Unity 实现转换bmp为png

.netframework的Bitmap类返回的pixel中alpha的值恒为255，因此需要自己读取对应的类。

BMP格式
https://en.wikipedia.org/wiki/BMP_file_format#Pixel_storage

参考
https://blog.csdn.net/sgs691459/article/details/111387043

实现结果

    private void ConvertBmp2Png(string path)
    {
        var fileNameList = Directory.GetFiles(path).ToList();
        var bmpFileList = fileNameList.Where(fileName => fileName.EndsWith(".bmp"));

        foreach(var bmpFileName in bmpFileList)
        {
            int width = -1;
            int height = -1;
            using (Bitmap bmp1 = new Bitmap(bmpFileName))
            {
                width = bmp1.Width;
                height = bmp1.Height;
            }


            FileStream fileStream = new FileStream(bmpFileName, FileMode.Open, FileAccess.Read);
            byte[] buffer = new byte[fileStream.Length];
            fileStream.Read(buffer, 0, buffer.Length);
            Bitmap bitmap = new Bitmap(width, height);

            for (int x = 0; x< width; x++)
            {
                for(int y = 0; y < height; y++)
                {
                    Color color;
                    if (y * width + x + 57 <= buffer.Length - 1) color = Color.FromArgb(Convert.ToInt32(buffer[(y * width + x) * 4 + 57]), Convert.ToInt32(buffer[(y * width + x) * 4 + 56]), Convert.ToInt32(buffer[(y * width + x) * 4 + 55]), Convert.ToInt32(buffer[(y * width + x) * 4 + 54]));
                    else color = Color.FromArgb(0, 0, 0, 0);
                    bitmap.SetPixel(x, y, color);
                }
            }

            bitmap.Save(bmpFileName.Replace(".bmp",".png"), ImageFormat.Png);
            AssetDatabase.Refresh();
        }
    }