解决gif 透明度问题

C++
for (MDWord i = 0; i < m_param.dwHeight; i++)
        {
                for (MDWord j = 0; j < m_param.dwWidth; j++)
                {
                        if (pTemp[((dwIndex + j) << 2) + 3] < 128)//原图BGRA，所以判断alpha的值
                        {
                                pAdjust[dwIndex + j] = 255;//255就是背景色的索引
                        }
                }
                dwIndex += m_param.dwWidth;
        }