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See more: C++ GDI+ font
This my friends, is a going to be long one...
I am getting some quite bizarre behaviour when I try to render text with GDI+ in a layered window.
The strange thing is that for some combinations of font/font-style/font-size, GDI+ changes the rendering method. For Tahoma-Bold fonts sizes between 8.49 and 16.49 (Pixel-Units) inclusive "fail". For other fonts and styles I get "fails" at different sizes.
output from code below[^]
For clarity I have provided a complete executable example further down. The two key parameters to play with are on line 23:
Color g_oTextColor( 255, 240, 0, 0 ); // Simply change Color to ( 254, 240, 0, 0 ) [to add slight transparency] and everything will work!
#define USE_LAYERED_WINDOW // or just comment this line out [to use a regular window], and everything will work!
When using layered windows and full opacity the fonts draw a transparent "hole" in the background. However if I add a slight transparency to the text colour (alpha-channel = 254) the fonts become opaque!! Or if I use regular (non-layered) windows the fonts render opaque. What is going on here??
But even without the layered/transparency problems it is clear that something strange is happening here. The fonts size 8.49 - 16.48 get rendered pixel perfect, the other fonts have slight blurry quality, especially the small ones. So it seems that the system takes a different approach to rendering these medium sizes. Can somebody shed some light on this, how can I possibly render for example fonts size 8.0 pixels without the blurriness above? I have tried all sorts of settings for SetTextRenderingHint() and SetTextContrast() but none looked crisp for fonts of size 8. I have tried Tahoma & Arial only...
Side question 1: I wanted to use pure GDI+ for the off-screen drawing but I could not get it work by simply creating Bitmap & Graphics objects. I still had to use old GDI stuff to create a DC and to select the HBitmap into it. How can I do it all in GDI+?
Side question 2 (Geeks only): I also tried to draw the fonts in good-old GDI but there I got some even more bizarre effects: (A) In a layered window the text became transparent but in an additive way. (So a red text would look fine if the window behind was dark but if the window behind it was whit the text disappeared completely!) Furthermore if I filled my own window with a semi-transparent square, then that behave as expected. (A red square would turn dark red if the window behind it was black, and the square would turn light red over a white window). And I can observe both these behaviours simultaneously in one layered window. And (B) as a highly undesired bonus the drawn text lost it's hit-test and became un-clickable? Any explanations out there?
And if you have read this far, thanks for enduring, and thanks for any answers!
// Create as a console application project
// + Unicode charset
// + Precompiled headers off
// + make sure to add linker input: gdiplus.lib

#ifndef _WIN32_WINNT        // Allow use of features specific to Windows XP or later.                   
#define _WIN32_WINNT 0x0501 // Change this to the appropriate value to target other versions of Windows.
// Standard and GDI+ stuffstuff 
#include <stdio.h>
#include <tchar.h>
#include <windows.h>
#include <iostream>
#include <cassert>
#include <Gdiplus.h>
using namespace Gdiplus;
GdiplusStartupInput g_oGdiPlusStartupInput;
ULONG_PTR g_pGdiPlusToken = NULL;

// #*#*#*#*#*#*#*#*# LINES TO CHANGE ---------->---------->---------->
Color g_oTextColor( 255, 240, 0, 0 ); // Simply change Color to ( 254, 240, 0, 0 ) [to add slight transparency] and everything will work!
#define USE_LAYERED_WINDOW // or just comment this line out [to use a regular window], and everything will work!

// Forward declarations
void RegWndClass();
void CreateWindows();
void Draw();
void MsgLoop();
// Other Globals
ATOM g_iWndClass = 0;
HWND g_hWndGdiPlus = NULL;
HWND g_hWndGdi = NULL;
const wchar_t* g_pWndClass = L"TST";
int g_iWidth = 200;
int g_iHeight = 200;
// Main entry-point
int _tmain( int argc, _TCHAR* argv[] )
    GdiplusStartup( &g_pGdiPlusToken, &g_oGdiPlusStartupInput, NULL );
    ::UnregisterClass( g_pWndClass, NULL );
    ::Sleep( 500 );

    GdiplusShutdown( g_pGdiPlusToken );
    return 0;
} // _tmain

void CreateWindows()
        // The key trick is to create a window with style WS_EX_LAYERED, but WITHOUT any subsequent calls to SetLayeredWindowAttributes()
        // This gives us a magic window that must be updated with UpdateLayeredWindow() ( and it does NOT recieve any WM_PAINT messages )
        // as brilliantly described in:
        g_hWndGdiPlus = ::CreateWindowEx( WS_EX_LAYERED, g_pWndClass, L"", WS_POPUP | WS_VISIBLE, 1000, 200, g_iWidth, g_iHeight, NULL, NULL, NULL, NULL ); 
        g_hWndGdiPlus = ::CreateWindowEx( 0, g_pWndClass, L"", WS_OVERLAPPEDWINDOW | WS_POPUP | WS_VISIBLE, 1000, 200, g_iWidth, g_iHeight, NULL, NULL, NULL, NULL ); 
    //g_hWndGdi = ::CreateWindowEx( WS_EX_LAYERED, g_pWndClass, L"", WS_POPUP | WS_VISIBLE, 720, 500, 200, 200, NULL, NULL, NULL, NULL ); 

} // CreateWindows

void Draw()
    // Init GDI+ surface
    HDC hOff = ::CreateCompatibleDC( NULL );
    Bitmap oDaBigOne( g_iWidth, g_iHeight, PixelFormat32bppARGB );
    HBITMAP hBMit =  NULL;
    Color oCol( 0, 0, 0, 0 );
    oDaBigOne.GetHBITMAP( oCol, &hBMit );
    HGDIOBJ hSave = ::SelectObject( hOff, hBMit );
        Graphics oGraph( hOff );
        Graphics oGraph( g_hWndGdiPlus );
    oGraph.Clear( Color( 255, 55, 155, 255 ) );
    // Draw text
    oGraph.SetTextRenderingHint( TextRenderingHintAntiAliasGridFit );
    oGraph.SetTextContrast( 0xffffffff );
    oGraph.SetCompositingMode( CompositingModeSourceOver );
    oGraph.SetCompositingQuality( CompositingQualityHighQuality );
    oGraph.SetPixelOffsetMode( PixelOffsetModeHighQuality );
    const FontFamily oFamily( L"Tahoma", NULL );
#if 1 // Use bold
    Font oF600( &oFamily, 6.00, FontStyle::FontStyleBold, Unit::UnitPixel );
    Font oF800( &oFamily, 8.00, FontStyle::FontStyleBold, Unit::UnitPixel );
    Font oF848( &oFamily, 8.48, FontStyle::FontStyleBold, Unit::UnitPixel );
    Font oF849( &oFamily, 8.49, FontStyle::FontStyleBold, Unit::UnitPixel );
    Font oF1200( &oFamily, 12.00, FontStyle::FontStyleBold, Unit::UnitPixel );
    Font oF1500( &oFamily, 15.00, FontStyle::FontStyleBold, Unit::UnitPixel );
    Font oF1648( &oFamily, 16.48, FontStyle::FontStyleBold, Unit::UnitPixel );
    Font oF1649( &oFamily, 16.49, FontStyle::FontStyleBold, Unit::UnitPixel );
#else // Use regular
    Font oF600( &oFamily, 6.00, FontStyle::FontStyleRegular, Unit::UnitPixel );
    Font oF800( &oFamily, 8.00, FontStyle::FontStyleRegular, Unit::UnitPixel );
    Font oF848( &oFamily, 8.48, FontStyle::FontStyleRegular, Unit::UnitPixel );
    Font oF849( &oFamily, 8.49, FontStyle::FontStyleRegular, Unit::UnitPixel );
    Font oF1200( &oFamily, 12.00, FontStyle::FontStyleRegular, Unit::UnitPixel );
    Font oF1500( &oFamily, 15.00, FontStyle::FontStyleRegular, Unit::UnitPixel );
    Font oF1648( &oFamily, 16.48, FontStyle::FontStyleRegular, Unit::UnitPixel );
    Font oF1649( &oFamily, 16.49, FontStyle::FontStyleRegular, Unit::UnitPixel );
    assert( oF600.GetLastStatus() == Ok ); // Make sure font is OK

    SolidBrush oBrush( g_oTextColor ); 
    double dy = 1.0;
    oGraph.DrawString( L"Size 6.00", -1, &oF600, PointF( 30.0, dy += 18.0 ), &oBrush );
    oGraph.DrawString( L"Size 8.00", -1, &oF800, PointF( 30.0, dy += 18.0 ), &oBrush );
    oGraph.DrawString( L"Size 8.48", -1, &oF848, PointF( 30.0, dy += 18.0 ), &oBrush );
    oGraph.DrawString( L"Size 8.49", -1, &oF849, PointF( 30.0, dy += 18.0 ), &oBrush );
    oGraph.DrawString( L"Size 12.00", -1, &oF1200, PointF( 30.0, dy += 18.0 ), &oBrush );
    oGraph.DrawString( L"Size 15.00", -1, &oF1500, PointF( 30.0, dy += 18.0 ), &oBrush );
    oGraph.DrawString( L"Size 16.48", -1, &oF1648, PointF( 30.0, dy += 18.0 ), &oBrush );
    oGraph.DrawString( L"Size 16.49", -1, &oF1649, PointF( 30.0, dy += 18.0 ), &oBrush );
    // Do da layered window magic stuff
    BLENDFUNCTION oBF = { 0 };
    oBF.BlendOp = AC_SRC_OVER;
    oBF.BlendFlags = 0;
    oBF.SourceConstantAlpha = 255;
    oBF.AlphaFormat = AC_SRC_ALPHA;
    SIZE oSize = { 0 }; = g_iWidth; = g_iHeight;
    POINT oPTZero = { 0 };
    RECT oRect = { 0 };
    ::GetWindowRect( g_hWndGdiPlus, &oRect );
    POINT oPTWnd = { 0 };
    oPTWnd.x = oRect.left;
    oPTWnd.y =;
    //HDC hDC = oGraph.GetHDC();
    BOOL bOK = ::UpdateLayeredWindow( g_hWndGdiPlus,
        NULL, //HDC hdcDst,
        &oPTWnd, // POINT &oPtNull,
        &oSize, // SIZE *psize,
        hOff, // HDC hdcSrc,
        &oPTZero, // POINT *pptSrc,
        RGB(255,255,255), // COLORREF crKey,
        &oBF, // BLENDFUNCTION *pblend,
        ULW_ALPHA // DWORD dwFlags
} // Draw

void MsgLoop()
    ::SetTimer( g_hWndGdiPlus, 0, 19999, NULL ); // Self-destruct timer

    MSG msg = { 0 };
    while ( ::GetMessage( &msg, NULL, 0, 0 ) )
} // MsgLoop

void RegWndClass()
        WNDCLASSEX wcex = { 0 };
        wcex.cbSize          = sizeof(WNDCLASSEX);           = CS_HREDRAW | CS_VREDRAW | CS_OWNDC;
        wcex.lpfnWndProc     = WndProc;
        wcex.cbClsExtra      = 0;
        wcex.cbWndExtra      = 8; // 8 bytes, to allow for 64-bit architecture
        wcex.hInstance       = NULL; // CHECK
        wcex.hIcon           = NULL;
        wcex.hCursor         = ::LoadCursor(NULL, IDC_ARROW);
        wcex.hbrBackground   = (HBRUSH)NULL_BRUSH; // CHECK
        wcex.lpszMenuName    = NULL;
        wcex.lpszClassName   = g_pWndClass;
        wcex.hIconSm         = NULL;
        g_iWndClass = ::RegisterClassEx(&wcex);
} // RegWndClass

    switch( uiMsg )
        case WM_TIMER:
            std::wstring s;
            std::wcout <<  L"Let´s quit" ;
            ::PostQuitMessage( 0 );
            return 0;
        case WM_PAINT:
            return DefWindowProc( hWnd, uiMsg, wParam, lParam );
    return DefWindowProc( hWnd, uiMsg, wParam, lParam );
} // WndProc
Posted 14-Apr-11 23:01pm

1 solution

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Solution 1

There is a little more to alpha blending than just setting the alpha values in some color values. The unexplained behavior you described may be the result of using another blending strategy (the default of the graphics device) than you expect.
I don't usually have much to do with this in GDI+, this is something I more often need in DirectX. Therefore I can't guarantee that this will help, but this page in the MSDN may be a good starting point:[^]
Edit: I quickly skimmed through the text and found thet the Graphics::SetCompositingMode() and Graphics::SetCompositingQuality() Methods may be what you are looking for.
Edit^2: I took another look at your code and saw that you already use SetCompositingMode(). You use CompositingMode.SourceOver, which may be the problem. Have you tried CompositingMode.SourceCopy? This way the alpha value would be determined by the drawn color alone and not by the destination color.
megaadam at 18-Apr-11 6:04am
Yes I have tried CompositingModeSourceCopy, and it looks look much worse :) It produces a transparent rectangle around each character, except the mid-size characters which still are transparent, but without the rectangle. But thanks anyway for taking a look at my problem.
And I am afraid that the MSDN-link you posted does not help me. This phenomenon does not occur when drawing thin lines. It only happens with typography.
CDP1802 at 18-Apr-11 6:08am
Sorry to hear that. Only two options for CompositionMode is a little spartan. From DirectX I am used to select and combine several options for source and destination blending, making it far easier to accomplish the desired effect.

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