpP0 = vpP2;
vpP2 = tmp;
}
// Divide into lines.
float lineCount = 1.0f;
if (vVertexID[1] > 0) {
// Quadratic curve.
vec2 dev = vpP0 - 2.0f * vpP1 + vpP2;
float devSq = dot(dev, dev);
if (devSq >= QUAD_CURVE_THRESHOLD) {
// Inverse square root is likely no slower and may be faster than regular square root
// (e.g. on x86).
lineCount += floor(inversesqrt(inversesqrt(QUAD_CURVE_TOLERANCE * devSq)));
}
}
// Tessellate into lines. This is subtle, so a diagram may help.
//
// Suppose we decided to divide this curve up into 4 lines. Then our abstract tessellated patch
// space will look like this:
//
// x₀ x₁ x₂ x₃ x₄ x₅ x₆ x₇
// ┌──┬──┬──┬──┬──┬──┬──┐
// │▒▒│ │▒▒│ │▒▒│ │▒▒│
// │▒▒│ │▒▒│ │▒▒│ │▒▒│
// └──┴──┴──┴──┴──┴──┴──┘
//
// The shaded areas are the only areas that will actually be drawn. They might look like this:
//
// x₅
// x₆ x₇
// x₃ ┌───────┐
// x₄ │▒▒▒▒▒▒▒│
// x₁ ┌─────┼───────┘
// x₂ │▒▒▒▒▒│
// ┌──┼─────┘
// │▒▒│
// │▒▒│
// x₀ │▒▒│
// ┌──┼──┘
// │▒▒│
// │▒▒│
// └──┘
//
// In this way, the unshaded areas become zero-size and are discarded by the rasterizer.
//
// Note that, in reality, it will often be the case that the quads overlap vertically by one
// pixel in the horizontal direction. In fact, this will occur whenever a line segment endpoint
// does not precisely straddle a pixel boundary. However, observe that we can guarantee that
// x₂ ≤ x₁, x₄ ≤ x₃, and so on, because there is never any horizontal space between endpoints.
// This means that all triangles inside the unshaded areas are guaranteed to be wound in the
// opposite direction from those inside the shaded areas. Because the OpenGL spec guarantees
// that, by default, all tessellated triangles are wound counterclockwise in abstract patch
// space, the triangles within the unshaded areas must be wound clockwise and are therefore
// candidates for backface culling. Backface culling is always enabled when running Pathfinder,
// so we're in the clear: the rasterizer will always discard the unshaded areas and render only
// the shaded ones.
gl_TessLevelInner[0] = vpP0.x == vpP2.x ? 0.0f : lineCount * 2.0f - 1.0f;
gl_TessLevelOuter[1] = gl_TessLevelOuter[3] = gl_TessLevelInner[0];
// Don't split vertically at all. We only tessellate horizontally.
gl_TessLevelInner[1] = gl_TessLevelOuter[0] = gl_TessLevelOuter[2] = 1.0f;
}

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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.
#version 410
layout(quads) in;
// The size of the atlas in pixels.
uniform uvec2 uAtlasSize;
// The starting point of the segment.
patch in vec2 vpP0;
// The control point, if this is a curve. If this is a line, this value must be ignored.
patch in vec2 vpP1;
// The endpoint of this segment.
patch in vec2 vpP2;
// 1.0 if this segment runs left to right; -1.0 otherwise.
patch in float vpDirection;
// The starting point of the segment.
flat out vec2 vP0;
// The endpoint of this segment.
flat out vec2 vP1;
// 1.0 if this segment runs left to right; -1.0 otherwise.
flat out float vDirection;
// The slope of this line.
flat out float vSlope;
// Minimum and maximum vertical extents, unrounded.
flat out vec2 vYMinMax;
void main() {
// Work out how many lines made up this segment, which line we're working on, and which
// endpoint of that line we're looking at.
uint tessPointCount = uint(gl_TessLevelInner[0] + 1.0f);
uint tessIndex = uint(round(gl_TessCoord.x * float(tessPointCount - 1)))
uint lineCount = tessPointCount / 2, lineIndex = tessIndex / 2, endpoint = tessIndex % 2;
// Compute our endpoints (trivial if this is part of a line, less trivial if this is part of a
// curve).
if (lineCount == 1) {
vP0 = vpP0;
vP1 = vpP2;
} else {
float t0 = float(lineIndex + 0) / float(lineCount);
float t1 = float(lineIndex + 1) / float(lineCount);
vP0 = mix(mix(vpP0, vpP1, t0), mix(vpP1, vpP2, t0), t0);
vP1 = mix(mix(vpP0, vpP1, t1), mix(vpP1, vpP2, t0), t1);
}
// Compute Y extents.
vYMinMax = vP0.y <= vP1.y ? vec2(vP0.y, vP1.y) : vec2(vP1.y, vP0.y);
// Compute our final position in atlas space, rounded out to the next pixel.
float x = pointIndex == 0 ? floor(vP0.x) : ceil(vP1.x);
float y = gl_TessCoord.y == 0.0f ? floor(vYMinMax.x) : ceil(vYMinMax.y) + 1.0f;
// Convert atlas space to device space.
gl_Position = vec4(vec2(x, y) / vec2(uAtlasSize) * 2.0f - 1.0f, 0.0f, 1.0f);
}

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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.
#version 410
// Information about the position of each glyph in the atlas.
layout(std140) struct ImageInfo {
// The left/top/right/bottom positions of the glyph in the atlas.
uvec4 atlasRect;
// The left/top/right/bottom offsets of the glyph from point (0, 0) in glyph space.
ivec4 extents;
// The font size in pixels.
float pointSize;
}
// The size of the atlas in pixels.
uniform uvec2 uAtlasSize;
layout(std140) uniform ubImageInfo {
ImageInfo uImageInfo[256];
};
// The position of each vertex in glyph space.
in ivec2 aPosition;
// Which image the vertex belongs to.
//
// TODO(pcwalton): See if this is faster as a binary search on the vertex ID.
in uint aImageIndex;
// The vertex ID, passed along onto the TCS.
flat out uint vVertexID;
void main() {
vVertexID = gl_VertexID;
ImageInfo imageInfo = uImageInfo[aImageIndex];
vec2 glyphPos = vec2(aPosition.x - imageInfo.extents.x, imageInfo.extents.w - aPosition.y);
vec2 atlasPos = glyphPos * EMS_PER_UNIT * pointSize + vec2(imageInfo.atlasRect.xy);
gl_Position = vec4(atlasPos, 0.0f, 1.0f);
}