Failed in Creating DDB Grayscale Bitmap with 24bit RGB by C++

Question

I am trying to create a DDB Grayscale Bitmap using 24BitRGB format by C++. But I always get a bitmap with black block at bottom. It will be very appreciated if someone can help me on this issue. Many thanks. The bitmap is : 1024x408 widthxheight Below is my code:

enter code here
#include "stdafx.h"
#include "Win32Project19.h"
#include <windows.h>
#include <wingdi.h>
#include <algorithm>

#define MAX_LOADSTRING 100

// 全局变量: 
HINSTANCE hInst;                                // 当前实例
WCHAR szTitle[MAX_LOADSTRING];                  // 标题栏文本
WCHAR szWindowClass[MAX_LOADSTRING];            // 主窗口类名

// 此代码模块中包含的函数的前向声明: 
ATOM                MyRegisterClass(HINSTANCE hInstance);
BOOL                InitInstance(HINSTANCE, int);
LRESULT CALLBACK    WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK    About(HWND, UINT, WPARAM, LPARAM);

int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
                     _In_opt_ HINSTANCE hPrevInstance,
                     _In_ LPWSTR    lpCmdLine,
                     _In_ int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    // TODO: 在此放置代码。

    // 初始化全局字符串
    LoadStringW(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
    LoadStringW(hInstance, IDC_WIN32PROJECT19, szWindowClass, MAX_LOADSTRING);
    MyRegisterClass(hInstance);

    // 执行应用程序初始化: 
    if (!InitInstance (hInstance, nCmdShow))
    {
        return FALSE;
    }

    HACCEL hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_WIN32PROJECT19));

    MSG msg;

    // 主消息循环: 
    while (GetMessage(&msg, nullptr, 0, 0))
    {
        if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    return (int) msg.wParam;
}



//
//  函数: MyRegisterClass()
//
//  目的: 注册窗口类。
//
ATOM MyRegisterClass(HINSTANCE hInstance)
{
    WNDCLASSEXW wcex;

    wcex.cbSize = sizeof(WNDCLASSEX);

    wcex.style          = CS_HREDRAW | CS_VREDRAW;
    wcex.lpfnWndProc    = WndProc;
    wcex.cbClsExtra     = 0;
    wcex.cbWndExtra     = 0;
    wcex.hInstance      = hInstance;
    wcex.hIcon          = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_WIN32PROJECT19));
    wcex.hCursor        = LoadCursor(nullptr, IDC_ARROW);
    wcex.hbrBackground  = (HBRUSH)(COLOR_WINDOW+1);
    wcex.lpszMenuName   = MAKEINTRESOURCEW(IDC_WIN32PROJECT19);
    wcex.lpszClassName  = szWindowClass;
    wcex.hIconSm        = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));

    return RegisterClassExW(&wcex);
}

//
//   函数: InitInstance(HINSTANCE, int)
//
//   目的: 保存实例句柄并创建主窗口
//
//   注释: 
//
//        在此函数中，我们在全局变量中保存实例句柄并
//        创建和显示主程序窗口。
//
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
   hInst = hInstance; // 将实例句柄存储在全局变量中

   HWND hWnd = CreateWindowW(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
      CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, nullptr, nullptr, hInstance, nullptr);

   if (!hWnd)
   {
      return FALSE;
   }

   ShowWindow(hWnd, nCmdShow);
   UpdateWindow(hWnd);

   return TRUE;
}

//
//  函数: WndProc(HWND, UINT, WPARAM, LPARAM)
//
//  目的:    处理主窗口的消息。
//
//  WM_COMMAND  - 处理应用程序菜单
//  WM_PAINT    - 绘制主窗口
//  WM_DESTROY  - 发送退出消息并返回
//
//
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{

    static HBITMAP hBitmap;
    HDC hdcMem, hdc;
    static BYTE* lpBits = new BYTE[1024*3*408];
    static BYTE* p;
    static int cxClient, cyClient;
    int i = 0;
    int c = 0;
    switch (message)
    {
        case WM_COMMAND:
           {
                int wmId = LOWORD(wParam);
                // 分析菜单选择: 
                switch (wmId)
                {
                     case IDM_ABOUT:
                     DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
                     break;
                     case IDM_EXIT:
                     DestroyWindow(hWnd);
                     break;
                     default:
                     return DefWindowProc(hWnd, message, wParam, lParam);
                  }
           }
               break;

          case WM_CREATE:

             cxClient = LOWORD(lParam);
             cyClient = HIWORD(lParam);
             for (int y = 0; y < 408; y++)
             {
                 for (int x = 0; x < 1024; x++)
                 {
                     lpBits[c + 0] = i;
                     lpBits[c + 1] = i;
                     lpBits[c + 2] = i;

                     c += 3;
                  }

                  i++;
                  if (i > 255)
                   i = 0;

              }
              break;

          case WM_PAINT:
           {
                 PAINTSTRUCT ps;
                 hdc = BeginPaint(hWnd, &ps);
                 RECT rect;
                 // TODO: Please add code here....
 
                 hBitmap = CreateCompatibleBitmap(hdc, 1024, 408);

                 SetBitmapBits(hBitmap, 1024 * 3 * 408, lpBits);
                 GetClientRect(hWnd, &rect);
                 hdcMem = CreateCompatibleDC(hdc);
                 SelectObject(hdcMem, hBitmap);
                 StretchBlt(hdc, rect.left, rect.top, rect.right, rect.bottom, hdcMem, 0, 0, 1024, 
                            408, SRCCOPY);
                 DeleteDC(hdcMem);
                 EndPaint(hWnd, &ps);
               }
               break;
             case WM_DESTROY:
                   DeleteObject(hBitmap);
                   PostQuitMessage(0);
               break;
             default:
                    return DefWindowProc(hWnd, message, wParam, lParam);
            }
           return 0;
}

I always get below results (always a black block in the bottom):

Answer 1

You are assuming that bitmap uses 3 bytes per pixel, but it appears that you need 4 (one for alpha channel).

I got your code to work by simply replacing all 3 with 4:

static BYTE* lpBits = new BYTE[WIDTH * 4 * HEIGHT];
...
c += 4;
...
SetBitmapBits(hBitmap, WIDTH * 4 * HEIGHT, lpBits);

Also, you create your hBitmap for each WM_PAINT message you get, but delete only once, in WM_DESTROY. That causes a resource leak, that may lead to other issues. Since hBitmap is static, you can create it like this:

    if (!hBitmap) {
        hBitmap = CreateCompatibleBitmap(hdc, WIDTH, HEIGHT);
        SetBitmapBits(hBitmap, WIDTH * 4 * HEIGHT, lpBits);
    }