png图片问题

#pragma once
#include<graphics.h>
void drawAlpha(IMAGE* picture, int  picture_x, int picture_y) //x为载入图片的X坐标，y为Y坐标
{

    // 变量初始化
    DWORD* dst = GetImageBuffer();    // GetImageBuffer()函数，用于获取绘图设备的显存指针，EASYX自带
    DWORD* draw = GetImageBuffer();
    DWORD* src = GetImageBuffer(picture); //获取picture的显存指针
    int picture_width = picture->getwidth(); //获取picture的宽度，EASYX自带
    int picture_height = picture->getheight(); //获取picture的高度，EASYX自带
    int graphWidth = getwidth();       //获取绘图区的宽度，EASYX自带
    int graphHeight = getheight();     //获取绘图区的高度，EASYX自带
    int dstX = 0;    //在显存里像素的角标

    // 实现透明贴图 公式： Cp=αp*FP+(1-αp)*BP ， 贝叶斯定理来进行点颜色的概率计算
    for (int iy = 0; iy < picture_height; iy++)
    {
        for (int ix = 0; ix < picture_width; ix++)
        {
            int srcX = ix + iy * picture_width; //在显存里像素的角标
            int sa = ((src[srcX] & 0xff000000) >> 24); //0xAArrggbb;AA是透明度
            int sr = ((src[srcX] & 0xff0000) >> 16); //获取RGB里的R
            int sg = ((src[srcX] & 0xff00) >> 8);   //G
            int sb = src[srcX] & 0xff;              //B
            if (ix >= 0 && ix <= graphWidth && iy >= 0 && iy <= graphHeight && dstX <= graphWidth * graphHeight)
            {
                dstX = (ix + picture_x) + (iy + picture_y) * graphWidth; //在显存里像素的角标
                int dr = ((dst[dstX] & 0xff0000) >> 16);
                int dg = ((dst[dstX] & 0xff00) >> 8);
                int db = dst[dstX] & 0xff;
                draw[dstX] = ((sr * sa / 255 + dr * (255 - sa) / 255) << 16)  //公式： Cp=αp*FP+(1-αp)*BP  ； αp=sa/255 , FP=sr , BP=dr
                    | ((sg * sa / 255 + dg * (255 - sa) / 255) << 8)         //αp=sa/255 , FP=sg , BP=dg
                    | (sb * sa / 255 + db * (255 - sa) / 255);              //αp=sa/255 , FP=sb , BP=db
            }
        }
    }
}