zenpixels_convert/

output.rs

//! Atomic output preparation for encoders.
//!
//! [`finalize_for_output`] converts pixel data and generates matching metadata
//! in a single atomic operation, preventing the most common color management
//! bug: pixel values that don't match the embedded color metadata.
//!
//! # Why atomicity matters
//!
//! The most common color management bug looks like this:
//!
//! ```rust,ignore
//! // BUG: pixels and metadata can diverge
//! let pixels = convert_to_p3(&buffer);
//! let metadata = OutputMetadata { icc: Some(srgb_icc), .. };
//! // ^^^ pixels are Display P3 but metadata says sRGB — wrong!
//! ```
//!
//! [`finalize_for_output`] prevents this by producing the pixels and metadata
//! together. The [`EncodeReady`] struct bundles both, and the only way to
//! create one is through this function. If the conversion fails, neither
//! pixels nor metadata are produced.
//!
//! # Usage
//!
//! ```rust,ignore
//! use zenpixels_convert::{
//!     finalize_for_output, OutputProfile, PixelFormat,
//! };
//!
//! let ready = finalize_for_output(
//!     &buffer,              // source pixel data
//!     &color_origin,        // how the source described its color
//!     OutputProfile::SameAsOrigin,  // re-embed original metadata
//!     PixelFormat::Rgb8,    // target byte layout
//!     &cms,                 // CMS impl (for ICC transforms)
//! )?;
//!
//! // Write pixels — these match the metadata
//! encoder.write_pixels(ready.pixels())?;
//!
//! // Embed color metadata — guaranteed to match the pixels
//! if let Some(icc) = &ready.metadata().icc {
//!     encoder.write_icc_chunk(icc)?;
//! }
//! if let Some(cicp) = &ready.metadata().cicp {
//!     encoder.write_cicp(cicp)?;
//! }
//! if let Some(hdr) = &ready.metadata().hdr {
//!     encoder.write_hdr_metadata(hdr)?;
//! }
//! ```
//!
//! # Output profiles
//!
//! [`OutputProfile`] controls what color space the output should be in:
//!
//! - **`SameAsOrigin`**: Re-embed the original ICC/CICP from the source file.
//!   Pixels are converted only if the target pixel format differs from the
//!   source. Used for transcoding without color changes. If the source had
//!   an ICC profile, it is passed through; if CICP, the CICP codes are
//!   preserved.
//!
//! - **`Named(cicp)`**: Convert to a well-known CICP profile (sRGB, Display P3,
//!   BT.2020, PQ, HLG). Uses hardcoded 3×3 gamut matrices and transfer
//!   function conversion via `RowConverter` — no CMS needed. Fast and
//!   deterministic.
//!
//! - **`Icc(bytes)`**: Convert to a specific ICC profile. Requires a
//!   [`ColorManagement`] implementation to build the source→destination
//!   transform. Use this for print workflows, custom profiles, or any
//!   profile that isn't a standard CICP combination.
//!
//! # CMS requirement
//!
//! The `cms` parameter is only used when:
//! - `OutputProfile::Icc` is selected, or
//! - `OutputProfile::SameAsOrigin` and the source has an ICC profile.
//!
//! For `OutputProfile::Named`, the CMS is unused — gamut conversion uses
//! hardcoded matrices. Codecs that don't need ICC support can pass a
//! no-op CMS implementation.

use alloc::sync::Arc;

use crate::cms::ColorManagement;
use crate::error::ConvertError;
use crate::hdr::HdrMetadata;
use crate::{
    Cicp, ColorAuthority, ColorOrigin, ColorPrimaries, PixelBuffer, PixelDescriptor, PixelFormat,
    PixelSlice, TransferFunction,
};
use whereat::{At, ResultAtExt};

/// Target output color profile.
#[derive(Clone, Debug)]
#[non_exhaustive]
pub enum OutputProfile {
    /// Re-encode with the original ICC/CICP from the source file.
    SameAsOrigin,
    /// Use a well-known CICP-described profile.
    Named(Cicp),
    /// Use specific ICC profile bytes.
    Icc(Arc<[u8]>),
}

// TODO(0.3.0): Add HdrPolicy enum and ConvertOutputOptions here once
// ConvertError is #[non_exhaustive] and can carry HdrTransferRequiresToneMapping.
// See imazen/zenpixels#10 for the full HDR provenance plan.

/// Metadata that the encoder should embed alongside the pixel data.
///
/// Generated atomically by [`finalize_for_output`] to guarantee that
/// the metadata matches the pixel values.
#[derive(Clone, Debug)]
#[non_exhaustive]
pub struct OutputMetadata {
    /// ICC profile bytes to embed, if any.
    pub icc: Option<Arc<[u8]>>,
    /// CICP code points to embed, if any.
    pub cicp: Option<Cicp>,
    /// HDR metadata to embed (content light level, mastering display), if any.
    pub hdr: Option<HdrMetadata>,
}

/// Pixel data bundled with matching metadata, ready for encoding.
///
/// The only way to create an `EncodeReady` is through [`finalize_for_output`],
/// which guarantees that the pixels and metadata are consistent.
///
/// Use [`into_parts()`](Self::into_parts) to destructure if needed, but
/// the default path keeps them coupled.
#[non_exhaustive]
pub struct EncodeReady {
    pixels: PixelBuffer,
    metadata: OutputMetadata,
}

impl EncodeReady {
    /// Borrow the pixel data.
    pub fn pixels(&self) -> PixelSlice<'_> {
        self.pixels.as_slice()
    }

    /// Borrow the output metadata.
    pub fn metadata(&self) -> &OutputMetadata {
        &self.metadata
    }

    /// Consume and split into pixel buffer and metadata.
    pub fn into_parts(self) -> (PixelBuffer, OutputMetadata) {
        (self.pixels, self.metadata)
    }
}

/// Atomically convert pixel data and generate matching encoder metadata.
///
/// This function does three things as a single operation:
///
/// 1. Determines the current pixel color state from `PixelDescriptor` +
///    optional ICC profile on `ColorContext`.
/// 2. Converts pixels to the target profile's space. For named profiles,
///    uses hardcoded matrices. For custom ICC profiles, uses the CMS.
/// 3. Bundles the converted pixels with matching metadata ([`EncodeReady`]).
///
/// # Arguments
///
/// - `buffer` — Source pixel data with its current descriptor.
/// - `origin` — How the source file described its color (for `SameAsOrigin`).
/// - `target` — Desired output color profile.
/// - `pixel_format` — Target pixel format for the output.
/// - `cms` — Color management system for ICC profile transforms.
///
/// # Errors
///
/// Returns [`ConvertError`] if:
/// - The target format requires a conversion that isn't supported.
/// - The CMS fails to build a transform for ICC profiles.
/// - Buffer allocation fails.
// TODO(0.3.0): Add HDR→SDR policy gate here once ConvertError has
// HdrTransferRequiresToneMapping. See imazen/zenpixels#10.
#[track_caller]
pub fn finalize_for_output<C: ColorManagement>(
    buffer: &PixelBuffer,
    origin: &ColorOrigin,
    target: OutputProfile,
    pixel_format: PixelFormat,
    cms: &C,
) -> Result<EncodeReady, At<ConvertError>> {
    let source_desc = buffer.descriptor();
    let target_desc = pixel_format.descriptor();

    // Determine output metadata based on target profile.
    let (metadata, needs_cms_transform) = match &target {
        OutputProfile::SameAsOrigin => {
            let metadata = OutputMetadata {
                icc: origin.icc.clone(),
                cicp: origin.cicp,
                hdr: None,
            };
            // SameAsOrigin = keep the source color space. No CMS conversion.
            // Pixel format changes (depth, layout) are handled by RowConverter.
            (metadata, false)
        }
        OutputProfile::Named(cicp) => {
            let metadata = OutputMetadata {
                icc: None,
                cicp: Some(*cicp),
                hdr: None,
            };
            (metadata, false)
        }
        OutputProfile::Icc(icc) => {
            let metadata = OutputMetadata {
                icc: Some(icc.clone()),
                cicp: None,
                hdr: None,
            };
            (metadata, true)
        }
    };

    // Apply CMS transform if needed, respecting color_authority.
    if needs_cms_transform
        && let Some(transform) =
            build_cms_transform(origin, &metadata, &source_desc, pixel_format, cms)?
    {
        let src_slice = buffer.as_slice();
        let mut out = PixelBuffer::try_new(buffer.width(), buffer.height(), target_desc)
            .map_err(|_| whereat::at!(ConvertError::AllocationFailed))?;

        {
            let mut dst_slice = out.as_slice_mut();
            for y in 0..buffer.height() {
                let src_row = src_slice.row(y);
                let dst_row = dst_slice.row_mut(y);
                transform.transform_row(src_row, dst_row, buffer.width());
            }
        }

        return Ok(EncodeReady {
            pixels: out,
            metadata,
        });
    }

    // Named profile conversion: use hardcoded matrices via RowConverter.
    let target_desc_full = target_desc
        .with_transfer(resolve_transfer(&target, &source_desc))
        .with_primaries(resolve_primaries(&target, &source_desc));

    if source_desc.layout_compatible(target_desc_full)
        && descriptors_match(&source_desc, &target_desc_full)
    {
        // No conversion needed — copy the buffer.
        let src_slice = buffer.as_slice();
        let bytes = src_slice.contiguous_bytes();
        let out = PixelBuffer::from_vec(
            bytes.into_owned(),
            buffer.width(),
            buffer.height(),
            target_desc_full,
        )
        .map_err(|_| whereat::at!(ConvertError::AllocationFailed))?;
        return Ok(EncodeReady {
            pixels: out,
            metadata,
        });
    }

    // Use RowConverter for format conversion.
    let mut converter = crate::RowConverter::new(source_desc, target_desc_full).at()?;
    let src_slice = buffer.as_slice();
    let mut out = PixelBuffer::try_new(buffer.width(), buffer.height(), target_desc_full)
        .map_err(|_| whereat::at!(ConvertError::AllocationFailed))?;

    {
        let mut dst_slice = out.as_slice_mut();
        for y in 0..buffer.height() {
            let src_row = src_slice.row(y);
            let dst_row = dst_slice.row_mut(y);
            converter.convert_row(src_row, dst_row, buffer.width());
        }
    }

    Ok(EncodeReady {
        pixels: out,
        metadata,
    })
}

/// Build a CMS transform from the origin's color metadata.
///
/// Respects [`ColorAuthority`]: when `Icc`, builds from ICC bytes; when `Cicp`,
/// builds from CICP codes via the CMS's `build_transform_from_cicp`. Falls
/// back to the non-authoritative field when the authoritative one is missing.
///
/// Returns `Ok(None)` when no source profile can be determined.
fn build_cms_transform<C: ColorManagement>(
    origin: &ColorOrigin,
    metadata: &OutputMetadata,
    source_desc: &PixelDescriptor,
    dst_format: PixelFormat,
    cms: &C,
) -> Result<Option<alloc::boxed::Box<dyn crate::cms::RowTransform>>, At<ConvertError>> {
    let src_format = source_desc.format;
    let Some(ref dst_icc) = metadata.icc else {
        return Ok(None);
    };

    // Try ICC path first (or second, depending on authority).
    let try_icc = |src_icc: &[u8]| -> Result<Option<_>, At<ConvertError>> {
        let transform = cms
            .build_transform_for_format(src_icc, dst_icc, src_format, dst_format)
            .map_err(|e| whereat::at!(ConvertError::CmsError(alloc::format!("{e:?}"))))?;
        Ok(Some(transform))
    };

    match origin.color_authority {
        ColorAuthority::Icc => {
            if let Some(ref src_icc) = origin.icc {
                return try_icc(src_icc);
            }
            // Fallback: ICC authority but no ICC bytes — can't build transform.
            Ok(None)
        }
        ColorAuthority::Cicp => {
            // CICP authority — but build_transform_from_cicp needs ICC bytes
            // on the dst side, so we still need ICC. Try src ICC if available.
            if let Some(ref src_icc) = origin.icc {
                return try_icc(src_icc);
            }
            Ok(None)
        }
    }
}

// TODO(0.3.0): restore origin_has_hdr_transfer / target_has_hdr_transfer
// helpers here for the HDR→SDR policy gate.

/// Resolve the target transfer function.
fn resolve_transfer(target: &OutputProfile, source: &PixelDescriptor) -> TransferFunction {
    match target {
        OutputProfile::SameAsOrigin => source.transfer(),
        OutputProfile::Named(cicp) => TransferFunction::from_cicp(cicp.transfer_characteristics)
            .unwrap_or(TransferFunction::Unknown),
        OutputProfile::Icc(_) => TransferFunction::Unknown,
    }
}

/// Resolve the target color primaries.
fn resolve_primaries(target: &OutputProfile, source: &PixelDescriptor) -> ColorPrimaries {
    match target {
        OutputProfile::SameAsOrigin => source.primaries,
        OutputProfile::Named(cicp) => {
            ColorPrimaries::from_cicp(cicp.color_primaries).unwrap_or(ColorPrimaries::Unknown)
        }
        OutputProfile::Icc(_) => ColorPrimaries::Unknown,
    }
}

/// Check if two descriptors match in all conversion-relevant fields.
fn descriptors_match(a: &PixelDescriptor, b: &PixelDescriptor) -> bool {
    a.format == b.format
        && a.transfer == b.transfer
        && a.primaries == b.primaries
        && a.signal_range == b.signal_range
}