Add "pixel filter" to only sample part of images

Mårten Åsberg committed on 2026-08-16 at 07:58

Colors/example/Program.cs +1 -1

diff --git a/Colors/example/Program.cs b/Colors/example/Program.cs
index 5251dde..d501187 100644
@@ -7,7 +7,7 @@ using SixLabors.ImageSharp.PixelFormats;
using var image = Image.Load<Rgba32>(args.Length > 0 ? args[0] : @".\images\example1.jpg");
image.ConvertToScaledOklch();
var palette = image.GetPalette();
var palette = image.GetPalette(new(image.Width / 3, 2 * image.Width / 3, image.Height / 3, 2 * image.Height / 3));
Console.WriteLine(
    " "

Colors/src/ColorAnalyzer.cs +2 -2

diff --git a/Colors/src/ColorAnalyzer.cs b/Colors/src/ColorAnalyzer.cs
index 5064119..ee4a917 100644
@@ -8,9 +8,9 @@ public static class ColorAnalyzer
{
    extension(Image<Rgba32> image)
    {
        public (Rgba32 color, float proportion)[] GetPalette()
        public (Rgba32 color, float proportion)[] GetPalette(PixelFilter pixelFilter)
        {
            var quantized = image.Quantize(6);
            var quantized = image.Quantize(pixelFilter, 6);
            var total = quantized.Values.Aggregate(0.0f, (t, v) => t + v);
            return [.. quantized.Select(kvp => (kvp.Key, kvp.Value / total)).OrderByDescending(p => p.Item2)];
        }

Colors/src/HistogramBuilder.cs +0 -28

diff --git a/Colors/src/HistogramBuilder.cs b/Colors/src/HistogramBuilder.cs
deleted file mode 100644
index 200568e..0000000
@@ -1,28 +0,0 @@
using System;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.PixelFormats;
namespace Mosaic.Colors;
internal class HistogramBuilder()
{
    public readonly Memory<int> Histogram = new int[MmcqQuantizer.HistogramSize];
    public void Populate(PixelAccessor<Rgba32> accessor)
    {
        var histogram = Histogram.Span;
        for (var y = 0; y < accessor.Height; y++)
        {
            var row = accessor.GetRowSpan(y);
            for (var x = 0; x < row.Length; x++)
            {
                var pixel = row[x];
                var r = pixel.R >> MmcqQuantizer.Shift;
                var g = pixel.G >> MmcqQuantizer.Shift;
                var b = pixel.B >> MmcqQuantizer.Shift;
                histogram[MmcqQuantizer.GetColorIndex(r, g, b)]++;
            }
        }
    }
}

Colors/src/MmcqQuantizer.cs +6 -14

diff --git a/Colors/src/MmcqQuantizer.cs b/Colors/src/MmcqQuantizer.cs
index 7e6f6aa..57b653f 100644
@@ -18,7 +18,7 @@ internal static class MmcqQuantizer
    extension(Image<Rgba32> image)
    {
        internal Dictionary<Rgba32, int> Quantize(int maxColorCount)
        internal Dictionary<Rgba32, int> Quantize(PixelFilter pixelFilter, int maxColorCount)
        {
            if (maxColorCount < 2 || 256 < maxColorCount)
            {
@@ -28,9 +28,8 @@ internal static class MmcqQuantizer
                );
            }
            var histogram = image.GetHistogram();
            var vBox = image.GetVbox(histogram);
            var vBox = image.GetVbox(pixelFilter);
            var histogram = vBox.Histogram;
            var queue = new PriorityQueue<VBox, VBox>(VBox.CountComparer);
            queue.Enqueue(vBox, vBox);
@@ -42,18 +41,11 @@ internal static class MmcqQuantizer
            return queue2.UnorderedItems.ToDictionary(p => p.Element.Average, p => p.Element.Count);
        }
        internal Memory<int> GetHistogram()
        {
            var builder = new HistogramBuilder();
            image.ProcessPixelRows(builder.Populate);
            return builder.Histogram;
        }
        internal VBox GetVbox(ReadOnlyMemory<int> histogram)
        internal VBox GetVbox(PixelFilter pixelFilter)
        {
            var builder = new VboxBuilder();
            var builder = new VboxBuilder(pixelFilter);
            image.ProcessPixelRows(builder.Populate);
            return builder.Build(histogram);
            return builder.Build();
        }
    }

Colors/src/PixelFilter.cs +13 -0

diff --git a/Colors/src/PixelFilter.cs b/Colors/src/PixelFilter.cs
new file mode 100644
index 0000000..6f6c307
@@ -0,0 +1,13 @@
namespace Mosaic.Colors;
public class PixelFilter(int minX, int maxX, int minY, int maxY)
{
    public int MinX { get; } = minX;
    private readonly int maxX = maxX;
    public int MinY { get; } = minY;
    private readonly int maxY = maxY;
    public int MaxY(int height) => int.Min(maxY, height);
    public int MaxX(int width) => int.Min(maxX, width);
}

Colors/src/VBox.cs +13 -7

diff --git a/Colors/src/VBox.cs b/Colors/src/VBox.cs
index 5faf1d3..f01f980 100644
@@ -5,21 +5,25 @@ using SixLabors.ImageSharp.PixelFormats;
namespace Mosaic.Colors;
internal class VboxBuilder
internal class VboxBuilder(PixelFilter filter)
{
    private int rMin = (int)MmcqQuantizer.MaxValue;
    public readonly Memory<int> Histogram = new int[MmcqQuantizer.HistogramSize];
    private int rMin = MmcqQuantizer.MaxValue;
    private int rMax = 0;
    private int gMin = (int)MmcqQuantizer.MaxValue;
    private int gMin = MmcqQuantizer.MaxValue;
    private int gMax = 0;
    private int bMin = (int)MmcqQuantizer.MaxValue;
    private int bMin = MmcqQuantizer.MaxValue;
    private int bMax = 0;
    public void Populate(PixelAccessor<Rgba32> accessor)
    {
        for (var y = 0; y < accessor.Height; y++)
        var histogram = Histogram.Span;
        for (var y = filter.MinY; y < filter.MaxY(accessor.Height); y++)
        {
            var row = accessor.GetRowSpan(y);
            for (var x = 0; x < row.Length; x++)
            for (var x = filter.MinX; x < filter.MaxX(row.Length); x++)
            {
                var pixel = row[x];
@@ -34,11 +38,13 @@ internal class VboxBuilder
                var b = pixel.B >> MmcqQuantizer.Shift;
                bMin = int.Min(bMin, b);
                bMax = int.Max(bMax, b);
                histogram[MmcqQuantizer.GetColorIndex(r, g, b)]++;
            }
        }
    }
    public VBox Build(ReadOnlyMemory<int> histogram) => new(rMin, rMax, gMin, gMax, bMin, bMax, histogram);
    public VBox Build() => new(rMin, rMax, gMin, gMax, bMin, bMax, Histogram);
}
internal class VBox