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LibGfx: Fix draw_triangle() precision issues

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This fixes some precision issues in Painter::draw_triangle()
that caused the Cube demo to leave pixels empty between triangles
under certain angles.

Also adds some extra comments for clarity and early returns when
parts of the triangle do not need to be drawn.
This commit is contained in:
Stephan Unverwerth 2021-03-05 23:29:58 +01:00 • committed by Andreas Kling
parent 25e4a3467d
commit 87d19273bc
1 changed files with 32 additions and 22 deletions
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54
Userland/Libraries/LibGfx/Painter.cpp
View file

@ -456,12 +456,11 @@ void Painter::draw_triangle(const IntPoint& a, const IntPoint& b, const IntPoint
{
VERIFY(scale() == 1); // FIXME: Add scaling support.
RGBA32 rgba = color.value();
IntPoint p0(a);
IntPoint p1(b);
IntPoint p2(c);
// sort points from top to bottom
if (p0.y() > p1.y())
swap(p0, p1);
if (p0.y() > p2.y())
@ -469,41 +468,52 @@ void Painter::draw_triangle(const IntPoint& a, const IntPoint& b, const IntPoint
if (p1.y() > p2.y())
swap(p1, p2);
// return if top and bottom points are on same line
if (p0.y() == p2.y())
return;
// return if top is below clip rect or bottom is above clip rect
auto clip = clip_rect();
if (p0.y() >= clip.bottom())
return;
if (p2.y() < clip.top())
return;
float dx01 = (float)(p1.x() - p0.x()) / (p1.y() - p0.y());
float dx02 = (float)(p2.x() - p0.x()) / (p2.y() - p0.y());
float dx12 = (float)(p2.x() - p1.x()) / (p2.y() - p1.y());
int rgba = color.value();
float dx02 = (float)(p2.x() - p0.x()) / (p2.y() - p0.y());
float x01 = p0.x();
float x02 = p0.x();
int top = p0.y();
if (top < clip.top()) {
x01 += dx01 * (clip.top() - top);
x02 += dx02 * (clip.top() - top);
top = clip.top();
}
if (p0.y() != p1.y()) { // p0 and p1 are on different lines
float dx01 = (float)(p1.x() - p0.x()) / (p1.y() - p0.y());
for (int y = top; y < p1.y() && y < clip.bottom(); ++y) { // XXX <=?
int start = x01 > x02 ? max((int)x02, clip.left()) : max((int)x01, clip.left());
int end = x01 > x02 ? min((int)x01, clip.right()) : min((int)x02, clip.right());
auto* scanline = m_target->scanline(y);
for (int x = start; x < end; x++) {
scanline[x] = rgba;
int top = p0.y();
if (top < clip.top()) {
x01 += dx01 * (clip.top() - top);
x02 += dx02 * (clip.top() - top);
top = clip.top();
}
for (int y = top; y < p1.y() && y < clip.bottom(); ++y) { // XXX <=?
int start = x01 > x02 ? max((int)x02, clip.left()) : max((int)x01, clip.left());
int end = x01 > x02 ? min((int)x01, clip.right()) : min((int)x02, clip.right());
auto* scanline = m_target->scanline(y);
for (int x = start; x < end; x++) {
scanline[x] = rgba;
}
x01 += dx01;
x02 += dx02;
}
x01 += dx01;
x02 += dx02;
}
x02 = p0.x() + dx02 * (p1.y() - p0.y());
float x12 = p1.x();
// return if middle point and bottom point are on same line
if (p1.y() == p2.y())
return;
top = p1.y();
float x12 = p1.x();
float dx12 = (float)(p2.x() - p1.x()) / (p2.y() - p1.y());
int top = p1.y();
if (top < clip.top()) {
x02 += dx02 * (clip.top() - top);
x12 += dx12 * (clip.top() - top);