How to display raw array of pixels to the screen? - c

Question

I am new to windows programming. I want to display the raw pixel array to the screen without using SetPixel function because it's too slow in my standards. I am using this question as my reference.
I made a small program below to fill the pixel array with random RGB values and display it to the screen. The result wasn't what I anticipated, I got the white canvas. I tried to change this line ptr++ = (b << 16) | (g << 8) | r; to ptr++ = 0x000000FF; expecting red canvas but I got the same result.
#include <stdlib.h>
#include <time.h>
#include <windows.h>
const int win_width = 500;
const int win_height = 450;
LRESULT CALLBACK WndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
RECT rect;
int width, height;
COLORREF *display;
switch (msg)
{
case WM_CREATE:
srand((unsigned int) time(NULL));
GetClientRect(hWnd, &rect);
width = rect.right - rect.left;
height = rect.bottom - rect.top;
display = (COLORREF *) malloc(sizeof(COLORREF) * width * height);
COLORREF *ptr = display;
for (int y = 0; y < height; ++y)
{
for (int x = 0; x < width; ++x)
{
int r = rand() % 256;
int g = rand() % 256;
int b = rand() % 256;
*ptr++ = (b << 16) | (g << 8) | r;
}
}
break;
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hDC, memDC;
HBITMAP hBmp, hOldBmp;
hDC = BeginPaint(hWnd, &ps);
memDC = CreateCompatibleDC(hDC);
hBmp = CreateBitmap(width, height, 1, 32, (void *) display);
hOldBmp = (HBITMAP) SelectObject(memDC, hBmp);
BitBlt(hDC, rect.left, rect.top, width, height, memDC, 0, 0, SRCCOPY);
SelectObject(memDC, hOldBmp);
DeleteObject(hBmp);
DeleteDC(memDC);
EndPaint(hWnd, &ps);
break;
}
case WM_CLOSE:
DestroyWindow(hWnd);
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hWnd, msg, wParam, lParam);
}
return 0;
}
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
const TCHAR szClassName[] = TEXT("MyClass");
WNDCLASS wc;
HWND hWnd;
MSG msg;
wc.style = CS_HREDRAW | CS_VREDRAW;
wc.lpfnWndProc = WndProc;
wc.cbClsExtra = 0;
wc.cbWndExtra = 0;
wc.hInstance = hInstance;
wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
wc.hCursor = LoadCursor(NULL, IDC_ARROW);
wc.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
wc.lpszMenuName = NULL;
wc.lpszClassName = szClassName;
wc.hIcon = LoadIcon(NULL, IDI_APPLICATION);
if (!RegisterClass(&wc))
{
MessageBox(NULL, TEXT("Window Registration Failed!"), TEXT("Error!"),
MB_ICONEXCLAMATION | MB_OK);
return 0;
}
hWnd = CreateWindow(szClassName,
TEXT("Random Pixels"),
WS_OVERLAPPEDWINDOW | WS_VISIBLE,
CW_USEDEFAULT, CW_USEDEFAULT, win_width, win_height,
NULL, NULL, hInstance, NULL);
if (hWnd == NULL)
{
MessageBox(NULL, TEXT("Window Creation Failed!"), TEXT("Error!"),
MB_ICONEXCLAMATION | MB_OK);
return 0;
}
ShowWindow(hWnd, nCmdShow);
UpdateWindow(hWnd);
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return (int) msg.wParam;
}
I know there's something wrong with my code inside WM_PAINT but I don't know how to fix it. I will appreciate any form of assistance. Thanks in advance.

Answer 1

The variable display that holds the pixel data has automatic lifetime. Its lifetime ends whenever control leaves WndProc. A consequence is, that every invocation of WndProc starts out with a new (indeterminate) value for display.
To solve this, display needs to have static storage duration. The easiest way to accomplish this is to replace
COLORREF *display;
with
static COLORREF *display;
This has 2 consequences:
The value stored in display survives separate invocations of WndProc.
The value stored in display is now properly zero-initialized.