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SnowViewModel.cs
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SnowViewModel.cs
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using System;
using System.Collections.Generic;
using System.IO;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Input;
using Avalonia;
using Avalonia.Media;
using Avalonia.Media.Imaging;
using Avalonia.Platform;
using SixLabors.ImageSharp;
using Color = Avalonia.Media.Color;
namespace AvaloniaCoreSnow
{
public class SnowViewModel
{
private const byte MaxSpeed = 200;
private int _flakeCount = 3000;
private Flake[] _flakes;
private readonly Random _rnd = new Random();
private readonly Action _invalidate;
private int _delayMs = 10;
public SnowViewModel(Action invalidate)
{
_invalidate = invalidate;
ResetCommand = new DelegateCommand(Reset);
// Bgra8888 is device-native and much faster.
Bitmap = new WriteableBitmap(new PixelSize(640, 480), new Vector(96, 96), PixelFormat.Bgra8888);
Reset();
Task.Run(MoveFlakes);
}
public WriteableBitmap Bitmap { get; }
public int FlakeCount
{
get => _flakeCount;
set => ResizeFlakes(value);
}
public int DelayMsInverted
{
get => MaxDelay - _delayMs;
set => _delayMs = MaxDelay - value;
}
public int MaxDelay => 16;
public ICommand ResetCommand { get; }
public IEnumerable<Color> Brushes => new[]
{
Colors.Red, Colors.Orange, Colors.Yellow, Colors.Green, Colors.Cyan, Colors.Blue,
Color.FromArgb(250, 0, 0, 0)
};
public Color SelectedBrush { get; set; } = Colors.Red;
public unsafe void PutPixel(double x, double y, int size)
{
// Convert relative to absolute.
var width = Bitmap.PixelSize.Width;
var height = Bitmap.PixelSize.Height;
var px = (int) (x * width);
var py = (int) (y * height);
var c = SelectedBrush;
var pixel = c.B + ((uint) c.G << 8) + ((uint) c.R << 16) + ((uint) c.A << 24);
using var buf = Bitmap.Lock();
for (var x0 = px - size; x0 <= px + size; x0++)
for (var y0 = py - size; y0 <= py + size; y0++)
{
if (x0 >= 0 && x0 < width && y0 >= 0 && y0 < height)
{
var ptr = (uint*) buf.Address;
ptr += (uint) (width * y0 + x0);
*ptr = pixel;
}
}
}
public unsafe void LoadFile(Stream file, double x, double y)
{
// Convert relative to absolute.
var width = Bitmap.PixelSize.Width;
var height = Bitmap.PixelSize.Height;
var px = (int)(x * width);
var py = (int)(y * height);
using var img = Image.Load(file);
using var buf = Bitmap.Lock();
var w = Math.Min(width - px, img.Width);
var h = Math.Min(height - py, img.Height);
var ptr = (uint*)buf.Address;
// Move snow out of the picture rectangle.
for (var i = 0; i < _flakes.Length; i++)
{
ref var f = ref _flakes[i];
if (f.X > px && f.Y > py && f.X < px + w && f.Y < py + h)
{
// Clear pixel and reset flake to top.
*(ptr + f.Y * width + f.X) = 0;
InitFlake(ref f);
}
}
// Load picture.
for (var i = 0; i < w; i++)
for (var j = 0; j < h; j++)
{
var pix = img[i, j];
// Alpha threshold: transparent pixels don't work with snow logic.
if (pix.A > 200)
{
var pixPtr = ptr + (j + py) * width + i + px;
*pixPtr = (uint) (pix.B | pix.G << 8 | pix.R << 16 | byte.MaxValue << 24);
}
}
}
private void Reset()
{
InitFlakes();
ResetBitmap();
}
private void InitFlakes()
{
_flakes = new Flake[_flakeCount];
for (var i = 0; i < _flakes.Length; i++)
{
ref var f = ref _flakes[i];
InitFlake(ref f);
f.Y = (short) _rnd.Next(40);
f.Y2 = 0;
}
}
private void InitFlake(ref Flake f)
{
var tone = (byte) _rnd.Next(MaxSpeed);
f.X = (short) _rnd.Next(Bitmap.PixelSize.Width);
f.Speed = tone;
f.Y = 0;
f.Y2 = 0;
}
private unsafe void ResizeFlakes(int newCount)
{
using var buf = Bitmap.Lock();
var ptr = (uint*)buf.Address;
var old = _flakes;
var oldCount = _flakeCount;
_flakes = new Flake[newCount];
if (newCount < oldCount)
{
// Remove extra flakes, trim array.
for (var i = newCount; i < oldCount; i++)
{
*(ptr + old[i].X + old[i].Y * Bitmap.PixelSize.Width) = 0;
}
Array.Copy(old, _flakes, newCount);
}
else
{
// Add more flakes.
Array.Copy(old, _flakes, oldCount);
for (var i = oldCount; i < newCount; i++)
{
InitFlake(ref _flakes[i]);
}
}
_flakeCount = newCount;
}
private unsafe void ResetBitmap()
{
using var buf = Bitmap.Lock();
var ptr = (uint*)buf.Address;
var w = Bitmap.PixelSize.Width;
var h = Bitmap.PixelSize.Height;
// Clear.
for (var i = 0; i < w * (h - 1); i++)
{
*(ptr + i) = 0;
}
// Draw bottom line.
for (var i = w * (h - 1); i < w * h ; i++)
{
*(ptr + i) = uint.MaxValue;
}
}
private unsafe void MoveFlakes()
{
while (true)
{
// MaxDelay means pause.
if (_delayMs < MaxDelay)
{
var bmp = Bitmap;
var w = bmp.PixelSize.Width;
using var buf = bmp.Lock();
var ptr = (uint*) buf.Address;
var flakes = _flakes;
for (var i = 0; i < flakes.Length; i++)
{
MoveFlake(ref flakes[i], ptr, w);
}
}
_invalidate();
Thread.Sleep(_delayMs);
}
// ReSharper disable once FunctionNeverReturns
}
private unsafe void MoveFlake(ref Flake f, uint* ptr, int width)
{
f.Y2 += f.Speed;
const short slowdown = 200;
if (f.Y2 < slowdown)
{
return;
}
// Erase old flake.
var oldPtr = ptr + width * f.Y + f.X;
*oldPtr = 0;
// New position.
f.Y2 = (short)(f.Y2 % slowdown);
f.Y++;
var newPtr = oldPtr + width;
var newAlphaPtr = (byte*) newPtr + 3;
// Check snow below us.
if (*newAlphaPtr == byte.MaxValue)
{
// Check pixels to the left or to the right: we might be on a slope.
if (f.X > 0 && *(newAlphaPtr - 4) != byte.MaxValue)
{
f.X--;
newPtr--;
}
else if (f.X + 1 < width && *(newAlphaPtr + 4) != byte.MaxValue)
{
f.X++;
newPtr++;
}
else
{
// Not on a slope, stop here and preserve the pixel.
InitFlake(ref f);
newPtr = ptr + width * f.Y + f.X;
// Mark as static by setting alpha to 255.
// Make persistent color lighter.
var clr = MaxSpeed * 0.8 + f.Speed * 0.2;
*oldPtr = GetGray((byte) clr) | 0xFF000000;
}
}
*newPtr = GetGray(f.Speed);
}
private static uint GetGray(byte tone)
{
var c = (byte) (byte.MaxValue - MaxSpeed + tone);
// Non-max alpha indicates moving pixel.
return (uint) (c | c << 8 | c << 16 | 0xFE000000);
}
private struct Flake
{
public short X;
public short Y;
public short Y2;
public byte Speed;
}
}
}