基于GDI显示png图像

intro

先前基于GDI已经能够显示BITMAP图像：windows下控制台程序实现窗口显示，其中BMP图像是使用LoadImage()这一Win32 API函数来做的。考虑到LoadImage()函数并不能读取png图像，因此需要libpng或stb等png编解码库的帮助。

网上找到相关代码不多，稍加修改可以运行，具备特点：

纯C，单个文件（依赖的libpng和zlib可以忽略）
直接读取png图像而不是通过读取.rc文件（资源文件）再读取png图像
png图像的读取：基于libpng（以及zlib），我直接用的opencv345 windows预编译报里的.h文件和库文件
入口函数为main()而非WinMain()，也即控制台程序，方便作为库函数、移植

不同于BMP的地方：在窗口处理函数的创建阶段有所不同：

case WM_CREATE:
	if (image_type == BMP) {
		my_window->hBmp = (HBITMAP)LoadImage(NULL, "D:/work/libfc/imgs/lena512.bmp", IMAGE_BITMAP, 0, 0, LR_LOADFROMFILE);
	}
	else if (image_type == PNG) {
		ReadPngData("D:/work/libfc/imgs/Lena.png", &im_width, &im_height, &my_window->imdata);
		my_window->hBmp = CreateBitmap(im_width, im_height, 32, 1, my_window->imdata);
	}

并没有处理alpha通道
如果图像有alpha通道，透明区域将显示为黑色，因为用的是BitBlt()。如果要使用alpha通道（例如做融合，或者显示为黑色以外的颜色），则应当使用AlphaBlend()和msing32库：
```
#pragma comment(lib, "msimg32.lib")
...

    BitBlt(hdc, 0, 0, bm.bmWidth, bm.bmHeight, my_window->dc, 0, 0, SRCCOPY);
    //AlphaBlend(hdc, 100, 0, bm.bmWidth, bm.bmHeight, my_window->dc, 0, 0, bm.bmWidth, bm.bmHeight, bf);
```
opencv中看起来也是把透明区域显示为黑色。
缺点：窗口client区域大小固定，没有能根据图像尺寸来变化

代码

#include <stdio.h>
#include <windows.h>
#include "png.h"


#define CRTDBG_MAP_ALLOC 
#include <crtdbg.h>


#pragma comment(lib, "D:/work/libfc/lib/libpng.lib")
#pragma comment(lib, "D:/work/libfc/lib/zlib.lib")

typedef struct MyRect {
	int x, y, width, height;
} MyRect;

const char* project_root = "D:/work/libfc";
char bitmap_im_pth[100];

typedef struct MyWindow {
	HDC dc;
	//HGDIOBJ image;
	HBITMAP hBmp;
	unsigned char* imdata;
} MyWindow;

MyWindow* my_window;
enum ImageType {BMP, PNG};

long ReadPngData(const char *szPath, int *pnWidth, int *pnHeight, unsigned char **cbData)
{
	FILE *fp = NULL;
	long file_size = 0, pos = 0, mPos = 0;
	int color_type = 0, x = 0, y = 0, block_size = 0;

	png_infop info_ptr;
	png_structp png_ptr;
	png_bytep *row_point = NULL;

	fp = fopen(szPath, "rb");
	if (!fp)    return -1;            //文件打开错误则返回 FILE_ERROR

	png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING, 0, 0, 0);        //创建png读取结构
	info_ptr = png_create_info_struct(png_ptr);        //png 文件信息结构
	png_init_io(png_ptr, fp);                //初始化文件 IO
	png_read_png(png_ptr, info_ptr, PNG_TRANSFORM_EXPAND, 0);                //读取png文件

	*pnWidth = png_get_image_width(png_ptr, info_ptr);        //获得图片宽度
	*pnHeight = png_get_image_height(png_ptr, info_ptr);        //获得图片高度
	color_type = png_get_color_type(png_ptr, info_ptr);        //获得图片色彩深度
	file_size = (*pnWidth) * (*pnHeight) * 4;                    //计算需要存储RGB(A)数据所需的内存大小
	*cbData = (unsigned char *)malloc(file_size);            //申请所需的内容, 并将 *cbData 指向申请的这块内容

	row_point = png_get_rows(png_ptr, info_ptr);            //读取RGB(A)数据

	block_size = color_type == 6 ? 4 : 3;                    //根据是否具有a通道判断每次所要读取的数据大小, 具有Alpha通道的每次读4位, 否则读3位

	//将读取到的RGB(A)数据按规定格式读到申请的内存中
	for (x = 0; x < *pnHeight; x++)
		for (y = 0; y < *pnWidth*block_size; y += block_size)
		{
			(*cbData)[pos++] = row_point[x][y + 2];        //B
			(*cbData)[pos++] = row_point[x][y + 1];        //G
			(*cbData)[pos++] = row_point[x][y + 0];        //R
			(*cbData)[pos++] = row_point[x][y + 3];        //alpha
		}

	png_destroy_read_struct(&png_ptr, &info_ptr, 0);
	fclose(fp);

	return file_size;
}


LRESULT __stdcall WindowProcedure(HWND window, unsigned int msg, WPARAM wp, LPARAM lp)
{
	int im_width, im_height;
	
	int image_type = PNG;
	switch (msg)
	{
	case WM_CREATE:
		if (image_type == BMP) {
			my_window->hBmp = (HBITMAP)LoadImage(NULL, "D:/work/libfc/imgs/lena512.bmp", IMAGE_BITMAP, 0, 0, LR_LOADFROMFILE);
		}
		else if (image_type == PNG) {
			ReadPngData("D:/work/libfc/imgs/Lena.png", &im_width, &im_height, &my_window->imdata);
			my_window->hBmp = CreateBitmap(im_width, im_height, 32, 1, my_window->imdata);
		}
		if (my_window->hBmp == NULL)
			MessageBox(window, "Could not load image!", "Error", MB_OK | MB_ICONEXCLAMATION);
		break;

	case WM_PAINT:
	{
		BITMAP bm;
		PAINTSTRUCT ps;

		HDC hdc = BeginPaint(window, &ps);
		SetStretchBltMode(hdc, COLORONCOLOR);

		my_window->dc = CreateCompatibleDC(hdc);
		HBITMAP hbmOld = SelectObject(my_window->dc, my_window->hBmp);

		GetObject(my_window->hBmp, sizeof(bm), &bm);

#if 1
		//原样拷贝，不支持拉伸
		BitBlt(hdc, 0, 0, bm.bmWidth, bm.bmHeight, my_window->dc, 0, 0, SRCCOPY);
#else
		RECT rcClient;
		GetClientRect(window, &rcClient);//获得客户区的大小
		int nWidth = rcClient.right - rcClient.left;//客户区的宽度
		int nHeight = rcClient.bottom - rcClient.top;//客户区的高度
		StretchBlt(hdc, 0, 0, nWidth, nHeight, hdcMem, 0, 0, bm.bmWidth, bm.bmHeight, SRCCOPY);//拉伸拷贝
#endif

		SelectObject(my_window->dc, hbmOld);
		DeleteDC(my_window->dc);

		EndPaint(window, &ps);
	}
	break;

	case WM_DESTROY:
		printf("
destroying window
");
		PostQuitMessage(0);
		return 0L;

	case WM_LBUTTONDOWN:
		printf("
mouse left button down at (%d, %d)
", LOWORD(lp), HIWORD(lp));

		// fall thru
	default:
		//printf(".");
		return DefWindowProc(window, msg, wp, lp);
	}
}

const char* szWindowClass = "myclass";


ATOM MyRegisterClass(HINSTANCE hInstance)
{
	WNDCLASSEX wc;
	wc.cbSize = sizeof(WNDCLASSEX);
	/* Win 3.x */
	wc.style = CS_DBLCLKS;
	wc.lpfnWndProc = WindowProcedure;
	wc.cbClsExtra = 0;
	wc.cbWndExtra = 0;
	wc.hInstance = GetModuleHandle(0);
	wc.hIcon = LoadIcon(0, IDI_APPLICATION);
	wc.hCursor = LoadCursor(0, IDC_ARROW);
	wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
	wc.lpszMenuName = 0;
	wc.lpszClassName = szWindowClass;
	/* Win 4.0 */
	wc.hIconSm = LoadIcon(0, IDI_APPLICATION);

	return RegisterClassEx(&wc);
}

BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
	MyRect rect;
	rect.x = 300;
	rect.y = 300;
	rect.width = 640;
	rect.height = 480;

	DWORD defStyle = WS_VISIBLE | WS_MINIMIZEBOX | WS_MAXIMIZEBOX | WS_SYSMENU;

	HWND hwnd = CreateWindowEx(0, szWindowClass, "title",
		defStyle, rect.x, rect.y,
		rect.width, rect.height, 0, 0, hInstance, 0);

	if (!hwnd)
	{
		return FALSE;
	}
	ShowWindow(hwnd, nCmdShow);
	UpdateWindow(hwnd);

	return TRUE;
}

void create_my_window(MyWindow** _my_window) {
	MyWindow* my_window = (MyWindow*)malloc(sizeof(MyWindow));
	my_window->dc = NULL;
	my_window->imdata = NULL;
	my_window->hBmp = NULL;

	*_my_window = my_window; // write back
}

void destroy_my_window(MyWindow* my_window) {
	if (my_window) {
		if (my_window->imdata) free(my_window->imdata);
		free(my_window);
	}
}

int main()
{
	printf("hello world!
");

	HINSTANCE hInstance = GetModuleHandle(0);
	int nCmdShow = SW_SHOWDEFAULT;

	MyRegisterClass(hInstance);
	create_my_window(&my_window);

	if (!InitInstance(hInstance, nCmdShow))
	{
		return FALSE;
	}
		
	MSG msg;
	while (GetMessage(&msg, 0, 0, 0)) {
		DispatchMessage(&msg);
	}

	destroy_my_window(my_window);


	return 0;
	
}

参考

使用 Libpng 配合 GDI 完成对 png 图片的解析与显示，讲解细致

How would I load a PNG image using Win32/GDI (no GDI+ if possible)? (没用）

使用libpng和GDI读取显示png图片 (代码无法运行)