render: add opaque CPU sampling and bounded mip chains (#1459)

Co-Authored-By: selune <0xlillith@users.noreply.github.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/render/src/texture_sampling/mips.rs

@@ -0,0 +1,191 @@
+//! Bounded linear area-filtered mip preparation for opaque CPU reference sampling.
+//! Call from the existing preparation host; row cancellation does not yield a browser event loop.
+use super::{SamplingError, TextureView};
+
+/// Derived pixel storage only, excluding the borrowed base and small level metadata.
+pub const MAX_MIP_TAIL_BYTES: usize = 32 * 1024 * 1024;
+/// Include this recipe in preparation/cache identity, alongside source/role/policies.
+/// V1 means floor-halved linear area filtering, opaque RGB and generated alpha=1.
+pub const MIP_RECIPE_ID: &str = "solidcraft.opaque-linear-area-mips/1";
+const MAX_LEVELS: usize = 12;
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum MipError {
+    Budget,
+    Allocation,
+    Cancelled,
+    Lod,
+    Sampling(SamplingError),
+}
+impl std::fmt::Display for MipError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::Budget => f.write_str("mip tail storage exceeds the admitted budget"),
+            Self::Allocation => f.write_str("cannot allocate mip tail storage"),
+            Self::Cancelled => f.write_str("mip preparation cancelled"),
+            Self::Lod => f.write_str("LOD must be finite and within the prepared level range"),
+            Self::Sampling(e) => e.fmt(f),
+        }
+    }
+}
+impl std::error::Error for MipError {}
+impl From<SamplingError> for MipError {
+    fn from(value: SamplingError) -> Self {
+        Self::Sampling(value)
+    }
+}
+
+struct Level {
+    rgba: Vec<f32>,
+    width: usize,
+    height: usize,
+}
+impl Level {
+    fn view(&self) -> TextureView<'_> {
+        // Private levels are created only from admitted pixels and convex averages.
+        // Reuse validation rather than rescan every stored pixel on each sample.
+        TextureView { rgba: &self.rgba, width: self.width, height: self.height }
+    }
+}
+
+/// Immutable derived levels; level zero borrows the admitted base without copying it.
+/// Generated levels have alpha=1, matching the explicitly opaque RGB sampler.
+pub struct MipChain<'a> {
+    base: TextureView<'a>,
+    tail: Vec<Level>,
+    tail_bytes: usize,
+    retained_bytes: usize,
+}
+impl<'a> MipChain<'a> {
+    /// Generate all floor-halved levels through 1x1, or return no chain on failure.
+    /// `max_tail_bytes` bounds derived pixel capacity, not total process memory.
+    /// Preparation is synchronous and must run in the caller-owned preparation host.
+    pub fn prepare(base: TextureView<'a>, max_tail_bytes: usize, cancelled: impl Fn() -> bool) -> Result<Self, MipError> {
+        if max_tail_bytes > MAX_MIP_TAIL_BYTES {
+            return Err(MipError::Budget);
+        }
+        check_cancel(&cancelled)?;
+        let required = required_bytes(base.width, base.height)?;
+        if required > max_tail_bytes {
+            return Err(MipError::Budget);
+        }
+        let mut chain = Self { base, tail: Vec::new(), tail_bytes: 0, retained_bytes: size_of::<Self>() };
+        chain.tail.try_reserve_exact(MAX_LEVELS - 1).map_err(|_| MipError::Allocation)?;
+        let metadata_bytes =
+            chain.tail.capacity().checked_mul(size_of::<Level>()).and_then(|n| n.checked_add(size_of::<Self>())).ok_or(MipError::Budget)?;
+        chain.retained_bytes = metadata_bytes;
+        while chain.level_count() < MAX_LEVELS {
+            check_cancel(&cancelled)?;
+            let source = chain.level(chain.level_count() - 1).ok_or(MipError::Budget)?;
+            if source.width == 1 && source.height == 1 {
+                break;
+            }
+            let width = (source.width / 2).max(1);
+            let height = (source.height / 2).max(1);
+            let length = width.checked_mul(height).and_then(|n| n.checked_mul(4)).ok_or(MipError::Budget)?;
+            let mut rgba = Vec::new();
+            rgba.try_reserve_exact(length).map_err(|_| MipError::Allocation)?;
+            let bytes = rgba.capacity().checked_mul(size_of::<f32>()).ok_or(MipError::Budget)?;
+            let next_bytes = chain.tail_bytes.checked_add(bytes).ok_or(MipError::Budget)?;
+            if next_bytes > max_tail_bytes {
+                return Err(MipError::Budget);
+            }
+            let retained_bytes = metadata_bytes.checked_add(next_bytes).ok_or(MipError::Budget)?;
+            for y in 0..height {
+                check_cancel(&cancelled)?;
+                for x in 0..width {
+                    let rgb = area_average(source, x, y, width, height)?;
+                    rgba.extend(rgb.map(|v| v as f32));
+                    rgba.push(1.0);
+                }
+            }
+            check_cancel(&cancelled)?;
+            chain.tail.push(Level { rgba, width, height });
+            chain.tail_bytes = next_bytes;
+            chain.retained_bytes = retained_bytes;
+        }
+        check_cancel(&cancelled)?;
+        Ok(chain)
+    }
+
+    pub fn level_count(&self) -> usize {
+        self.tail.len() + 1
+    }
+    pub fn tail_bytes(&self) -> usize {
+        self.tail_bytes
+    }
+    /// Retained pixel/level Vec capacities plus the chain value. Charge to the
+    /// existing host pool; borrowed base ownership and allocator overhead are separate.
+    pub fn retained_bytes(&self) -> usize {
+        self.retained_bytes
+    }
+    /// Borrow a prepared level without allocation or another full-image admission scan.
+    pub fn level(&self, index: usize) -> Option<TextureView<'_>> {
+        if index == 0 { Some(self.base) } else { self.tail.get(index - 1).map(Level::view) }
+    }
+    pub fn dimensions(&self, index: usize) -> Option<[u32; 2]> {
+        self.level(index).map(|v| [v.width as u32, v.height as u32])
+    }
+
+    /// Explicit LOD only: bilinear within each level, linear between adjacent levels.
+    /// This does not choose a footprint, provide anisotropy or certify GPU parity.
+    /// Out-of-range LOD refuses rather than silently clamping to a different policy.
+    pub fn sample_repeat_lod_opaque(&self, uv: [f64; 2], lod: f64) -> Result<[f64; 3], MipError> {
+        let last = self.level_count() - 1;
+        if !lod.is_finite() || lod < 0.0 || lod > last as f64 {
+            return Err(MipError::Lod);
+        }
+        let low = lod.floor() as usize;
+        let high = (low + 1).min(last);
+        let t = lod - low as f64;
+        let a = self.level(low).ok_or(MipError::Lod)?.sample_repeat_opaque(uv)?;
+        let b = self.level(high).ok_or(MipError::Lod)?.sample_repeat_opaque(uv)?;
+        let mut rgb = [0.0; 3];
+        for ((output, a), b) in rgb.iter_mut().zip(a).zip(b) {
+            *output = a * (1.0 - t) + b * t;
+        }
+        Ok(rgb)
+    }
+}
+
+fn check_cancel(cancelled: &impl Fn() -> bool) -> Result<(), MipError> {
+    if cancelled() { Err(MipError::Cancelled) } else { Ok(()) }
+}
+
+fn required_bytes(mut width: usize, mut height: usize) -> Result<usize, MipError> {
+    let mut bytes = 0usize;
+    for _ in 1..MAX_LEVELS {
+        if width == 1 && height == 1 {
+            return Ok(bytes);
+        }
+        width = (width / 2).max(1);
+        height = (height / 2).max(1);
+        let level_bytes = width.checked_mul(height).and_then(|n| n.checked_mul(4 * size_of::<f32>())).ok_or(MipError::Budget)?;
+        bytes = bytes.checked_add(level_bytes).ok_or(MipError::Budget)?;
+    }
+    if width == 1 && height == 1 { Ok(bytes) } else { Err(MipError::Budget) }
+}
+
+fn area_average(source: TextureView<'_>, x: usize, y: usize, width: usize, height: usize) -> Result<[f64; 3], MipError> {
+    // Integer overlap numerators avoid dropping the final row/column at odd sizes.
+    // All factors are bounded by the validated 2048 edge; at most 3x3 source cells.
+    let x_start = x * source.width;
+    let x_end = (x + 1) * source.width;
+    let y_start = y * source.height;
+    let y_end = (y + 1) * source.height;
+    let mut rgb = [0.0; 3];
+    for sy in y_start / height..y_end.div_ceil(height) {
+        let wy = ((sy + 1) * height).min(y_end) - (sy * height).max(y_start);
+        for sx in x_start / width..x_end.div_ceil(width) {
+            let wx = ((sx + 1) * width).min(x_end) - (sx * width).max(x_start);
+            let weight = wx as f64 * wy as f64 / (source.width as f64 * source.height as f64);
+            for (channel, output) in rgb.iter_mut().enumerate() {
+                *output += source.channel(sx, sy, channel)? * weight;
+            }
+        }
+    }
+    Ok(rgb)
+}
+
+#[cfg(test)]
+mod tests;

crates/render/src/texture_sampling/mips/tests.rs

@@ -0,0 +1,115 @@
+use super::*;
+use std::cell::Cell;
+
+fn close(actual: [f64; 3], expected: [f64; 3]) {
+    for (a, b) in actual.into_iter().zip(expected) {
+        assert!((a - b).abs() < 1e-7, "{actual:?} != {expected:?}");
+    }
+}
+
+#[test]
+fn linear_checkerboard_mean_and_fractional_lod() {
+    let pixels = [0., 0., 0., 0., 1., 1., 1., 1., 1., 1., 1., 1., 0., 0., 0., 0.];
+    let base = TextureView::new(&pixels, [2, 2]).unwrap();
+    let chain = MipChain::prepare(base, 16, || false).unwrap();
+    assert_eq!(chain.level_count(), 2);
+    assert_eq!(chain.tail_bytes(), 16);
+    assert_eq!(chain.retained_bytes(), chain.tail_bytes() + chain.tail.capacity() * size_of::<Level>() + size_of::<MipChain<'_>>());
+    assert_eq!(MIP_RECIPE_ID, "solidcraft.opaque-linear-area-mips/1");
+    close(chain.sample_repeat_lod_opaque([0.25, 0.25], 0.).unwrap(), [0.; 3]);
+    close(chain.sample_repeat_lod_opaque([0.25, 0.25], 1.).unwrap(), [0.5; 3]);
+    close(chain.sample_repeat_lod_opaque([0.25, 0.25], 0.25).unwrap(), [0.125; 3]);
+    close(chain.sample_repeat_lod_opaque([0.25, 0.25], 0.5).unwrap(), [0.25; 3]);
+}
+
+#[test]
+fn odd_edges_contribute_by_area_instead_of_being_dropped_or_overweighted() {
+    // Five pixels reduce to two equal-width cells: [0,0,0] and [0,0,1].
+    // The middle pixel straddles the boundary, the last pixel weighs 1/2.5.
+    let mut pixels = [0., 0., 0., 1.].repeat(5);
+    pixels[16] = 1.;
+    let chain = MipChain::prepare(TextureView::new(&pixels, [5, 1]).unwrap(), 48, || false).unwrap();
+    assert_eq!(chain.dimensions(1), Some([2, 1]));
+    close(chain.level(1).unwrap().sample_repeat_opaque([0.75, 0.5]).unwrap(), [0.4, 0., 0.]);
+    close(chain.sample_repeat_lod_opaque([0., 0.], 2.).unwrap(), [0.2, 0., 0.]);
+}
+
+#[test]
+fn rectangular_odd_pyramid_preserves_independent_mean() {
+    let mut pixels = Vec::new();
+    for i in 0..15 {
+        pixels.extend([i as f32 / 16., 0.25, (14 - i) as f32 / 16., (i % 2) as f32]);
+    }
+    let chain = MipChain::prepare(TextureView::new(&pixels, [5, 3]).unwrap(), 48, || false).unwrap();
+    assert_eq!(chain.level_count(), 3);
+    assert_eq!(chain.dimensions(0), Some([5, 3]));
+    assert_eq!(chain.dimensions(1), Some([2, 1]));
+    assert_eq!(chain.dimensions(2), Some([1, 1]));
+    assert_eq!(chain.dimensions(usize::MAX), None);
+    close(chain.sample_repeat_lod_opaque([-9.5, 12.], 2.).unwrap(), [7. / 16., 0.25, 7. / 16.]);
+}
+
+#[test]
+fn single_texel_borrows_base_and_needs_no_pixel_budget() {
+    let pixels = [0.25, 0.5, 0.75, 0.];
+    let chain = MipChain::prepare(TextureView::new(&pixels, [1, 1]).unwrap(), 0, || false).unwrap();
+    assert_eq!(chain.level_count(), 1);
+    assert_eq!(chain.tail_bytes(), 0);
+    assert!(std::ptr::eq(chain.level(0).unwrap().rgba.as_ptr(), pixels.as_ptr()));
+    close(chain.sample_repeat_lod_opaque([-0.1, 0.7], 0.).unwrap(), [0.25, 0.5, 0.75]);
+}
+
+#[test]
+fn constant_images_and_single_axes_are_invariant_at_all_lods() {
+    for dimensions in [[1, 7], [7, 1], [7, 5]] {
+        let pixels = [0.25, 0.5, 0.75, 0.].repeat((dimensions[0] * dimensions[1]) as usize);
+        let chain = MipChain::prepare(TextureView::new(&pixels, dimensions).unwrap(), MAX_MIP_TAIL_BYTES, || false).unwrap();
+        for quarter in 0..=(chain.level_count() - 1) * 4 {
+            close(chain.sample_repeat_lod_opaque([-2.34, 1.92], quarter as f64 / 4.).unwrap(), [0.25, 0.5, 0.75]);
+        }
+    }
+}
+
+#[test]
+fn hostile_lod_and_uv_refuse_without_changing_the_chain() {
+    let pixels = [0., 0., 0., 1.].repeat(4);
+    let chain = MipChain::prepare(TextureView::new(&pixels, [2, 2]).unwrap(), 16, || false).unwrap();
+    for lod in [f64::NAN, f64::INFINITY, f64::NEG_INFINITY, -0.001, 1.001, f64::MAX] {
+        assert_eq!(chain.sample_repeat_lod_opaque([0., 0.], lod), Err(MipError::Lod));
+    }
+    assert_eq!(chain.sample_repeat_lod_opaque([f64::NAN, 0.], 0.5), Err(MipError::Sampling(SamplingError::Coordinates)));
+    close(chain.sample_repeat_lod_opaque([0.25, 0.25], 0.).unwrap(), [0.; 3]);
+}
+
+#[test]
+fn exact_budget_and_global_cap_are_admitted_before_work() {
+    let pixels = [0., 0., 0., 1.].repeat(4);
+    let base = TextureView::new(&pixels, [2, 2]).unwrap();
+    assert!(matches!(MipChain::prepare(base, 15, || false), Err(MipError::Budget)));
+    assert!(MipChain::prepare(base, 16, || false).is_ok());
+    assert!(matches!(MipChain::prepare(base, MAX_MIP_TAIL_BYTES + 1, || false), Err(MipError::Budget)));
+    assert!(matches!(MipChain::prepare(base, usize::MAX, || false), Err(MipError::Budget)));
+    assert_eq!(required_bytes(2048, 2048).unwrap(), 22_369_616);
+    assert!(required_bytes(2048, 2048).unwrap() < MAX_MIP_TAIL_BYTES);
+}
+
+#[test]
+fn cancellation_before_during_and_after_preparation_never_returns_a_partial_chain() {
+    let pixels = [0.25, 0.5, 0.75, 0.].repeat(64);
+    let base = TextureView::new(&pixels, [8, 8]).unwrap();
+    let successful_calls = Cell::new(0);
+    MipChain::prepare(base, MAX_MIP_TAIL_BYTES, || {
+        successful_calls.set(successful_calls.get() + 1);
+        false
+    })
+    .unwrap();
+    for stop in 1..=successful_calls.get() {
+        let calls = Cell::new(0);
+        let result = MipChain::prepare(base, MAX_MIP_TAIL_BYTES, || {
+            calls.set(calls.get() + 1);
+            calls.get() == stop
+        });
+        assert!(matches!(result, Err(MipError::Cancelled)));
+    }
+    assert!(MipChain::prepare(base, MAX_MIP_TAIL_BYTES, || false).is_ok());
+}

crates/render/src/texture_sampling/tests.rs

@@ -0,0 +1,94 @@
+use super::*;
+
+const PIXELS: [f32; 16] = [1., 0., 0., 0., 0., 1., 0., 1., 0., 0., 1., 1., 1., 1., 1., 1.];
+
+fn close(actual: [f64; 3], expected: [f64; 3]) {
+    for (a, b) in actual.into_iter().zip(expected) {
+        assert!((a - b).abs() < 1e-9, "{actual:?} != {expected:?}");
+    }
+}
+
+#[test]
+fn centers_orientation_and_explicit_opaque_alpha() {
+    let view = TextureView::new(&PIXELS, [2, 2]).unwrap();
+    for (uv, rgb) in [([0.25, 0.25], [1., 0., 0.]), ([0.75, 0.25], [0., 1., 0.]), ([0.25, 0.75], [0., 0., 1.]), ([0.75, 0.75], [1., 1., 1.])] {
+        close(view.sample_repeat_opaque(uv).unwrap(), rgb);
+    }
+    close(view.sample_repeat_opaque([0.5, 0.5]).unwrap(), [0.5; 3]);
+    close(view.sample_repeat_opaque([0.5, 0.25]).unwrap(), [0.5, 0.5, 0.]);
+    close(view.sample_repeat_opaque([0.375, 0.625]).unwrap(), [0.375, 0.25, 0.75]);
+}
+
+#[test]
+fn linear_black_white_mixing_is_not_encoded_srgb_averaging() {
+    let pixels = [0., 0., 0., 1., 1., 1., 1., 1.];
+    let view = TextureView::new(&pixels, [2, 1]).unwrap();
+    // Linear 0.5, which would encode to about sRGB 0.735, not encoded 0.5.
+    close(view.sample_repeat_opaque([0.5, 0.5]).unwrap(), [0.5; 3]);
+}
+
+#[test]
+fn signed_repeats_and_both_periodic_seams() {
+    let view = TextureView::new(&PIXELS, [2, 2]).unwrap();
+    for uv in [[0., 0.], [1., 1.], [-1., -1.], [MAX_REPEAT_COORDINATE, -MAX_REPEAT_COORDINATE]] {
+        close(view.sample_repeat_opaque(uv).unwrap(), [0.5; 3]);
+    }
+    close(view.sample_repeat_opaque([-0.75, -0.25]).unwrap(), [0., 0., 1.]);
+    for uv in [[0.13, 0.92], [0., 0.25], [0.75, 0.]] {
+        close(view.sample_repeat_opaque(uv).unwrap(), view.sample_repeat_opaque([uv[0] - 7., uv[1] + 9.]).unwrap());
+    }
+    for epsilon in [-1e-12, 0., 1e-12] {
+        close(view.sample_repeat_opaque([epsilon, 0.25]).unwrap(), [0.5, 0.5, 0.]);
+        close(view.sample_repeat_opaque([0.25, epsilon]).unwrap(), [0.5, 0., 0.5]);
+    }
+}
+
+#[test]
+fn constant_and_single_texel_invariance() {
+    for dimensions in [[1, 1], [1, 3], [3, 1], [3, 5]] {
+        let pixels = [0.25, 0.5, 0.75, 0.].repeat((dimensions[0] * dimensions[1]) as usize);
+        let view = TextureView::new(&pixels, dimensions).unwrap();
+        for uv in [[0., 0.], [-12.125, 3.25], [1., 1.], [MAX_REPEAT_COORDINATE - 0.125, 0.5]] {
+            close(view.sample_repeat_opaque(uv).unwrap(), [0.25, 0.5, 0.75]);
+        }
+    }
+}
+
+#[test]
+fn rejects_hostile_storage_and_channels() {
+    for dimensions in [[0, 1], [1, 0], [MAX_TEXTURE_EDGE + 1, 1], [u32::MAX, u32::MAX]] {
+        assert_eq!(TextureView::new(&[], dimensions).unwrap_err(), SamplingError::Dimensions);
+    }
+    for pixels in [&[][..], &[0.; 3][..], &[0.; 5][..]] {
+        assert_eq!(TextureView::new(pixels, [1, 1]).unwrap_err(), SamplingError::Length);
+    }
+    for invalid in [f32::NAN, f32::INFINITY, f32::NEG_INFINITY, -0.01, 1.01] {
+        for channel in 0..4 {
+            let mut pixels = [0.; 4];
+            pixels[channel] = invalid;
+            assert_eq!(TextureView::new(&pixels, [1, 1]).unwrap_err(), SamplingError::Values);
+        }
+    }
+}
+
+#[test]
+fn rejects_nonfinite_and_extreme_coordinates() {
+    let view = TextureView::new(&PIXELS, [2, 2]).unwrap();
+    for invalid in [f64::NAN, f64::INFINITY, f64::NEG_INFINITY, f64::MAX, MAX_REPEAT_COORDINATE + 0.001, -MAX_REPEAT_COORDINATE - 0.001] {
+        for uv in [[invalid, 0.], [0., invalid]] {
+            assert_eq!(view.sample_repeat_opaque(uv), Err(SamplingError::Coordinates));
+        }
+    }
+}
+
+#[test]
+fn reference_bound_retains_fractional_centers_and_maximum_edge() {
+    let view = TextureView::new(&PIXELS, [2, 2]).unwrap();
+    close(view.sample_repeat_opaque([MAX_REPEAT_COORDINATE - 0.75, -MAX_REPEAT_COORDINATE + 0.25]).unwrap(), [1., 0., 0.]);
+    let mut pixels = [0., 0., 0., 1.].repeat(MAX_TEXTURE_EDGE as usize);
+    pixels[0] = 1.;
+    let view = TextureView::new(&pixels, [MAX_TEXTURE_EDGE, 1]).unwrap();
+    let half_texel = 0.5 / f64::from(MAX_TEXTURE_EDGE);
+    close(view.sample_repeat_opaque([half_texel, 0.]).unwrap(), [1., 0., 0.]);
+    close(view.sample_repeat_opaque([half_texel - MAX_REPEAT_COORDINATE, 0.]).unwrap(), [1., 0., 0.]);
+}