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LibGfx: Use the first/last color for positions before/after a gradient

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This fixes a few rendering bugs especially where the first or last
color stop had a zero alpha value. Note we can't just set the first
and last values on the gradient line to the first/last colors since
that might not be correct (e.g. have a transition hint).
This commit is contained in:
MacDue 2023-01-23 00:35:57 +00:00 committed by Linus Groh
parent 6255ca4a42
commit a43d892c4e
Notes: sideshowbarker 2024-07-17 10:31:19 +09:00 
Author: https://github.com/MacDue
Commit: https://github.com/SerenityOS/serenity/commit/a43d892c4e
Pull-request: https://github.com/SerenityOS/serenity/pull/17143
Reviewed-by: https://github.com/linusg
1 changed files with 23 additions and 14 deletions
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37
Userland/Libraries/LibGfx/GradientPainting.cpp
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@ -55,6 +55,8 @@ public:
GradientLine(int gradient_length, Span<ColorStop const> color_stops, Optional<float> repeat_length, UsePremultipliedAlpha use_premultiplied_alpha = UsePremultipliedAlpha::Yes)
: m_repeating { repeat_length.has_value() }
, m_start_offset { round_to<int>((m_repeating ? color_stops.first().position : 0.0f) * gradient_length) }
, m_color_stops { color_stops }
, m_use_premultiplied_alpha { use_premultiplied_alpha }
{
// Avoid generating excessive amounts of colors when the not enough shades to fill that length.
auto necessary_length = min<int>((color_stops.size() - 1) * 255, gradient_length);
@ -63,14 +65,6 @@ public:
auto color_count = round_to<int>(repeat_length.value_or(1.0f) * necessary_length);
m_gradient_line_colors.resize(color_count);
auto color_blend = [&](Color a, Color b, float amount) {
// Note: color.mixed_with() performs premultiplied alpha mixing when necessary as defined in:
// https://drafts.csswg.org/css-images/#coloring-gradient-line
if (use_premultiplied_alpha == UsePremultipliedAlpha::Yes)
return a.mixed_with(b, amount);
return a.interpolate(b, amount);
};
for (int loc = 0; loc < color_count; loc++) {
auto relative_loc = float(loc + m_start_offset) / necessary_length;
Color gradient_color = color_blend(color_stops[0].color, color_stops[1].color,
@ -85,9 +79,22 @@ public:
}
}
Color color_blend(Color a, Color b, float amount) const
{
// Note: color.mixed_with() performs premultiplied alpha mixing when necessary as defined in:
// https://drafts.csswg.org/css-images/#coloring-gradient-line
if (m_use_premultiplied_alpha == UsePremultipliedAlpha::Yes)
return a.mixed_with(b, amount);
return a.interpolate(b, amount);
};
Color get_color(i64 index) const
{
return m_gradient_line_colors[clamp(index, 0, m_gradient_line_colors.size() - 1)];
if (index < 0)
return m_color_stops.first().color;
if (index >= static_cast<i64>(m_gradient_line_colors.size()))
return m_color_stops.last().color;
return m_gradient_line_colors[index];
}
Color sample_color(float loc) const
@ -106,7 +113,7 @@ public:
auto color = get_color(repeat_wrap_if_required(int_loc));
// Blend between the two neighbouring colors (this fixes some nasty aliasing issues at small angles)
if (blend >= 0.004f)
color = color.mixed_with(get_color(repeat_wrap_if_required(int_loc + 1)), blend);
color = color_blend(color, get_color(repeat_wrap_if_required(int_loc + 1)), blend);
return color;
}
@ -123,11 +130,13 @@ public:
}
private:
bool m_repeating;
int m_start_offset;
bool m_repeating { false };
int m_start_offset { 0 };
float m_sample_scale { 1 };
Vector<Color, 1024> m_gradient_line_colors;
Span<ColorStop const> m_color_stops {};
UsePremultipliedAlpha m_use_premultiplied_alpha { UsePremultipliedAlpha::Yes };
Vector<Color, 1024> m_gradient_line_colors;
bool m_requires_blending = false;
};
@ -385,7 +394,7 @@ void CanvasRadialGradientPaintStyle::paint(IntRect physical_bounding_box, PaintF
}
// This is just an approximate upperbound (the gradient line class will shorten this if necessary).
int gradient_length = center_dist + end_radius + start_radius;
int gradient_length = AK::ceil(center_dist + end_radius + start_radius);
GradientLine gradient_line(gradient_length, color_stops(), repeat_length(), UsePremultipliedAlpha::No);
auto radius2 = end_radius * end_radius;