How to make a fade effect in WINAPI GDI? - c

Question

Below I have a code snippet showing what I have tried.
#include <stdio.h>
#include <stdlib.h>
#include <windows.h>
#include <windowsx.h>
#define WC_MAIN "MainClass"
#define WC_NUMBER "NumberClass"
LRESULT CALLBACK NumberProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
LRESULT CALLBACK MainWndProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam);
void InitClasses(void)
{
WNDCLASS wc;
memset(&wc, 0, sizeof(wc));
wc.hbrBackground = GetStockObject(WHITE_BRUSH);
wc.hCursor = LoadCursor(NULL, IDC_ARROW);
wc.lpfnWndProc = MainWndProc;
wc.lpszClassName = WC_MAIN;
RegisterClass(&wc);
wc.lpfnWndProc = NumberProc;
wc.lpszClassName = WC_NUMBER;
RegisterClass(&wc);
}
#define NUMBER_SPEED 2
#define NUMBER_TICK_SPEED 25
#define NUMBER_TICKS 55
typedef struct {
UINT ticks;
HBITMAP buffer;
} NUMBERINFO;
HWND CreateNumber(HWND parent, const char *text, int x, int y, COLORREF color)
{
NUMBERINFO *ni = malloc(sizeof(*ni));
HDC hdc = GetDC(NULL);
HFONT oldFont = NULL;
SIZE s;
GetTextExtentPoint32(hdc, text, strlen(text), &s);
SelectObject(hdc, oldFont);
BITMAPINFO bmi;
bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);
bmi.bmiHeader.biWidth = s.cx;
bmi.bmiHeader.biHeight = s.cy;
bmi.bmiHeader.biPlanes = 1;
bmi.bmiHeader.biBitCount = 32; // four 8-bit components
bmi.bmiHeader.biCompression = BI_RGB;
bmi.bmiHeader.biSizeImage = s.cx * s.cy * 4;
COLORREF *pvBits;
HBITMAP buffer = CreateDIBSection(hdc, &bmi, DIB_RGB_COLORS, (void**) &pvBits, NULL, 0);
HDC bufferDc = CreateCompatibleDC(hdc);
HBITMAP oldBmp = SelectObject(bufferDc, buffer);
oldFont = NULL;
SetTextAlign(bufferDc, TA_TOP | TA_LEFT);
SetBkMode(bufferDc, TRANSPARENT);
SetTextColor(bufferDc, color);
TextOut(bufferDc, 0, 0, text, strlen(text));
SelectObject(bufferDc, oldFont);
SelectObject(bufferDc, oldBmp);
DeleteDC(bufferDc);
ReleaseDC(NULL, hdc);
ni->buffer = buffer;
return CreateWindow(WC_NUMBER, text, WS_VISIBLE | WS_CHILD, x - (s.cx >> 1), y - (s.cy >> 1), s.cx, s.cy, parent, NULL, NULL, ni);
}
LRESULT CALLBACK NumberProc(HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
NUMBERINFO *info = (NUMBERINFO*) GetWindowLongPtr(hWnd, GWLP_USERDATA);
switch(msg)
{
case WM_CREATE:
info = ((CREATESTRUCT*) lParam)->lpCreateParams;
SetWindowLongPtr(hWnd, GWLP_USERDATA, (LONG_PTR) info);
SetTimer(hWnd, 0, NUMBER_TICK_SPEED, NULL);
info->ticks = NUMBER_TICKS;
return 0;
case WM_DESTROY: return 0;
case WM_TIMER:
{
RECT rc;
GetWindowRect(hWnd, &rc);
HWND parent = GetParent(hWnd);
MapWindowPoints(HWND_DESKTOP, parent, (POINT*) &rc, 2);
rc.top -= NUMBER_SPEED;
if(!--info->ticks)
{
DestroyWindow(hWnd);
return 0;
}
SetWindowPos(hWnd, NULL, rc.left, rc.top, rc.right - rc.left, rc.bottom - rc.top - NUMBER_SPEED, SWP_NOREDRAW | SWP_NOCOPYBITS);
// redraw parent call erases last shown number
RedrawWindow(GetParent(hWnd), &rc, NULL, RDW_ERASE | RDW_INVALIDATE | RDW_UPDATENOW);
return 0;
}
case WM_ERASEBKGND: return 1;
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hWnd, &ps);
RECT r;
GetClientRect(hWnd, &r);
HDC bufferDc = CreateCompatibleDC(hdc);
HBITMAP oldBmp = SelectObject(bufferDc, info->buffer);
BLENDFUNCTION bfn;
bfn.BlendOp = AC_SRC_OVER;
bfn.BlendFlags = 0;
bfn.SourceConstantAlpha = info->ticks * 0xFF / NUMBER_TICKS;
bfn.AlphaFormat = 0;
//TransparentBlt(hdc, 0, 0, r.right, r.bottom, bufferDc, 0, 0, r.right, r.bottom, 0);
AlphaBlend(hdc, 0, 0, r.right, r.bottom, bufferDc, 0, 0, r.right, r.bottom, bfn);
SelectObject(bufferDc, oldBmp);
DeleteDC(bufferDc);
EndPaint(hWnd, &ps);
return 0;
}
}
return DefWindowProc(hWnd, msg, wParam, lParam);
}
LRESULT CALLBACK MainWndProc (HWND hWnd, UINT msg, WPARAM wParam, LPARAM lParam)
{
switch(msg)
{
case WM_DESTROY: PostQuitMessage(0); return 0;
case WM_ERASEBKGND: return 1;
case WM_PAINT:
{
InvalidateRect(hWnd, NULL, FALSE);
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hWnd, &ps);
// draw gradient (red to green)
RECT r;
GetClientRect(hWnd, &r);
GRADIENT_RECT gr;
gr.UpperLeft = 0;
gr.LowerRight = 1;
TRIVERTEX pVertex[2] = {
{ 0, 0, 0xFF00, 0x0000, 0x0000, 0xFF00 },
{ r.right, r.bottom, 0x0000, 0xFF00, 0x0000, 0xFF00 }
};
GradientFill(hdc, pVertex, 2, &gr, 2, GRADIENT_FILL_RECT_H);
EndPaint(hWnd, &ps);
return 0;
}
}
return DefWindowProc(hWnd, msg, wParam, lParam);
}
int main()
{
InitClasses();
HWND mainWindow = CreateWindow(WC_MAIN, "Title", WS_OVERLAPPEDWINDOW, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL, NULL, GetModuleHandle(NULL), NULL);
ShowWindow(mainWindow, 1);
UpdateWindow(mainWindow);
MSG msg;
while(GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
// adding a number at mouse position if pressed (testing)
if(msg.message == WM_LBUTTONDOWN)
{
RECT r;
GetClientRect(mainWindow, &r);
int x = GET_X_LPARAM(msg.lParam);
int y = GET_Y_LPARAM(msg.lParam);
CreateNumber(mainWindow, "123", x, y, 0xFFFF00);
}
}
return 0;
}
This draws some text with any color and every time a timer ticks, the text becomes more faded (more transparent), but the black background of the bitmap the text was drawn on, is drawn, but I want no background to be present, just the text.
I would assume I need a combination of TransparentBlt and AlphaBlend.
How would I go on to solve this?

Answer 1

The solution was quite simple. It was to use AC_SRC_ALPHA for the AlphaFormat member of BLENDFUNCTION and in order for that to work, set the alpha values of the HBITMAP, the buffer.
After the buffer (the one inside of NUMBERINFO) is created (in CreateNumber; check question code for reference), one must loop over all the colors and set the alpha value to 255 where needed.
for(int i = s.cx * s.cy; i--; pvBits++)
{
if(*pvBits)
*pvBits |= 0xFF000000; // make non black pixels opaque
}
And setting the flag in WM_PAINT: bfn.AlphaFormat = AC_SRC_ALPHA;