GradientFill API doesn't perform as it should

Question

5.00/5 (1 vote)

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INTRODUCTION AND RELEVANT INFORMATION:

I want to create a static control with gradient background.

I want to do it the following way:

Create the gradient at the main window’s background, and then place transparent static control on top of that background.

To do that, I have created RECT variable in WM_PAINT handler that positions the gradient at the position where static control should be.

Also, I try to use double buffering to avoid flickering ( I have also handled WM_ERASEBKGND, removed flags CS_VREDRAW and CS_HREDRAW from window class ).

I have also handled WM_SIZE message to invalidate the window, and to reposition static control properly.

In my WM_CTLCOLORSTATIC handler, I have returned NULL_BRUSH.

I have made a demo application to illustrate this, via application wizard in Visual Studio.

I work on Windows XP, using pure Win32 API and C++.

Bellow I will submit the link to the demo project:

http://www.filedropper.com/gradientstaticcontrol[^]

PROBLEM:

When I use GradientFill API in WM_PAINT handler, I get a bigger rectangle on the screen than it should be.

Bellow picture illustrates this result:

http://pbrd.co/1cMWhip[^]

If I try to fill the same rectangle with some solid brush, everything works fine.

Bellow picture illustrates this result:

http://pbrd.co/1bjz7cT[^]

MY EFFORTS TO SOLVE PROBLEM:

I have placed breakpoints at the places where rectangle’s coordinates are set and have seen that there is no problem with setting the rectangles coordinates.

GradientFill returns TRUE, so there should be no problems.

QUESTION:

How to fix this?

Thank you in advance.

Regards.

Posted 10-Nov-13 16:40pm

AlwaysLearningNewStuff

Updated 10-Nov-13 17:18pm

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enhzflep · Accepted Answer · 2013-11-10T19:03:00

There's just two changes you need to make. In each case, it's because you've forgotten that when drawing to the HDC of a control, the RECT always has the top and left members set to 0.

However, when drawing this rect onto the background of the dialog, the top and left members are no longer 0.

The changes you need to make are:

1.
WAS:

C++

vertexS[1].y     = ( rect.bottom - rect.top ) / 2;

IS:

C++

vertexS[1].y     = rect.top + ( rect.bottom - rect.top ) / 2;

2.
WAS:

C++

vertex1S[0].y     = ( rect.bottom - rect.top ) / 2;

IS:

C++

vertex1S[0].y     = rect.top + ( rect.bottom - rect.top ) / 2;

I strongly recommend that you use the functions I've mentioned in the past, repeated here for clarity.

C++

void GradientFillRect(HDC hdc, RECT fillMe, COLORREF colStart, COLORREF colEnd, bool isVertical)
{
    TRIVERTEX vertex[2];
    GRADIENT_RECT gRect;
    gRect.UpperLeft = 0;
    gRect.LowerRight = 1;
    vertex[0].x = fillMe.left;
    vertex[0].y = fillMe.top;
    vertex[0].Red = GetRValue(colStart) << 8;
    vertex[0].Green = GetGValue(colStart) << 8;
    vertex[0].Blue = GetBValue(colStart) << 8;
    vertex[0].Alpha = 255;

    vertex[1].x = fillMe.right;
    vertex[1].y = fillMe.bottom;
    vertex[1].Red = GetRValue(colEnd) << 8;
    vertex[1].Green = GetGValue(colEnd) << 8;
    vertex[1].Blue = GetBValue(colEnd) << 8;
    vertex[1].Alpha = 255;

    if (isVertical)
        GradientFill(hdc, vertex, 2, &gRect, 1, GRADIENT_FILL_RECT_V );
    else
        GradientFill(hdc, vertex, 2, &gRect, 1, GRADIENT_FILL_RECT_H );
}

C++

void drawBigBtn(HDC dst, RECT btnRect, wchar_t *wndText, HFONT btnFont)
{
    RECT topRect, botRect;
    topRect = botRect = btnRect;
    topRect.bottom = botRect.top = ((btnRect.bottom - btnRect.top)/2) + btnRect.top;
    GradientFillRect(dst, topRect, RGB(0x95,0xb3,0xd7), RGB(0x4f,0x8b,0xb0), true);
    GradientFillRect(dst, botRect, RGB(0x4f,0x8b,0xb0), RGB(0x95,0xb3,0xd7), true);

    HBRUSH blueBrush, oldBrush;
    blueBrush = CreateSolidBrush(RGB(79,129,189));
    oldBrush = (HBRUSH) SelectObject(dst, blueBrush);
    FrameRect(dst, &btnRect, blueBrush);
    SelectObject(dst, oldBrush);
    DeleteObject(blueBrush);


    RECT textRect = btnRect;
    textRect.top = textRect.bottom - 40;
    SetBkMode(dst, TRANSPARENT);

   int textLen = wcslen(wndText);
   HFONT oldFont;
   if (btnFont != NULL)
   {
        oldFont = (HFONT)SelectObject( dst, btnFont );
        DrawTextEx( dst, wndText, textLen, &textRect, DT_CENTER | DT_VCENTER | DT_SINGLELINE | DT_WORDBREAK, 0 );
        SelectObject(dst, oldFont);
   }
   else
        DrawTextEx( dst, wndText, textLen, &textRect, DT_CENTER | DT_VCENTER | DT_SINGLELINE | DT_WORDBREAK, 0 );
}