tiny.gui.paint.gradient

1const std = @import("std");23const gui = @import("../root.zig");45const Color = gui.model.UiColor;6const Command = gui.paint.command.Command;7const Point = gui.model.UiPoint;89pub fn colorAt(paint: Command, x: f32, y: f32) Color {10    return Sampler.init(paint).colorAt(x, y);11}1213pub fn parameter(paint: Command, x: f32, y: f32) f32 {14    return Sampler.init(paint).parameterAt(x, y);15}1617pub const Sampler = struct {18    start: Point,19    start_color: PremultipliedColor,20    end_color: PremultipliedColor,21    dx: f32,22    dy: f32,23    length_squared: f32,24    degenerate: bool,2526    pub fn init(paint: Command) Sampler {27        const dx = paint.gradient_end.x - paint.gradient_start.x;28        const dy = paint.gradient_end.y - paint.gradient_start.y;29        const length_squared = dx * dx + dy * dy;30        return .{31            .start = paint.gradient_start,32            .start_color = PremultipliedColor.init(paint.color),33            .end_color = PremultipliedColor.init(paint.color_end),34            .dx = dx,35            .dy = dy,36            .length_squared = length_squared,37            .degenerate = !std.math.isFinite(length_squared) or38                length_squared <= std.math.floatEps(f32),39        };40    }4142    pub fn colorAt(self: Sampler, x: f32, y: f32) Color {43        return self.colorAtRow(x, self.rowProjection(y));44    }4546    pub fn parameterAt(self: Sampler, x: f32, y: f32) f32 {47        return self.parameterAtRow(x, self.rowProjection(y));48    }4950    pub fn rowProjection(self: Sampler, y: f32) f32 {51        return (y - self.start.y) * self.dy;52    }5354    pub fn colorAtRow(self: Sampler, x: f32, row_projection: f32) Color {55        const t = self.parameterAtRow(x, row_projection);56        const alpha_value = interpolateValue(self.start_color.a, self.end_color.a, t);57        if (alpha_value <= 0) return .{ .a = 0 };58        return .{59            .r = unpremultipliedValue(self.start_color.r, self.end_color.r, t, alpha_value),60            .g = unpremultipliedValue(self.start_color.g, self.end_color.g, t, alpha_value),61            .b = unpremultipliedValue(self.start_color.b, self.end_color.b, t, alpha_value),62            .a = roundedChannel(alpha_value),63        };64    }6566    pub fn parameterAtRow(self: Sampler, x: f32, row_projection: f32) f32 {67        if (self.degenerate) return 1;68        const projected = ((x - self.start.x) * self.dx + row_projection) /69            self.length_squared;70        if (!std.math.isFinite(projected)) return 1;71        return std.math.clamp(projected, @as(f32, 0), @as(f32, 1));72    }73};7475const PremultipliedColor = struct {76    r: f32,77    g: f32,78    b: f32,79    a: f32,8081    fn init(color: Color) PremultipliedColor {82        const alpha: f32 = @floatFromInt(color.a);83        return .{84            .r = @as(f32, @floatFromInt(color.r)) * alpha,85            .g = @as(f32, @floatFromInt(color.g)) * alpha,86            .b = @as(f32, @floatFromInt(color.b)) * alpha,87            .a = alpha,88        };89    }90};9192pub fn colorAtParameter(start: Color, end: Color, parameter_value: f32) Color {93    const t = std.math.clamp(parameter_value, @as(f32, 0), @as(f32, 1));94    const alpha_value = interpolate(start.a, end.a, t);95    if (alpha_value <= 0) return .{ .a = 0 };96    return .{97        .r = unpremultiplied(start.r, start.a, end.r, end.a, t, alpha_value),98        .g = unpremultiplied(start.g, start.a, end.g, end.a, t, alpha_value),99        .b = unpremultiplied(start.b, start.a, end.b, end.a, t, alpha_value),100        .a = roundedChannel(alpha_value),101    };102}103104fn unpremultiplied(start: u8, start_alpha: u8, end: u8, end_alpha: u8, t: f32, alpha_value: f32) u8 {105    const start_value = @as(f32, @floatFromInt(start)) * @as(f32, @floatFromInt(start_alpha));106    const end_value = @as(f32, @floatFromInt(end)) * @as(f32, @floatFromInt(end_alpha));107    return unpremultipliedValue(start_value, end_value, t, alpha_value);108}109110fn unpremultipliedValue(start: f32, end: f32, t: f32, alpha_value: f32) u8 {111    return roundedChannel(interpolateValue(start, end, t) / alpha_value);112}113114fn interpolate(start: u8, end: u8, t: f32) f32 {115    return interpolateValue(@floatFromInt(start), @floatFromInt(end), t);116}117118fn interpolateValue(start: f32, end: f32, t: f32) f32 {119    return start + (end - start) * t;120}121122fn roundedChannel(value: f32) u8 {123    return @intFromFloat(std.math.clamp(@round(value), @as(f32, 0), @as(f32, 255)));124}125126test "linear gradient clamps and reverses direction" {127    const paint = Command{128        .kind = .linear_gradient,129        .rect = .{ .width = 12, .height = 4 },130        .clip = .{ .width = 12, .height = 4 },131        .color = .{ .r = 10, .g = 20, .b = 30, .a = 255 },132        .color_end = .{ .r = 210, .g = 120, .b = 80, .a = 255 },133        .gradient_start = .{ .x = 10 },134        .gradient_end = .{ .x = 2 },135    };136    try std.testing.expectEqual(paint.color, colorAt(paint, 12, 2));137    try std.testing.expectEqual(paint.color_end, colorAt(paint, 0, 2));138    try std.testing.expectEqual(@as(u8, 110), colorAt(paint, 6, 2).r);139}140141test "linear gradient degenerate vector resolves to end color" {142    const paint = Command{143        .kind = .linear_gradient,144        .rect = .{ .width = 4, .height = 4 },145        .clip = .{ .width = 4, .height = 4 },146        .color = .{ .r = 255, .a = 255 },147        .color_end = .{ .b = 255, .a = 120 },148        .gradient_start = .{ .x = 2, .y = 2 },149        .gradient_end = .{ .x = 2, .y = 2 },150    };151    try std.testing.expectEqual(paint.color_end, colorAt(paint, 1, 3));152}153154test "linear gradient premultiplies translucent interpolation" {155    const color = colorAtParameter(156        .{ .r = 255, .g = 40, .b = 20, .a = 255 },157        .{ .r = 0, .g = 0, .b = 0, .a = 0 },158        0.5,159    );160    try std.testing.expectEqual(@as(u8, 255), color.r);161    try std.testing.expectEqual(@as(u8, 40), color.g);162    try std.testing.expectEqual(@as(u8, 20), color.b);163    try std.testing.expectEqual(@as(u8, 128), color.a);164}

1pub const gradient = @import("gradient.zig");