oxipng/src/reduction/mod.rs

use crate::evaluate::Evaluator;
use crate::png::PngImage;
use crate::Deadline;
use crate::Deflaters;
use crate::Options;
use std::sync::Arc;

pub mod alpha;
use crate::alpha::*;
pub mod bit_depth;
use crate::bit_depth::*;
pub mod color;
use crate::color::*;
pub mod palette;
use crate::palette::*;

pub(crate) fn perform_reductions(
    mut png: Arc<PngImage>,
    opts: &Options,
    deadline: &Deadline,
    eval: &Evaluator,
) -> Arc<PngImage> {
    let mut evaluation_added = false;

    // At low compression levels, skip some transformations which are less likely to be effective
    // This currently affects optimization presets 0-2
    let cheap = match opts.deflate {
        Deflaters::Libdeflater { compression } => compression < 12 && opts.fast_evaluation,
        _ => false,
    };

    // Interlacing must be processed first in order to evaluate the rest correctly
    if let Some(interlacing) = opts.interlace {
        if let Some(reduced) = png.change_interlacing(interlacing) {
            png = Arc::new(reduced);
        }
    }

    // If alpha optimization is enabled, clean the alpha channel before continuing
    // This can allow some color type reductions which may not have been possible otherwise
    if opts.optimize_alpha && !deadline.passed() {
        if let Some(reduced) = cleaned_alpha_channel(&png) {
            png = Arc::new(reduced);
        }
    }

    // Attempt to reduce 16-bit to 8-bit
    // This is just removal of bytes and does not need to be evaluated
    if opts.bit_depth_reduction && !deadline.passed() {
        if let Some(reduced) = reduced_bit_depth_16_to_8(&png, opts.scale_16) {
            png = Arc::new(reduced);
        }
    }

    // Attempt to reduce RGB to grayscale
    // This is just removal of bytes and does not need to be evaluated
    if opts.color_type_reduction && opts.grayscale_reduction && !deadline.passed() {
        if let Some(reduced) = reduced_rgb_to_grayscale(&png) {
            png = Arc::new(reduced);
        }
    }

    // Attempt to expand the bit depth to 8
    // This does need to be evaluated but will be done so later when it gets reduced again
    if opts.bit_depth_reduction && !deadline.passed() {
        if let Some(reduced) = expanded_bit_depth_to_8(&png) {
            png = Arc::new(reduced);
        }
    }

    // Attempt to reduce the palette
    // This may change bytes but should always be beneficial
    if opts.palette_reduction && !deadline.passed() {
        if let Some(reduced) = reduced_palette(&png, opts.optimize_alpha) {
            png = Arc::new(reduced);
        }
    }

    // Now retain the current png for the evaluator baseline
    // It will only be entered into the evaluator if there are also others to evaluate
    let mut baseline = png.clone();

    // Attempt alpha removal
    if opts.color_type_reduction && !deadline.passed() {
        if let Some(reduced) = reduced_alpha_channel(&png, opts.optimize_alpha) {
            png = Arc::new(reduced);
            // For small differences, if a tRNS chunk is required then enter this into the evaluator
            // Otherwise it is mostly just removal of bytes and should become the baseline
            if png.ihdr.color_type.has_trns() && baseline.data.len() - png.data.len() <= 1000 {
                eval.try_image(png.clone());
                evaluation_added = true;
            } else {
                baseline = png.clone();
            }
        }
    }

    // Attempt to sort the palette
    if opts.palette_reduction && !deadline.passed() {
        if let Some(reduced) = sorted_palette(&png) {
            png = Arc::new(reduced);
            eval.try_image(png.clone());
            evaluation_added = true;
        }
    }

    // Attempt to convert from indexed to channels
    // This may give a better result due to dropping the PLTE chunk
    if !cheap && opts.color_type_reduction && !deadline.passed() {
        if let Some(reduced) = indexed_to_channels(&png, opts.grayscale_reduction) {
            // This result should not be passed on to subsequent reductions
            eval.try_image(Arc::new(reduced));
            evaluation_added = true;
        }
    }

    // Attempt to reduce to indexed
    let mut indexed = None;
    if opts.color_type_reduction && !deadline.passed() {
        if let Some(reduced) = reduced_to_indexed(&png, opts.grayscale_reduction) {
            // Make sure the palette gets sorted (but don't bother evaluating both results)
            let new = Arc::new(sorted_palette(&reduced).unwrap_or(reduced));
            // For relatively small differences, enter this into the evaluator
            // Otherwise we're confident enough for it to become the baseline
            if png.data.len() - new.data.len() <= INDEXED_MAX_DIFF {
                eval.try_image(new.clone());
                evaluation_added = true;
            } else {
                baseline = new.clone();
            }
            indexed = Some(new);
        }
    }

    // Attempt to sort the palette using an alternative method
    if !cheap && opts.palette_reduction && !deadline.passed() {
        if let Some(reduced) = sorted_palette_battiato(&png) {
            eval.try_image(Arc::new(reduced));
            evaluation_added = true;
        }
    }

    // Attempt to reduce to a lower bit depth
    if opts.bit_depth_reduction && !deadline.passed() {
        // Try reducing the previous png, falling back to the indexed one if it exists
        // This allows a grayscale depth reduction to be preferred over an indexed depth reduction
        let reduced = reduced_bit_depth_8_or_less(&png)
            .or_else(|| indexed.and_then(|png| reduced_bit_depth_8_or_less(&png)));
        if let Some(reduced) = reduced {
            eval.try_image(Arc::new(reduced));
            evaluation_added = true;
        }
    }

    if evaluation_added {
        eval.try_image(baseline.clone());
    }
    baseline
}