SKBitmap Crop Whitespace

README.md

Crop whitespace around a bitmap

Based on the original stackoverflow post: https://stackoverflow.com/questions/248141/remove-surrounding-whitespace-from-an-image

Updated to work with SKBitmap.

BitmapExtensions.cs

using SkiaSharp;
using System;

//
// Based on the original stackoverflow post:  https://stackoverflow.com/questions/248141/remove-surrounding-whitespace-from-an-image
//
namespace BlahBlah
{

  public static class BitmapExtensions
  {
    public static SKBitmap TrimWhitespace(this SKBitmap bmp)
    {
      int w = bmp.Width;
      int h = bmp.Height;
      
      // get all the pixels here - this can take a while so dont want it in the loops below
      // maybe theres a more efficient way?  loading all the pixels could be greedy
      var pixels = bmp.Pixels;

      bool IsTransparent(SKColor color)
      {
        return (color.Red == 0 && color.Green == 0 && color.Blue == 0 && color.Alpha == 0) || 
          (color == SKColors.Transparent);
      }

      Func<int, bool> allWhiteRow = row =>
      {
        for (int i = 0; i < w; ++i)
        {
          var px = row * w + i;
          if (!IsTransparent(pixels[px]))
            return false;
        }
        return true;
      };

      Func<int, bool> allWhiteColumn = col =>
      {
        for (int i = 0; i < h; ++i)
        {
          var px = col * h + i;
          if (!IsTransparent(pixels[px]))
            return false;
        }
        return true;
      };

      int topmost = 0;
      for (int row = 0; row < h; ++row)
      {
        if (allWhiteRow(row))
          topmost = row;
        else break;
      }

      int bottommost = 0;
      for (int row = h - 1; row >= 0; --row)
      {
        if (allWhiteRow(row))
          bottommost = row;
        else break;
      }

      int leftmost = 0, rightmost = 0;
      for (int col = 0; col < w; ++col)
      {
        if (allWhiteColumn(col))
          leftmost = col;
        else
          break;
      }

      for (int col = w - 1; col >= 0; --col)
      {
        if (allWhiteColumn(col))
          rightmost = col;
        else
          break;
      }

      if (rightmost == 0) rightmost = w; // As reached left
      if (bottommost == 0) bottommost = h; // As reached top.

      int croppedWidth = rightmost - leftmost;
      int croppedHeight = bottommost - topmost;

      if (croppedWidth == 0) // No border on left or right
      {
        leftmost = 0;
        croppedWidth = w;
      }

      if (croppedHeight == 0) // No border on top or bottom
      {
        topmost = 0;
        croppedHeight = h;
      }

      try
      {
        var target = new SKBitmap(croppedWidth, croppedHeight);

        using var canvas = new SKCanvas(target);
        using var img = SKImage.FromBitmap(bmp);
        canvas.DrawImage(img,
          new SKRect(leftmost, topmost, rightmost, bottommost),
          new SKRect(0, 0, croppedWidth, croppedHeight));

        return target;
      }
      catch (Exception ex)
      {
        throw new Exception(
          string.Format("Values are topmost={0} btm={1} left={2} right={3} croppedWidth={4} croppedHeight={5}", topmost, bottommost, leftmost, rightmost, croppedWidth, croppedHeight),
          ex);
      }
    }

  }
}