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use super::SwRenderer;
use crate::render::font::sdf::sample_signed_distance;
use crate::types::{Color, Fixed};
impl SwRenderer<'_> {
/// Rasterize one SDF glyph into the current target.
///
/// `cx`, `cy` are the top-left of the glyph in physical pixels;
/// `target_size` is the rendered square size in physical pixels —
/// the atlas (`source_size` square) is bilinear-resampled to it, so
/// any target works, not just integer multiples. Coverage comes
/// from a one-output-pixel-wide linear ramp around the zero
/// distance, matching one-pixel anti-aliasing without explicit
/// super-sampling.
#[allow(clippy::too_many_arguments)]
pub(super) fn blit_sdf_glyph(
&mut self,
atlas: &[u8],
source_size: u16,
bit_depth: u8,
spread: u16,
cx: i32,
cy: i32,
target_size: u16,
phys_bounds: (i32, i32, i32, i32),
color: &Color,
opa: u8,
) {
let (clip_x, clip_y, clip_x2, clip_y2) = phys_bounds;
let target_size = target_size.max(1) as i32;
// Source pixels per output pixel. At an integer N× this is
// exactly 1/N, so the integer-scale path stays byte-identical.
let inv_scale = Fixed::from_int(source_size as i32) / Fixed::from_int(target_size);
let half_texel = Fixed::ONE / 2;
// AA ramp half-width = half an output pixel, in source pixels.
let edge_half = inv_scale / 2;
for dy in 0..target_size {
let py = cy + dy;
if py < clip_y || py >= clip_y2 {
continue;
}
// Output-pixel-center to source-texel-index transform: the
// -half_texel shift maps texel index i to its centre at
// continuous i+0.5, so a 1:1 atlas samples texel centres
// exactly. Without it, sampling lands on texel boundaries
// and bilinear averages a solid stem's interior with its
// outside neighbour, halving the coverage.
let sy = (Fixed::from_int(dy) + half_texel) * inv_scale - half_texel;
for dx in 0..target_size {
let px = cx + dx;
if px < clip_x || px >= clip_x2 {
continue;
}
let sx = (Fixed::from_int(dx) + half_texel) * inv_scale - half_texel;
let dist = sample_signed_distance(atlas, source_size, bit_depth, spread, sx, sy);
let cov = if dist <= -edge_half {
continue;
} else if dist >= edge_half {
Fixed::ONE
} else {
(dist + edge_half) / (edge_half * 2)
};
let final_alpha = (cov * Fixed::from_int(opa as i32)).to_int().clamp(0, 255) as u8;
if final_alpha == 0 {
continue;
}
self.target.blend_pixel_int(px, py, color, final_alpha);
}
}
}
}
#[cfg(test)]
mod tests {
use super::*;
use crate::render::backends::sw::SwRenderer;
use crate::render::texture::{ColorFormat, Texture};
use alloc::vec;
fn pixel_alpha(buf: &[u8], stride: usize, x: usize, y: usize) -> u8 {
// RGBA8888: byte 3 of (y * stride + x) * 4.
buf[(y * stride + x) * 4 + 3]
}
/// Build a 4×4 4-bit atlas where the inner 2×2 is "deep inside"
/// (q = 15) and the outer ring is "deep outside" (q = 0). Blitting
/// it should paint the centre opaque and leave the corners blank.
fn build_inside_outside_atlas() -> alloc::vec::Vec<u8> {
let mut atlas = vec![0u8; 8];
let set = |buf: &mut [u8], x: usize, y: usize, q: u8| {
let idx = y * 4 + x;
let byte_idx = idx >> 1;
if idx & 1 == 0 {
buf[byte_idx] = (buf[byte_idx] & 0xF0) | (q & 0x0F);
} else {
buf[byte_idx] = (buf[byte_idx] & 0x0F) | ((q & 0x0F) << 4);
}
};
for y in 0..4 {
for x in 0..4 {
let inside = (1..=2).contains(&x) && (1..=2).contains(&y);
set(&mut atlas, x, y, if inside { 15 } else { 0 });
}
}
atlas
}
#[test]
fn blit_inside_pixel_opaque_outside_blank() {
let mut buf = vec![0u8; 8 * 8 * 4];
let tex = Texture::new(&mut buf, 8, 8, ColorFormat::RGBA8888);
let mut backend = SwRenderer::new(tex);
let atlas = build_inside_outside_atlas();
let color = Color::rgba(255, 255, 255, 255);
backend.blit_sdf_glyph(&atlas, 4, 4, 1, 0, 0, 4, (0, 0, 8, 8), &color, 255);
// Centre of the inside region (source x=1.5..2.5) → opaque.
assert!(
pixel_alpha(&buf, 8, 1, 1) >= 200,
"expected high alpha inside, got {}",
pixel_alpha(&buf, 8, 1, 1)
);
// Corner pixel: source 0..1 is in the deep-outside region →
// blank.
assert_eq!(
pixel_alpha(&buf, 8, 0, 0),
0,
"expected blank corner, got {}",
pixel_alpha(&buf, 8, 0, 0)
);
}
/// Build an 8×8 4-bit atlas with a 1px-wide vertical stem at
/// column 4. Inside the stem distance is +; one px either side is
/// the edge; beyond is outside. This is the thin-stroke case that
/// a fixed-width coverage ramp renders at ~50% (the faded-stem
/// bug); the gradient-normalized ramp must resolve it to solid.
fn build_thin_stem_atlas(spread: u16) -> alloc::vec::Vec<u8> {
let n = 8usize;
let mut atlas = vec![0u8; n * n / 2];
let set = |buf: &mut [u8], x: usize, y: usize, q: u8| {
let idx = y * n + x;
let bi = idx >> 1;
if idx & 1 == 0 {
buf[bi] = (buf[bi] & 0xF0) | (q & 0x0F);
} else {
buf[bi] = (buf[bi] & 0x0F) | ((q & 0x0F) << 4);
}
};
// Signed distance to the stem at column 4 (stem occupies x∈[4,5)).
// Encode like the host quantizer: q = round(d/spread*7.5 + 7.5).
for y in 0..n {
for x in 0..n {
let cx = x as f32 + 0.5;
let d = 0.5 - (cx - 4.5).abs();
let dc = d.clamp(-(spread as f32), spread as f32);
let q = (dc / spread as f32 * 7.5 + 7.5).round().clamp(0.0, 15.0) as u8;
set(&mut atlas, x, y, q);
}
}
atlas
}
#[test]
fn thin_stem_renders_solid_not_half_coverage() {
let spread = 2u16;
let atlas = build_thin_stem_atlas(spread);
let mut buf = vec![0u8; 8 * 8 * 4];
let tex = Texture::new(&mut buf, 8, 8, ColorFormat::RGBA8888);
let mut backend = SwRenderer::new(tex);
let color = Color::rgba(255, 255, 255, 255);
backend.blit_sdf_glyph(&atlas, 8, 4, spread, 0, 0, 8, (0, 0, 8, 8), &color, 255);
// The stem column (x=4) must read as solid, not the ~137/255
// (≈54%) the fixed-ramp produced. Gradient normalization should
// push the stem-center coverage high.
let stem = pixel_alpha(&buf, 8, 4, 3);
assert!(
stem >= 200,
"thin stem should render near-solid, got alpha {stem}",
);
}
#[test]
fn blit_respects_clip_bounds() {
let mut buf = vec![0u8; 8 * 8 * 4];
let tex = Texture::new(&mut buf, 8, 8, ColorFormat::RGBA8888);
let mut backend = SwRenderer::new(tex);
let atlas = build_inside_outside_atlas();
let color = Color::rgba(255, 255, 255, 255);
// Tight clip: only y < 2 allowed.
backend.blit_sdf_glyph(&atlas, 4, 4, 1, 0, 0, 4, (0, 0, 8, 2), &color, 255);
// Pixel below clip stays blank even though the atlas would
// otherwise paint it opaque.
assert_eq!(pixel_alpha(&buf, 8, 1, 2), 0);
assert_eq!(pixel_alpha(&buf, 8, 1, 3), 0);
}
#[test]
fn blit_upscales_to_non_integer_target() {
// 4px source atlas rendered to a 10px target (2.5×, not an
// integer multiple) — the path the zoom animation relies on.
// The inside region (source 1..3) maps to roughly target
// 2.5..7.5, so the centre is solid and it paints a 10px box.
let mut buf = vec![0u8; 16 * 16 * 4];
let tex = Texture::new(&mut buf, 16, 16, ColorFormat::RGBA8888);
let mut backend = SwRenderer::new(tex);
let atlas = build_inside_outside_atlas();
let color = Color::rgba(255, 255, 255, 255);
backend.blit_sdf_glyph(&atlas, 4, 4, 1, 0, 0, 10, (0, 0, 16, 16), &color, 255);
// Centre of the 10px render is deep inside → opaque.
assert!(
pixel_alpha(&buf, 16, 5, 5) >= 200,
"expected solid centre at 2.5x, got {}",
pixel_alpha(&buf, 16, 5, 5)
);
// Far corner is outside → blank.
assert_eq!(pixel_alpha(&buf, 16, 0, 0), 0);
// Coverage reaches into the lower rows, proving it scaled past
// the source's 4px extent.
assert!(
pixel_alpha(&buf, 16, 5, 7) > 0,
"render did not reach 10px tall"
);
}
}