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LibGfx: Avoid fill_path() crashes due to rounding errors

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It seems for very narrow tall paths it is possible for the dxdy value
to round to a value (that after many repeated additions) overshoots
the desired end x. This caused a (rather rare) crash on this 3D canvas
demo: https://www.kevs3d.co.uk/dev/html5logo/. For now, lets just avoid
a crash here. This does not make any noticeable visual differences on
the demos I tired.
This commit is contained in:
MacDue 2023-06-12 20:14:05 +01:00 committed by Andreas Kling
parent 147c3b3d97
commit 976d9b32d6
Notes: sideshowbarker 2024-07-17 08:37:36 +09:00 
Author: https://github.com/MacDue
Commit: https://github.com/SerenityOS/serenity/commit/976d9b32d6
Pull-request: https://github.com/SerenityOS/serenity/pull/19369
Reviewed-by: https://github.com/Hendiadyoin1
1 changed files with 36 additions and 19 deletions
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55
Userland/Libraries/LibGfx/EdgeFlagPathRasterizer.cpp
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@ -5,6 +5,7 @@
*/
#include <AK/Array.h>
#include <AK/Debug.h>
#include <AK/IntegralMath.h>
#include <AK/Types.h>
#include <LibGfx/AntiAliasingPainter.h>
@ -51,14 +52,19 @@ static Vector<Detail::Edge> prepare_edges(ReadonlySpan<FloatLine> lines, unsigne
if (p0.y() == p1.y())
continue;
auto dx = p1.x() - p0.x();
auto dy = p1.y() - p0.y();
float dxdy = float(dx) / dy;
float x = p0.x();
auto min_y = static_cast<int>(p0.y());
auto max_y = static_cast<int>(p1.y());
float start_x = p0.x();
float end_x = p1.x();
auto dx = end_x - start_x;
auto dy = max_y - min_y;
auto dxdy = dx / dy;
edges.unchecked_append(Detail::Edge {
x,
static_cast<int>(p0.y()),
static_cast<int>(p1.y()),
start_x,
min_y,
max_y,
dxdy,
winding,
nullptr });
@ -132,18 +138,32 @@ void EdgeFlagPathRasterizer<SamplesPerPixel>::fill_internal(Painter& painter, Pa
max_scanline = max(max_scanline, end_scanline);
}
Detail::Edge* active_edges = nullptr;
// FIXME: We could probably clip some of the egde plotting if we know it won't be shown.
// Though care would have to be taken to ensure the active edges are correct at the first drawn scaline.
auto for_each_sample = [&](Detail::Edge& edge, int start_subpixel_y, int end_subpixel_y, auto callback) {
for (int y = start_subpixel_y; y < end_subpixel_y; y++) {
int xi = static_cast<int>(edge.x + SubpixelSample::nrooks_subpixel_offsets[y]);
if (xi < 0 || xi >= (int)m_scanline.size()) {
// FIXME: For very low dxdy values, floating point error can push the sample outside the scanline.
// This does not seem to make a visible difference most of the time (and is more likely from generated
// paths, such as this 3D canvas demo: https://www.kevs3d.co.uk/dev/html5logo/).
dbgln_if(FILL_PATH_DEBUG, "fill_path: Sample out of bounds: {} not in [0, {})", xi, m_scanline.size());
return;
}
SampleType sample = 1 << y;
callback(xi, y, sample);
edge.x += edge.dxdy;
}
};
Detail::Edge* active_edges = nullptr;
if (winding_rule == Painter::WindingRule::EvenOdd) {
auto plot_edge = [&](Detail::Edge& edge, int start_subpixel_y, int end_subpixel_y) {
for (int y = start_subpixel_y; y < end_subpixel_y; y++) {
int xi = static_cast<int>(edge.x + SubpixelSample::nrooks_subpixel_offsets[y]);
SampleType sample = 1 << y;
for_each_sample(edge, start_subpixel_y, end_subpixel_y, [&](int xi, int, SampleType sample) {
m_scanline[xi] ^= sample;
edge.x += edge.dxdy;
}
});
};
for (int scanline = min_scanline; scanline <= max_scanline; scanline++) {
active_edges = plot_edges_for_scanline(scanline, plot_edge, active_edges);
@ -157,13 +177,10 @@ void EdgeFlagPathRasterizer<SamplesPerPixel>::fill_internal(Painter& painter, Pa
m_windings.resize(m_size.width());
auto plot_edge = [&](Detail::Edge& edge, int start_subpixel_y, int end_subpixel_y) {
for (int y = start_subpixel_y; y < end_subpixel_y; y++) {
int xi = static_cast<int>(edge.x + SubpixelSample::nrooks_subpixel_offsets[y]);
SampleType sample = 1 << y;
for_each_sample(edge, start_subpixel_y, end_subpixel_y, [&](int xi, int y, SampleType sample) {
m_scanline[xi] |= sample;
m_windings[xi].counts[y] += edge.winding;
edge.x += edge.dxdy;
}
});
};
for (int scanline = min_scanline; scanline <= max_scanline; scanline++) {
active_edges = plot_edges_for_scanline(scanline, plot_edge, active_edges);