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Transparency is a fun and neat thing to do to windows to spice up your application a little bit.
Before Win2k, it was a hassle to even attempt to set some sort of alpha-channeling in windows, but now that
feature is included with the SDK. You can do it using what is called Layered Windows. The function
that we use which is new to the Win2k SDK is SetLayeredWindowAttributes, it is defined on MSDN as:
BOOL SetLayeredWindowAttributes(
HWND hwnd, // handle to the layered window
COLORREF crKey, // specifies the color key
BYTE bAlpha, // value for the blend function
DWORD dwFlags // action
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We do not need to worry about the crKey argument, only hwnd, dwFlags, and bAlpha. bAlpha is a value between 0 and 255, 0 being completely
transparent, and 255 being completely opaque. In our example, we will use a slider bar to set the Alpha level.
Note: This function only works with layered windows, so we must use SetWindowLong to add the
WS_EX_LAYERED style to our form.
| trans_unit.h |
//---------------------------------------------------------------------------
#ifndef trans_unitH
#define trans_unitH
//---------------------------------------------------------------------------
#include <winuser.h>
#include <Classes.hpp>
#include <Controls.hpp>
#include <StdCtrls.hpp>
#include <Forms.hpp>
#include <ComCtrls.hpp>
//---------------------------------------------------------------------------
class TForm1 : public TForm
{
__published: // IDE-managed Components
TTrackBar *TrackBar1;
TButton *Button1;
void __fastcall TrackBar1Change(TObject *Sender);
private: // User declarations
int alpha;
void __fastcall SetAlpha(int alph);
public: // User declarations
__fastcall TForm1(TComponent* Owner);
__property int Alpha = {read=alpha,write=SetAlpha};
//this property makes setting transparency easier
};
//---------------------------------------------------------------------------
extern PACKAGE TForm1 *Form1;
//---------------------------------------------------------------------------
#endif
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| trans_unit.cpp |
//---------------------------------------------------------------------------
#include <vcl.h>
#pragma hdrstop
#include "trans_unit.h"
//---------------------------------------------------------------------------
#pragma package(smart_init)
#pragma resource "*.dfm"
TForm1 *Form1;
//---------------------------------------------------------------------------
__fastcall TForm1::TForm1(TComponent* Owner)
: TForm(Owner)
{
SetWindowLong(Handle,GWL_EXSTYLE, //make our form a layered window
GetWindowLong(Handle,GWL_EXSTYLE)|WS_EX_LAYERED);
Alpha = 255; //make sure our form starts out opaque
}
//---------------------------------------------------------------------------
void __fastcall TForm1::TrackBar1Change(TObject *Sender)
{
Alpha = TrackBar1->Position; //set the transparency according to the slider
}
//---------------------------------------------------------------------------
void __fastcall TForm1::SetAlpha(int alph)
{
alpha = alph;
SetLayeredWindowAttributes(Handle,NULL,alpha,LWA_ALPHA);
//set the alpha level of our form
}
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