diff --git a/src/convert.rs b/src/convert.rs
new file mode 100644
index 0000000..95f68be
--- /dev/null
+++ b/src/convert.rs
@@ -0,0 +1,501 @@
+//! BGRx → RGB24 conversion with optional downscaling (box filter for integer factors,
+//! area averaging otherwise).
+
+/// Fixed-point unit of the area-average weights.
+const WEIGHT_BITS: u32 = 16;
+const WEIGHT_ONE: u32 = 1 << WEIGHT_BITS;
+
+#[derive(Debug, Clone, thiserror::Error, PartialEq, Eq)]
+pub enum ConvertError {
+    #[error("empty frame {width}x{height}")]
+    Empty { width: usize, height: usize },
+    #[error("stride {stride} is smaller than width*4 ({width}*4)")]
+    StrideTooSmall { stride: usize, width: usize },
+    #[error("{width}x{height} frame with stride {stride} at offset {offset} does not fit into {mapped} mapped bytes")]
+    OutOfBounds {
+        width: usize,
+        height: usize,
+        stride: usize,
+        offset: usize,
+        mapped: usize,
+    },
+    #[error("chunk size {chunk_size} is smaller than stride*height ({needed})")]
+    ChunkTooSmall { chunk_size: usize, needed: usize },
+}
+
+/// A bounds-checked view of a packed BGRx frame.
+#[derive(Debug, Clone, Copy)]
+pub struct BgrxFrame<'a> {
+    /// Starts at the first pixel of row 0.
+    data: &'a [u8],
+    width: usize,
+    height: usize,
+    stride: usize,
+}
+
+impl<'a> BgrxFrame<'a> {
+    /// Validates the frame geometry against the mapped memory and the chunk size.
+    pub fn new(
+        mapped: &'a [u8],
+        offset: usize,
+        chunk_size: usize,
+        stride: usize,
+        width: usize,
+        height: usize,
+    ) -> Result<Self, ConvertError> {
+        if width == 0 || height == 0 {
+            return Err(ConvertError::Empty { width, height });
+        }
+        let row_bytes = width
+            .checked_mul(4)
+            .ok_or(ConvertError::StrideTooSmall { stride, width })?;
+        if stride < row_bytes {
+            return Err(ConvertError::StrideTooSmall { stride, width });
+        }
+        let out_of_bounds = ConvertError::OutOfBounds {
+            width,
+            height,
+            stride,
+            offset,
+            mapped: mapped.len(),
+        };
+        let end = stride
+            .checked_mul(height - 1)
+            .and_then(|v| v.checked_add(row_bytes))
+            .and_then(|v| v.checked_add(offset))
+            .ok_or_else(|| out_of_bounds.clone())?;
+        if end > mapped.len() {
+            return Err(out_of_bounds);
+        }
+        match stride.checked_mul(height) {
+            Some(needed) if chunk_size >= needed => {}
+            needed => {
+                return Err(ConvertError::ChunkTooSmall {
+                    chunk_size,
+                    needed: needed.unwrap_or(usize::MAX),
+                })
+            }
+        }
+        Ok(Self {
+            data: &mapped[offset..end],
+            width,
+            height,
+            stride,
+        })
+    }
+
+    pub fn width(&self) -> usize {
+        self.width
+    }
+
+    pub fn height(&self) -> usize {
+        self.height
+    }
+
+    fn row(&self, y: usize) -> &'a [u8] {
+        let start = y * self.stride;
+        &self.data[start..start + self.width * 4]
+    }
+}
+
+/// Largest size that fits into `max_w`x`max_h` with the aspect ratio of `src_w`x`src_h`.
+/// Never upscales.
+pub fn fit_within(src_w: u32, src_h: u32, max_w: u32, max_h: u32) -> (u32, u32) {
+    if src_w <= max_w && src_h <= max_h {
+        return (src_w, src_h);
+    }
+    let scale = f64::min(
+        f64::from(max_w) / f64::from(src_w),
+        f64::from(max_h) / f64::from(src_h),
+    );
+    let w = (f64::from(src_w) * scale)
+        .round()
+        .clamp(1.0, f64::from(max_w)) as u32;
+    let h = (f64::from(src_h) * scale)
+        .round()
+        .clamp(1.0, f64::from(max_h)) as u32;
+    (w, h)
+}
+
+/// Source taps of one axis for area averaging: for destination index `i`,
+/// `taps[starts[i]..starts[i + 1]]` holds `(source index, weight)` with weights summing
+/// to `WEIGHT_ONE`.
+#[derive(Debug, Default)]
+struct AxisTaps {
+    starts: Vec<usize>,
+    taps: Vec<(usize, u32)>,
+}
+
+impl AxisTaps {
+    fn build(src: usize, dst: usize) -> Self {
+        let scale = src as f64 / dst as f64;
+        let mut starts = Vec::with_capacity(dst + 1);
+        let mut taps = Vec::new();
+        for i in 0..dst {
+            starts.push(taps.len());
+            let begin = i as f64 * scale;
+            let end = (i + 1) as f64 * scale;
+            let first = begin.floor() as usize;
+            let last = (end.ceil() as usize).min(src);
+            let from = taps.len();
+            for j in first..last {
+                let overlap = end.min(j as f64 + 1.0) - begin.max(j as f64);
+                let weight = (overlap / scale * f64::from(WEIGHT_ONE)).round() as u32;
+                if weight > 0 {
+                    taps.push((j, weight));
+                }
+            }
+            // Make the weights sum to exactly WEIGHT_ONE so flat areas stay flat.
+            let sum: u32 = taps[from..].iter().map(|t| t.1).sum();
+            if let Some(max) = taps[from..].iter_mut().max_by_key(|t| t.1) {
+                max.1 = (max.1 + WEIGHT_ONE).saturating_sub(sum);
+            } else {
+                taps.push((first.min(src - 1), WEIGHT_ONE));
+            }
+        }
+        starts.push(taps.len());
+        Self { starts, taps }
+    }
+
+    fn get(&self, i: usize) -> &[(usize, u32)] {
+        &self.taps[self.starts[i]..self.starts[i + 1]]
+    }
+}
+
+/// Reusable converter; keeps its scratch buffers and filter taps between frames.
+#[derive(Debug, Default)]
+pub struct Converter {
+    geometry: (usize, usize, usize, usize),
+    x_taps: AxisTaps,
+    y_taps: AxisTaps,
+    row: Vec<u32>,
+    acc: Vec<u64>,
+}
+
+impl Converter {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Converts `src` to RGB24 of `dst_w`x`dst_h` into `out` (resized as needed).
+    pub fn convert(&mut self, src: &BgrxFrame<'_>, dst_w: usize, dst_h: usize, out: &mut Vec<u8>) {
+        assert!(dst_w > 0 && dst_h > 0, "destination must not be empty");
+        out.resize(dst_w * dst_h * 3, 0);
+        let (sw, sh) = (src.width, src.height);
+        if (sw, sh) == (dst_w, dst_h) {
+            copy_rgb(src, out);
+        } else if sw % dst_w == 0 && sh % dst_h == 0 {
+            self.box_filter(src, sw / dst_w, sh / dst_h, dst_w, out);
+        } else {
+            self.area_average(src, dst_w, dst_h, out);
+        }
+    }
+
+    fn box_filter(
+        &mut self,
+        src: &BgrxFrame<'_>,
+        fx: usize,
+        fy: usize,
+        dst_w: usize,
+        out: &mut [u8],
+    ) {
+        let n = (fx * fy) as u32;
+        let sums = &mut self.row;
+        sums.clear();
+        sums.resize(dst_w * 3, 0);
+        for (dy, out_row) in out.chunks_exact_mut(dst_w * 3).enumerate() {
+            sums.fill(0);
+            for sy in dy * fy..(dy + 1) * fy {
+                let row = src.row(sy);
+                for (block, sum) in row.chunks_exact(fx * 4).zip(sums.chunks_exact_mut(3)) {
+                    for px in block.chunks_exact(4) {
+                        sum[0] += u32::from(px[2]);
+                        sum[1] += u32::from(px[1]);
+                        sum[2] += u32::from(px[0]);
+                    }
+                }
+            }
+            for (o, s) in out_row.iter_mut().zip(sums.iter()) {
+                *o = ((s + n / 2) / n) as u8;
+            }
+        }
+    }
+
+    fn area_average(&mut self, src: &BgrxFrame<'_>, dst_w: usize, dst_h: usize, out: &mut [u8]) {
+        let geometry = (src.width, src.height, dst_w, dst_h);
+        if self.geometry != geometry {
+            self.x_taps = AxisTaps::build(src.width, dst_w);
+            self.y_taps = AxisTaps::build(src.height, dst_h);
+            self.geometry = geometry;
+        }
+        self.row.clear();
+        self.row.resize(dst_w * 3, 0);
+        self.acc.clear();
+        self.acc.resize(dst_w * 3, 0);
+
+        let round = 1u64 << (2 * WEIGHT_BITS - 1);
+        for (dy, out_row) in out.chunks_exact_mut(dst_w * 3).enumerate() {
+            self.acc.fill(0);
+            for &(sy, wy) in self.y_taps.get(dy) {
+                let src_row = src.row(sy);
+                // Horizontal pass: values in [0, 255 * WEIGHT_ONE].
+                for (dx, rgb) in self.row.chunks_exact_mut(3).enumerate() {
+                    let (mut r, mut g, mut b) = (0u32, 0u32, 0u32);
+                    for &(sx, wx) in self.x_taps.get(dx) {
+                        let px = &src_row[sx * 4..sx * 4 + 4];
+                        r += wx * u32::from(px[2]);
+                        g += wx * u32::from(px[1]);
+                        b += wx * u32::from(px[0]);
+                    }
+                    rgb[0] = r;
+                    rgb[1] = g;
+                    rgb[2] = b;
+                }
+                let wy = u64::from(wy);
+                for (a, v) in self.acc.iter_mut().zip(self.row.iter()) {
+                    *a += wy * u64::from(*v);
+                }
+            }
+            for (o, a) in out_row.iter_mut().zip(self.acc.iter()) {
+                *o = ((a + round) >> (2 * WEIGHT_BITS)) as u8;
+            }
+        }
+    }
+}
+
+fn copy_rgb(src: &BgrxFrame<'_>, out: &mut [u8]) {
+    for (y, out_row) in out.chunks_exact_mut(src.width * 3).enumerate() {
+        for (px, rgb) in src.row(y).chunks_exact(4).zip(out_row.chunks_exact_mut(3)) {
+            rgb[0] = px[2];
+            rgb[1] = px[1];
+            rgb[2] = px[0];
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Builds a BGRx buffer with padding and offset filled with garbage.
+    fn make_buffer(
+        width: usize,
+        height: usize,
+        stride: usize,
+        offset: usize,
+        pixel: impl Fn(usize, usize) -> [u8; 3],
+    ) -> Vec<u8> {
+        let mut buf = vec![0xEE; offset + stride * height];
+        for y in 0..height {
+            for x in 0..width {
+                let [r, g, b] = pixel(x, y);
+                let i = offset + y * stride + x * 4;
+                buf[i..i + 4].copy_from_slice(&[b, g, r, 0xFF]);
+            }
+        }
+        buf
+    }
+
+    fn convert(
+        buf: &[u8],
+        offset: usize,
+        stride: usize,
+        w: usize,
+        h: usize,
+        dw: usize,
+        dh: usize,
+    ) -> Vec<u8> {
+        let frame = BgrxFrame::new(buf, offset, buf.len() - offset, stride, w, h).unwrap();
+        let mut out = Vec::new();
+        Converter::new().convert(&frame, dw, dh, &mut out);
+        out
+    }
+
+    /// Reference area average in floating point.
+    fn reference(
+        w: usize,
+        h: usize,
+        dw: usize,
+        dh: usize,
+        pixel: impl Fn(usize, usize) -> [u8; 3],
+    ) -> Vec<u8> {
+        let (sx, sy) = (w as f64 / dw as f64, h as f64 / dh as f64);
+        let overlap =
+            |a0: f64, a1: f64, j: usize| (a1.min(j as f64 + 1.0) - a0.max(j as f64)).max(0.0);
+        let mut out = Vec::new();
+        for dy in 0..dh {
+            for dx in 0..dw {
+                let (x0, x1, y0, y1) = (
+                    dx as f64 * sx,
+                    (dx + 1) as f64 * sx,
+                    dy as f64 * sy,
+                    (dy + 1) as f64 * sy,
+                );
+                let mut acc = [0f64; 3];
+                let xs = x0.floor() as usize..(x1.ceil() as usize).min(w);
+                for y in y0.floor() as usize..(y1.ceil() as usize).min(h) {
+                    for x in xs.clone() {
+                        let wgt = overlap(x0, x1, x) * overlap(y0, y1, y);
+                        let p = pixel(x, y);
+                        for c in 0..3 {
+                            acc[c] += wgt * f64::from(p[c]);
+                        }
+                    }
+                }
+                for a in acc {
+                    out.push((a / (sx * sy)).round() as u8);
+                }
+            }
+        }
+        out
+    }
+
+    fn pattern(x: usize, y: usize) -> [u8; 3] {
+        [
+            (x * 37 + y * 11) as u8,
+            (x * 5 + y * 71) as u8,
+            ((x ^ y) * 29) as u8,
+        ]
+    }
+
+    #[test]
+    fn identity_with_padding_and_offset() {
+        let (w, h, stride, offset) = (3, 2, 16, 5);
+        let buf = make_buffer(w, h, stride, offset, pattern);
+        let out = convert(&buf, offset, stride, w, h, w, h);
+        let expected: Vec<u8> = (0..h)
+            .flat_map(|y| (0..w).flat_map(move |x| pattern(x, y)))
+            .collect();
+        assert_eq!(out, expected);
+    }
+
+    #[test]
+    fn identity_odd_size() {
+        let (w, h) = (7, 5);
+        let buf = make_buffer(w, h, w * 4, 0, pattern);
+        let out = convert(&buf, 0, w * 4, w, h, w, h);
+        let expected: Vec<u8> = (0..h)
+            .flat_map(|y| (0..w).flat_map(move |x| pattern(x, y)))
+            .collect();
+        assert_eq!(out, expected);
+    }
+
+    #[test]
+    fn box_filter_factor_2() {
+        let values = [
+            [0u8, 10, 20, 30],
+            [40, 50, 60, 70],
+            [100, 100, 0, 0],
+            [100, 101, 0, 0],
+        ];
+        let buf = make_buffer(4, 4, 16, 0, |x, y| [values[y][x], 255 - values[y][x], 7]);
+        let out = convert(&buf, 0, 16, 4, 4, 2, 2);
+        // (0+10+40+50)/4 = 25, (20+30+60+70)/4 = 45, (100+100+100+101)/4 = 100.25, 0
+        let r: Vec<u8> = out.chunks(3).map(|p| p[0]).collect();
+        assert_eq!(r, [25, 45, 100, 0]);
+        let g: Vec<u8> = out.chunks(3).map(|p| p[1]).collect();
+        assert_eq!(g, [230, 210, 155, 255]);
+        assert!(out.chunks(3).all(|p| p[2] == 7));
+    }
+
+    #[test]
+    fn box_filter_factor_3_with_padding() {
+        let (w, h, stride, offset) = (6, 3, 28, 8);
+        let buf = make_buffer(w, h, stride, offset, |x, y| [(x + y * 6) as u8 * 10, 0, 0]);
+        let out = convert(&buf, offset, stride, w, h, 2, 1);
+        // Left block: x 0..3, all rows: mean of {0,10,20,60,70,80,120,130,140} = 70
+        // Right block: +30 = 100
+        assert_eq!(out, [70, 0, 0, 100, 0, 0]);
+    }
+
+    #[test]
+    fn area_average_non_integer_known_values() {
+        let values = [0u8, 90, 180];
+        let buf = make_buffer(3, 1, 12, 0, |x, _| [values[x], values[x], values[x]]);
+        let out = convert(&buf, 0, 12, 3, 1, 2, 1);
+        assert_eq!(out, [30, 30, 30, 150, 150, 150]);
+    }
+
+    #[test]
+    fn area_average_matches_reference() {
+        for &(w, h, dw, dh) in &[
+            (5, 3, 2, 2),
+            (322, 182, 320, 181),
+            (17, 9, 4, 3),
+            (10, 10, 3, 7),
+        ] {
+            let buf = make_buffer(w, h, w * 4, 0, pattern);
+            let out = convert(&buf, 0, w * 4, w, h, dw, dh);
+            let expected = reference(w, h, dw, dh, pattern);
+            assert_eq!(out.len(), expected.len());
+            for (i, (a, b)) in out.iter().zip(expected.iter()).enumerate() {
+                assert!(
+                    a.abs_diff(*b) <= 1,
+                    "{w}x{h}->{dw}x{dh} byte {i}: {a} vs {b}"
+                );
+            }
+        }
+    }
+
+    #[test]
+    fn flat_color_stays_flat() {
+        let buf = make_buffer(641, 359, 641 * 4, 0, |_, _| [200, 17, 255]);
+        let out = convert(&buf, 0, 641 * 4, 641, 359, 320, 180);
+        assert!(out.chunks(3).all(|p| p == [200, 17, 255]));
+    }
+
+    #[test]
+    fn converter_reuse_across_geometries() {
+        let mut conv = Converter::new();
+        let mut out = Vec::new();
+        for &(w, h, dw, dh) in &[(9, 9, 4, 4), (8, 8, 4, 4), (9, 9, 4, 4), (5, 5, 5, 5)] {
+            let buf = make_buffer(w, h, w * 4, 0, pattern);
+            let frame = BgrxFrame::new(&buf, 0, buf.len(), w * 4, w, h).unwrap();
+            conv.convert(&frame, dw, dh, &mut out);
+            let expected = reference(w, h, dw, dh, pattern);
+            assert!(out
+                .iter()
+                .zip(expected.iter())
+                .all(|(a, b)| a.abs_diff(*b) <= 1));
+        }
+    }
+
+    #[test]
+    fn bounds_violations_are_errors() {
+        let buf = vec![0u8; 64];
+        assert!(matches!(
+            BgrxFrame::new(&buf, 0, 64, 16, 0, 4),
+            Err(ConvertError::Empty { .. })
+        ));
+        assert!(matches!(
+            BgrxFrame::new(&buf, 0, 64, 15, 4, 4),
+            Err(ConvertError::StrideTooSmall { .. })
+        ));
+        assert!(matches!(
+            BgrxFrame::new(&buf, 1, 64, 16, 4, 4),
+            Err(ConvertError::OutOfBounds { .. })
+        ));
+        assert!(matches!(
+            BgrxFrame::new(&buf, 0, 63, 16, 4, 4),
+            Err(ConvertError::ChunkTooSmall { .. })
+        ));
+        assert!(BgrxFrame::new(&buf, 0, 64, 16, 4, 4).is_ok());
+        // Last row without padding still fits.
+        assert!(BgrxFrame::new(&buf[..60], 0, 64, 16, 3, 4).is_ok());
+        // Overflowing geometry must not panic.
+        assert!(BgrxFrame::new(&buf, usize::MAX, 64, 16, 4, 4).is_err());
+        assert!(BgrxFrame::new(&buf, 0, usize::MAX, usize::MAX, 4, usize::MAX).is_err());
+        assert!(BgrxFrame::new(&buf, 0, 64, 16, usize::MAX, 1).is_err());
+    }
+
+    #[test]
+    fn fit_within_keeps_aspect() {
+        assert_eq!(fit_within(3840, 2160, 320, 180), (320, 180));
+        assert_eq!(fit_within(640, 360, 320, 180), (320, 180));
+        assert_eq!(fit_within(320, 134, 320, 180), (320, 134));
+        assert_eq!(fit_within(322, 182, 321, 181), (320, 181));
+        assert_eq!(fit_within(1080, 1920, 320, 180), (101, 180));
+        assert_eq!(fit_within(100, 100, 320, 180), (100, 100));
+        assert_eq!(fit_within(10000, 1, 320, 180), (320, 1));
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 8f52000..d126966 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,6 +2,7 @@
 //! HyperHDR's flatbuffers server.
 
 pub mod config;
+pub mod convert;
 pub mod hyperhdr;
 pub mod pacing;
 pub mod slot;