RotMG-inspired sprite upscaler. Converts 16x16 sprites to 80x80 with border and drop shadow.

Program.cs

using System;
using System.Diagnostics;
using System.IO;
using System.Threading.Tasks;
using SkiaSharp;

namespace AGES_Sprite_Processor
{
    internal class Program
    {
        const string inputFilePath  = @"C:\Users\Alde\Documents\GitHub\AGES\AGES-Demo\Assets\Entities\Runtime\Sprites\Items";
        const string outputFilePath = "Output/";

        private static SKColor DropShadowColor = new(0, 0, 0, 145);

        public static void Main(string[] args)
        {
            // Ensure file exists
            if (!Directory.Exists(inputFilePath))
            {
                Console.WriteLine($"Error, directory {inputFilePath} does not exist.");
                return;
            }

            var filesInDirectory = Directory.GetFiles(inputFilePath, "*.png", SearchOption.AllDirectories);

            Parallel.ForEach(filesInDirectory, s => { UpscaleSprite(s, outputFilePath); });

            // Open image in explorer
            Process.Start("explorer", $"/select,\"{AppDomain.CurrentDomain.BaseDirectory}");
        }
        
        private static void UpscaleSprite(string s, string outputFilePath)
        {
            // Load the image
            using var bitmap = SKBitmap.Decode(s);

            // Upscale the image
            var scaledBitmap = bitmap.Resize(new SKImageInfo(bitmap.Width * 4, bitmap.Height * 4), SKFilterQuality.None);

            // Calculate the padding for the shadow
            var shadowRadius = 2;
            var paddingX = shadowRadius + 4;
            var paddingY = shadowRadius + 4;

            // Create a new bitmap with added padding for the border
            var bitmapWithBorder = new SKBitmap(scaledBitmap.Width + 2 * paddingX, scaledBitmap.Height + 2 * paddingY);

            using var canvas = new SKCanvas(bitmapWithBorder);

            // Draw the image 8 times to create a border
            var paint = new SKPaint();
            paint.Color = new SKColor(0, 0, 0);
            for (int dx = -1; dx <= 1; dx++)
            {
                for (int dy = -1; dy <= 1; dy++)
                {
                    if (dx != 0 || dy != 0)
                    {
                        canvas.DrawBitmap(scaledBitmap, dx + paddingX, dy + paddingY, paint);
                    }
                }
            }

            // Replace all non-transparent pixels with black
            for (int x = 0; x < bitmapWithBorder.Width; x++)
            {
                for (int y = 0; y < bitmapWithBorder.Height; y++)
                {
                    var pixel = bitmapWithBorder.GetPixel(x, y);
                    if (pixel.Alpha != 0)
                    {
                        bitmapWithBorder.SetPixel(x, y, new SKColor(0, 0, 0));
                    }
                }
            }

            // Draw the main image over the top with padding
            canvas.DrawBitmap(scaledBitmap, paddingX, paddingY, paint);

            // Create bitmap for shadow with extra padding
            var shadowWidth = bitmapWithBorder.Width + 2 * shadowRadius;
            var shadowHeight = bitmapWithBorder.Height + 2 * shadowRadius;
            var shadowBitmap = new SKBitmap(shadowWidth, shadowHeight);

            // Create black paint
            var shadowPaint = new SKPaint
            {
                Color = new SKColor(0, 0, 0),
                ImageFilter = SKImageFilter.CreateDropShadow(0, 0, shadowRadius, shadowRadius, DropShadowColor)
            };

            using var shadowCanvas = new SKCanvas(shadowBitmap);

            // Draw shadow with padding
            shadowCanvas.DrawBitmap(bitmapWithBorder, shadowRadius, shadowRadius, shadowPaint);

            // Draw the main bitmap over the top with padding
            shadowCanvas.DrawBitmap(bitmapWithBorder, shadowRadius, shadowRadius, paint);

            // Save the image to disk
            using var image = SKImage.FromBitmap(shadowBitmap);
            using var data = image.Encode();

            var fileName = Path.GetFileNameWithoutExtension(s);
            var fileFolder = Path.GetDirectoryName(s);
            var subFolder = fileFolder.Replace(inputFilePath, "");
            var outputFolder = $"{outputFilePath}{subFolder}";
            var outputFile = $"{outputFolder}{Path.DirectorySeparatorChar}{fileName}-output.png";

            Console.WriteLine($"Would have outputted to {outputFile}");

            // Create the directory if it doesn't exist
            if (!Directory.Exists(outputFolder))
                Directory.CreateDirectory(outputFolder);

            File.WriteAllBytes(outputFile, data.ToArray());
        }
    }
}