How to Scroll a Bitmap
This section describes changes you can make to an application's main window procedure to enable the user to scroll a bitmap.
The example includes a menu item that copies the screen content to a bitmap, and displays the bitmap in the client area. The example also processes the WM_HSCROLL and WM_VSCROLL messages that are generated by the scroll bars so that the user may scroll the bitmap horizontally and vertically. Unlike the example for scrolled text, the bitmap example employs the BitBlt function to draw the invalid portion of the client area.
What you need to know
Technologies
Prerequisites
- C/C++
- Windows User Interface Programming
Instructions
Processing the WM_CREATE Message
When the WM_CREATE message is processed, the variables that are required for scrolling are initialized. Use the CreateCompatibleDC function to create a compatible device context (DC), the CreateBitmap function to create a bitmap, and the SelectObject function to select the bitmap for the DC. Note that a compatible DC is also known as a memory DC.
The device-specific information about the display device is retrieved. If a compatible DC is created for the screen, as in the example, use the GetDeviceCaps function to get this information. The information includes the number of adjacent color bits per pixel, the number of color planes, and the height and width of the DC.
Processing the WM_SIZE Message
Processing of the WM_SIZE message requires adjusting the scrolling range and position, so that it reflects the dimensions of the client area and the bitmap that will be displayed.
The SetScrollInfo function sets the minimum and maximum position values, the page size, and the scrolling position for a scroll bar.
Processing the WM_HSCROLL and WM_VSCROLL Messages
When the WM_HSCROLL and WM_VSCROLL messages are processed, the scroll bar request code is examined and the scrolling position is set to a new value that reflects the scrolling action of the user. If the scrolling position is within the scrolling range, the window is scrolled to the new position by using the ScrollWindow function. The position of the scroll box is then adjusted by using the SetScrollInfo function.
After a window is scrolled, part of its client area is made invalid. To ensure that the invalid region is updated, use the UpdateWindow function to generate a WM_PAINT message. When processing the WM_PAINT message, an application must repaint the invalid region at the bottom of the client area. When scrolling or resizing the client area, the example uses the BitBlt function to copy the appropriate portion of the bitmap to the invalid portion of the client area.
Example of Scrolling a Bitmap
The following example enables the user to capture the screen content into a bitmap and scroll the bitmap in the client area. Screen content is captured when the user right-clicks the client area.
LRESULT CALLBACK MyBitmapWindowProc(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's 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);
}
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