Ｃ＃コード断片/共通/画像にマスクをかける（ビットマップデータを直接扱う）

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using System;
using System.IO;
using System.Drawing;
using System.Drawing.Imaging;
 
namespace MaskTest {
    class Program {
        static void Main(string[] args) {
            foreach (string file in Directory.GetFiles(".", "*.bmp")) {
                //元画像読み込み
                Bitmap srcBmp = (Bitmap)Bitmap.FromFile(file);
 
                //元画像を32bppに変換
                Bitmap tmpBmp = new Bitmap(srcBmp.Width, srcBmp.Height, PixelFormat.Format32bppArgb);
                Graphics tmpG = Graphics.FromImage(tmpBmp);
                tmpG.DrawImage(srcBmp, new Rectangle(0, 0, tmpBmp.Width, tmpBmp.Height),
                    new Rectangle(0, 0, srcBmp.Width, srcBmp.Height), GraphicsUnit.Pixel);
 
                //左側画像作成
                Bitmap dstBmp = new Bitmap(srcBmp.Width / 2, srcBmp.Height, PixelFormat.Format32bppArgb);
                Graphics dstG = Graphics.FromImage(dstBmp);
                dstG.DrawImage(srcBmp, new Rectangle(0, 0, dstBmp.Width, dstBmp.Height),
                    0, 0, srcBmp.Width / 2, srcBmp.Height, GraphicsUnit.Pixel);
 
                //右側画像作成
                Bitmap mskBmp = new Bitmap(srcBmp.Width / 2, srcBmp.Height, PixelFormat.Format32bppArgb);
                Graphics mskG = Graphics.FromImage(mskBmp);
                mskG.DrawImage(srcBmp, new Rectangle(0, 0, dstBmp.Width, dstBmp.Height),
                    srcBmp.Width/2 , 0, srcBmp.Width / 2, srcBmp.Height, GraphicsUnit.Pixel);
 
                //マスク処理
                Mask(dstBmp, mskBmp,true);
 
                //保存
                dstBmp.Save(file.ToLower().Replace(".bmp", ".png"), ImageFormat.Png);
            }
        }
 
        public unsafe static void Mask(Bitmap srcBmp,Bitmap mskBmp,bool isNegate){
            int w = srcBmp.Width;
            int h = srcBmp.Height;
 
            if (mskBmp.Width != w || mskBmp.Height != h) {
                throw new ArgumentException("bitmap size unmatch");
            }
            if (srcBmp.PixelFormat != PixelFormat.Format32bppArgb ||
                mskBmp.PixelFormat != PixelFormat.Format32bppArgb) {
                throw new ArgumentException("bitmap must be 32bpp");
            }
 
            //ビットマップをロックしてポインタを取得
            BitmapData srcData = srcBmp.LockBits(new Rectangle(0, 0, w, h),
                ImageLockMode.WriteOnly, PixelFormat.Format32bppArgb);
            BitmapData mskData = mskBmp.LockBits(new Rectangle(0, 0, w, h),
                ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
            byte* pSrcBase = (byte*)srcData.Scan0.ToPointer();
            byte* pMskBase = (byte*)mskData.Scan0.ToPointer();
 
            //ピクセル毎にマスク処理
            for (int y = 0; y < h; y++) {
                for (int x = 0; x < w; x++) {
                    byte* pSrc = pSrcBase + (y * w + x) * 4;
                    byte* pMsk = pMskBase + (y * w + x) * 4;
 
                    //mask RGB-> src A  罠：Argbといいつつ格納の順番は逆のBGRA
                    int rgbsum = (*(pMsk + 0) + *(pMsk + 1) + *(pMsk + 2))/3;
                    if (isNegate) {
                        rgbsum = 255 - rgbsum;
                    }
                    *(pSrc + 3) = (byte)rgbsum;
                }
            }
 
            //アンロック
            srcBmp.UnlockBits(srcData);
            mskBmp.UnlockBits(mskData);
        }
    }
}