MSDN example for scrolling large image crashes

Question

INTRODUCTION AND RELATED INFORMATION:

I am trying to draw an image in my main window in full size. Currently I need to display EMF.

After browsing over the internet, I found this MSDN example .

Problem:

I tried and found out that this does not work.

After right-clicking, as the instructions say, a screenshot of the desktop is correctly displayed in the main window.

However , when the user resizes the window, they draw artifacts similar to those shown below. The effect below also occurs when the user scrolls a bit and then resizes the window.

Here is the image:

This is my first time trying to scroll an image and use scrollbars, so it’s really hard for me to find a solution.

QUESTIONS:

How to fix article code to remove this visual artifact?

Can you recommend another example, for example, a tutorial / sample code / etc, which I can learn?

Can you provide “verbal” instructions / recommendations that can help me?

To make your task even easier, here is the smallest code that reproduces the problem. Note that I just copied / pasted the window procedure and added the minimal code for the program to create a working demo:

#include <windows.h> LRESULT CALLBACK WndProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) { HDC hdc; PAINTSTRUCT ps; SCROLLINFO si; // These variables are required by BitBlt. static HDC hdcWin; // window DC static HDC hdcScreen; // DC for entire screen static HDC hdcScreenCompat; // memory DC for screen static HBITMAP hbmpCompat; // bitmap handle to old DC static BITMAP bmp; // bitmap data structure static BOOL fBlt; // TRUE if BitBlt occurred static BOOL fScroll; // TRUE if scrolling occurred static BOOL fSize; // TRUE if fBlt & WM_SIZE // These variables are required for horizontal scrolling. static int xMinScroll; // minimum horizontal scroll value static int xCurrentScroll; // current horizontal scroll value static int xMaxScroll; // maximum horizontal scroll value // These variables are required for vertical scrolling. static int yMinScroll; // minimum vertical scroll value static int yCurrentScroll; // current vertical scroll value static int yMaxScroll; // maximum vertical scroll value switch (uMsg) { case WM_CREATE: // Create a normal DC and a memory DC for the entire // screen. The normal DC provides a snapshot of the // screen contents. The memory DC keeps a copy of this // snapshot in the associated bitmap. hdcScreen = CreateDC(L"DISPLAY", (PCTSTR)NULL, (PCTSTR)NULL, (CONST DEVMODE *) NULL); hdcScreenCompat = CreateCompatibleDC(hdcScreen); // Retrieve the metrics for the bitmap associated with the // regular device context. bmp.bmBitsPixel = (BYTE)GetDeviceCaps(hdcScreen, BITSPIXEL); bmp.bmPlanes = (BYTE)GetDeviceCaps(hdcScreen, PLANES); bmp.bmWidth = GetDeviceCaps(hdcScreen, HORZRES); bmp.bmHeight = GetDeviceCaps(hdcScreen, VERTRES); // The width must be byte-aligned. bmp.bmWidthBytes = ((bmp.bmWidth + 15) &~15) / 8; // Create a bitmap for the compatible DC. hbmpCompat = CreateBitmap(bmp.bmWidth, bmp.bmHeight, bmp.bmPlanes, bmp.bmBitsPixel, (CONST VOID *) NULL); // Select the bitmap for the compatible DC. SelectObject(hdcScreenCompat, hbmpCompat); // Initialize the flags. fBlt = FALSE; fScroll = FALSE; fSize = FALSE; // Initialize the horizontal scrolling variables. xMinScroll = 0; xCurrentScroll = 0; xMaxScroll = 0; // Initialize the vertical scrolling variables. yMinScroll = 0; yCurrentScroll = 0; yMaxScroll = 0; break; case WM_SIZE: { int xNewSize; int yNewSize; xNewSize = LOWORD(lParam); yNewSize = HIWORD(lParam); if (fBlt) fSize = TRUE; // The horizontal scrolling range is defined by // (bitmap_width) - (client_width). The current horizontal // scroll value remains within the horizontal scrolling range. xMaxScroll = max(bmp.bmWidth - xNewSize, 0); xCurrentScroll = min(xCurrentScroll, xMaxScroll); si.cbSize = sizeof(si); si.fMask = SIF_RANGE | SIF_PAGE | SIF_POS; si.nMin = xMinScroll; si.nMax = bmp.bmWidth; si.nPage = xNewSize; si.nPos = xCurrentScroll; SetScrollInfo(hwnd, SB_HORZ, &si, TRUE); // The vertical scrolling range is defined by // (bitmap_height) - (client_height). The current vertical // scroll value remains within the vertical scrolling range. yMaxScroll = max(bmp.bmHeight - yNewSize, 0); yCurrentScroll = min(yCurrentScroll, yMaxScroll); si.cbSize = sizeof(si); si.fMask = SIF_RANGE | SIF_PAGE | SIF_POS; si.nMin = yMinScroll; si.nMax = bmp.bmHeight; si.nPage = yNewSize; si.nPos = yCurrentScroll; SetScrollInfo(hwnd, SB_VERT, &si, TRUE); break; } case WM_PAINT: { PRECT prect; hdc = BeginPaint(hwnd, &ps); // If the window has been resized and the user has // captured the screen, use the following call to // BitBlt to paint the window client area. if (fSize) { BitBlt(ps.hdc, 0, 0, bmp.bmWidth, bmp.bmHeight, hdcScreenCompat, xCurrentScroll, yCurrentScroll, SRCCOPY); fSize = FALSE; } // If scrolling has occurred, use the following call to // BitBlt to paint the invalid rectangle. // // The coordinates of this rectangle are specified in the // RECT structure to which prect points. // // Note that it is necessary to increment the seventh // argument (prect->left) by xCurrentScroll and the // eighth argument (prect->top) by yCurrentScroll in // order to map the correct pixels from the source bitmap. if (fScroll) { prect = &ps.rcPaint; BitBlt(ps.hdc, prect->left, prect->top, (prect->right - prect->left), (prect->bottom - prect->top), hdcScreenCompat, prect->left + xCurrentScroll, prect->top + yCurrentScroll, SRCCOPY); fScroll = FALSE; } EndPaint(hwnd, &ps); break; } case WM_HSCROLL: { int xDelta; // xDelta = new_pos - current_pos int xNewPos; // new position int yDelta = 0; switch (LOWORD(wParam)) { // User clicked the scroll bar shaft left of the scroll box. case SB_PAGEUP: xNewPos = xCurrentScroll - 50; break; // User clicked the scroll bar shaft right of the scroll box. case SB_PAGEDOWN: xNewPos = xCurrentScroll + 50; break; // User clicked the left arrow. case SB_LINEUP: xNewPos = xCurrentScroll - 5; break; // User clicked the right arrow. case SB_LINEDOWN: xNewPos = xCurrentScroll + 5; break; // User dragged the scroll box. case SB_THUMBPOSITION: xNewPos = HIWORD(wParam); break; default: xNewPos = xCurrentScroll; } // New position must be between 0 and the screen width. xNewPos = max(0, xNewPos); xNewPos = min(xMaxScroll, xNewPos); // If the current position does not change, do not scroll. if (xNewPos == xCurrentScroll) break; // Set the scroll flag to TRUE. fScroll = TRUE; // Determine the amount scrolled (in pixels). xDelta = xNewPos - xCurrentScroll; // Reset the current scroll position. xCurrentScroll = xNewPos; // Scroll the window. (The system repaints most of the // client area when ScrollWindowEx is called; however, it is // necessary to call UpdateWindow in order to repaint the // rectangle of pixels that were invalidated.) ScrollWindowEx(hwnd, -xDelta, -yDelta, (CONST RECT *) NULL, (CONST RECT *) NULL, (HRGN)NULL, (PRECT)NULL, SW_INVALIDATE); UpdateWindow(hwnd); // Reset the scroll bar. si.cbSize = sizeof(si); si.fMask = SIF_POS; si.nPos = xCurrentScroll; SetScrollInfo(hwnd, SB_HORZ, &si, TRUE); break; } case WM_VSCROLL: { int xDelta = 0; int yDelta; // yDelta = new_pos - current_pos int yNewPos; // new position switch (LOWORD(wParam)) { // User clicked the scroll bar shaft above the scroll box. case SB_PAGEUP: yNewPos = yCurrentScroll - 50; break; // User clicked the scroll bar shaft below the scroll box. case SB_PAGEDOWN: yNewPos = yCurrentScroll + 50; break; // User clicked the top arrow. case SB_LINEUP: yNewPos = yCurrentScroll - 5; break; // User clicked the bottom arrow. case SB_LINEDOWN: yNewPos = yCurrentScroll + 5; break; // User dragged the scroll box. case SB_THUMBPOSITION: yNewPos = HIWORD(wParam); break; default: yNewPos = yCurrentScroll; } // New position must be between 0 and the screen height. yNewPos = max(0, yNewPos); yNewPos = min(yMaxScroll, yNewPos); // If the current position does not change, do not scroll. if (yNewPos == yCurrentScroll) break; // Set the scroll flag to TRUE. fScroll = TRUE; // Determine the amount scrolled (in pixels). yDelta = yNewPos - yCurrentScroll; // Reset the current scroll position. yCurrentScroll = yNewPos; // Scroll the window. (The system repaints most of the // client area when ScrollWindowEx is called; however, it is // necessary to call UpdateWindow in order to repaint the // rectangle of pixels that were invalidated.) ScrollWindowEx(hwnd, -xDelta, -yDelta, (CONST RECT *) NULL, (CONST RECT *) NULL, (HRGN)NULL, (PRECT)NULL, SW_INVALIDATE); UpdateWindow(hwnd); // Reset the scroll bar. si.cbSize = sizeof(si); si.fMask = SIF_POS; si.nPos = yCurrentScroll; SetScrollInfo(hwnd, SB_VERT, &si, TRUE); break; } case WM_RBUTTONDOWN: { // Get the compatible DC of the client area. hdcWin = GetDC(hwnd); // Fill the client area to remove any existing contents. RECT rect; GetClientRect(hwnd, &rect); FillRect(hdcWin, &rect, (HBRUSH)(COLOR_WINDOW + 1)); // Copy the contents of the current screen // into the compatible DC. BitBlt(hdcScreenCompat, 0, 0, bmp.bmWidth, bmp.bmHeight, hdcScreen, 0, 0, SRCCOPY); // Copy the compatible DC to the client area. BitBlt(hdcWin, 0, 0, bmp.bmWidth, bmp.bmHeight, hdcScreenCompat, 0, 0, SRCCOPY); ReleaseDC(hwnd, hdcWin); fBlt = TRUE; break; } case WM_DESTROY: PostQuitMessage(0); return 0; } return DefWindowProc(hwnd, uMsg, wParam, lParam); } // WinMain int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow) { WNDCLASSEX wc; HWND hwnd; MSG Msg; // register main window class wc.cbSize = sizeof(WNDCLASSEX); wc.style = 0; wc.lpfnWndProc = WndProc; wc.cbClsExtra = 0; wc.cbWndExtra = 0; wc.hInstance = hInstance; wc.hIcon = LoadIcon(hInstance, IDI_APPLICATION); wc.hCursor = LoadCursor(NULL, IDC_ARROW); wc.hbrBackground = GetSysColorBrush(COLOR_WINDOW); wc.lpszMenuName = NULL; wc.lpszClassName = L"Main_Window"; wc.hIconSm = LoadIcon(hInstance, IDI_APPLICATION); if (!RegisterClassEx(&wc)) { MessageBox(NULL, L"Window Registration Failed!", L"Error!", MB_ICONEXCLAMATION | MB_OK); return 0; } // create main window hwnd = CreateWindowEx(0, L"Main_Window", L"Scrollable map", WS_OVERLAPPEDWINDOW | WS_VSCROLL | WS_HSCROLL, 50, 50, 400, 400, NULL, NULL, hInstance, 0); ShowWindow(hwnd, nCmdShow); UpdateWindow(hwnd); while (GetMessage(&Msg, NULL, 0, 0) > 0) { TranslateMessage(&Msg); DispatchMessage(&Msg); } return Msg.wParam; } 

Answer 1

From your description, the problem is the simplicity of the fact that your window will not redraw when resized.

Either use InvalidateRect to force redrawing the newly opened areas when your window is enlarged, or set the CS_VREDRAW and CS_HREDRAW in your window class to automatically recolor the client area.