FyroxEngine/Fyrox · error

Vertex size cannot be larger than 256 bytes!

Error message

Vertex size cannot be larger than 256 bytes!

What it means

MeshBuffer::modify_vertices (and similar iterator helpers) copies the vertex data into a fixed-capacity ArrayVec of 256 bytes per vertex. Any vertex layout whose size exceeds 256 bytes cannot fit, so try_extend_from_slice fails and the code panics. This is a hard engine-side vertex size limit.

Solutions

1. Reduce the vertex layout below 256 bytes: drop unused attributes or shrink formats (e.g. f32 -> u8 normalized)
2. Move bulk per-vertex data into textures or instanced attributes instead
3. Split the mesh into multiple buffers if the layout genuinely needs more data

Example fix

// before
layout.push(VertexAttribute::Float4x4 /* + many others, > 256 bytes */);

// after
// keep total layout under 256 bytes; move matrices to instance data
layout.push(VertexAttribute::Float3);

let size: usize = layout.iter().map(|a| a.size() as usize).sum();
assert!(size <= 256, "vertex layout too large: {} bytes", size);

fn vertex_size_ok(layout: &[VertexAttribute]) -> bool { layout.iter().map(|a| a.size() as usize).sum::<usize>() <= 256 }

// check layout size before modify_vertices; panics here are by-design limits
if !vertex_size_ok(&layout) { /* reduce layout or split mesh */ }

When it happens

Trigger: Calling modify_vertices/modify on a vertex buffer whose VertexFormat layout totals more than 256 bytes per vertex (e.g. many custom attributes added via set_vertex_format).

Common situations: Custom shaders/materials with excessive per-vertex data (dozens of float4 attributes); procedurally generated buffers that pack large amounts of data per vertex instead of using textures or storage buffers.

Understand the failure class

Background: "value must be between 0 and 1" / "out of range" / "must not be negative" errors: fixing range-validation failures across open-source libraries — this error's family across 42 libraries.

Related errors

• Vertex size must match!
• Animation pool must be empty on load!
• Cast to failed!
• An object at index must be returned to a pool it was taken…
• Attempt to spawn an object at pool record with payload!…

AI-assisted analysis of FyroxEngine/Fyrox@76c91aad8e (2026-09-10). Data as JSON: /api/errors/58eeedd031d146e6. Report an issue: GitHub.

Appendix: source

Thrown at fyrox-impl/src/scene/mesh/buffer.rs:1256

}

/// A trait for read-only vertex data accessor.
pub trait VertexReadTrait {
    #[doc(hidden)]
    fn data_layout_ref(&self) -> (&[u8], &[Option<VertexAttribute>]);

    /// Clones the vertex and applies the given transformer closure to it and returns a stack-allocated
    /// data buffer representing the transformed vertex.
    #[inline(always)]
    fn transform<F>(&self, func: &mut F) -> ArrayVec<u8, 256>
    where
        F: FnMut(VertexViewMut),
    {
        let (data, layout) = self.data_layout_ref();
        let mut transformed = ArrayVec::new();
        transformed
            .try_extend_from_slice(data)
            .expect("Vertex size cannot be larger than 256 bytes!");
        func(VertexViewMut {
            vertex_data: &mut transformed,
            sparse_layout: layout,
        });
        transformed
    }

    /// Tries to read an attribute with given usage as a pair of two f32.
    #[inline(always)]
    fn read_2_f32(&self, usage: VertexAttributeUsage) -> Result<Vector2<f32>, VertexFetchError> {
        let (data, layout) = self.data_layout_ref();
        if let Some(attribute) = layout.get(usage as usize).unwrap() {
            let x = LittleEndian::read_f32(&data[(attribute.offset as usize)..]);
            let y = LittleEndian::read_f32(&data[(attribute.offset as usize + 4)..]);
            Ok(Vector2::new(x, y))
        } else {
            Err(VertexFetchError::NoSuchAttribute(usage))
        }

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