text_raster_policy.hpp

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#pragma once
 
// How a render surface maps glyph coverage to pixels.
//
// Skia applies a contrast and gamma "preblend" to glyph masks, and it reads
// both values from the surface the text is drawn into: SkFont has no gamma
// field, a saveLayer inherits its parent's values, and the strike cache keys
// glyphs on them. A text-raster choice is therefore a property of a SURFACE,
// fixed when the surface is created, and every surface a frame's text can pass
// through (the frame target, offscreen and scene targets, retained layers) has
// to be created with the same one, or text painted through a cached layer
// comes out a different weight from the same text painted directly.
//
// Measured on macOS (CoreText through Skia, raster and DPR 2) against Chrome:
//   - `legacy` (Skia's defaults: contrast 0.5, sRGB gamma) with hinting slight
//     reproduces Chromium's `-webkit-font-smoothing: auto` exactly.
//   - `chromium` (contrast 0, gamma 1) with hinting none reproduces
//     `-webkit-font-smoothing: antialiased` exactly, about 23% lighter than
//     `legacy`. An `auto` node drawn on a `chromium` surface is about 2% heavier
//     than Chromium's.
// No single surface reproduces both modes, so a surface that holds any
// `antialiased` text is created `chromium` (select_text_raster_policy).
//
// Hinting is per font, not per surface, and is chosen by the text painter.
 
#include <cstdint>
#include <span>
 
namespace pulp::canvas {

enum class TextRasterPolicy : std::uint8_t {
    legacy,
    chromium,
};

enum class FontSmoothing : std::uint8_t {
    auto_smoothing,
    antialiased,
};

struct TextRasterTuple {
    float contrast;
    float gamma;
};

inline constexpr TextRasterTuple kLegacyTextRaster{0.5f, 0.0f};
inline constexpr TextRasterTuple kChromiumTextRaster{0.0f, 1.0f};
 
constexpr TextRasterTuple text_raster_tuple(TextRasterPolicy policy) {
    return policy == TextRasterPolicy::chromium ? kChromiumTextRaster : kLegacyTextRaster;
}

constexpr TextRasterPolicy select_text_raster_policy(std::span<const FontSmoothing> nodes) {
    for (const FontSmoothing smoothing : nodes) {
        if (smoothing == FontSmoothing::antialiased)
            return TextRasterPolicy::chromium;
    }
    return TextRasterPolicy::legacy;
}
 
} // namespace pulp::canvas