.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();
}
}