Clear text from a device context.

Mr Simple

Hi, I have written text to a device context using "dc.DrawText()" onto a transparent background. The text needs to move, but it leaves the old text on the DC, which just causes a long black smudge. As I mentioned, the background is transparent, so I cant just blit the background again. I have noticed that calling ShowWindow(SW_HIDE); then re-showing it clears the text, however I make the text move from within the OnPaint method, which obviously causes problems if I hide/show it constantly. Does anyone have any ideas as to how I can remove the text? Thanks. MS.

CPallini

Mr Simple wrote:

however I make the text move from within the OnPaint method

Is it a good idea? :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.

Mr Simple

Well, by 'move' i mean more 'repostitioned'. The location of the text is based upon a number of variables, the window itself is a large mishaped window. How do you think I should be drawing the text?

CPallini

Well. I don't know exactly your needs. Anyway, I think the current text position has to be assigned outside the OnPaint method (for instance inside a timer) and then you have to invalidate both the latest text rectangle and the current text one and, finally, you have to call UpdateWindow to let OnPaint doing its job. :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.

Nelek

Are you using Invalidate () before calling UpdateWindow () ?? It mades the drawing surface to be erased and then repainted, so the old text will be away, and just the new "repositioned" will be drawn.

Greetings. -------- M.D.V. ;) If something has a solution... Why do we have to worry about?. If it has no solution... For what reason do we have to worry about? Help me to understand what I'm saying, and I'll explain it better to you “The First Rule of Program Optimization: Don't do it. The Second Rule of Program Optimization (for experts only!): Don't do it yet.” - Michael A. Jackson

Mr Simple

How do you suggest I set the text from outside the paint method? Another reason I chose to use OnPaint is that I dont know how to do it without having the DC variable to use the .DrawText with! @Nelek, I have tried both Invalidate (entire), InvalidateRect (text in rectangles), and InvalidateRgn (dc region) all to no avail! Thank you for your time. MS

CPallini

Of course drawing should happen inside OnPaint method. Now suppose ('cause I don't know yet what are you trying to do...) you're doing a kind of animation of your text and, say rcCur is the current text rectangle. Inside the timer procedure you can: (1) Compute next position text rectangle rcNext. (2) Invalidate both rcCur, rcNext rectangles. (3) Set rcCur = rcNext. (4) call UpdateWindow. Point (4) makes OnPaint being called and your text position updated. Hope that helps. :)

If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler. -- Alfonso the Wise, 13th Century King of Castile.

Mark Salsbery

Also, keep in mind - you don't need to preserve a DC between WM_PAINT messages. The DC does not remember where you draw text - that's up to you to store. A DC is more like a canvas - when you paint on it, that's it. If you want to "move" something you've painted, you need to paint over the old item and repaint the item at it's new location. Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

Mr Simple

Hi, I am not (knowingly/intentionally) preserving the DC. I am able to get this to work over sections of the DC are not transparent (the redrawn image covers the old text) but I am unable to draw over the transparent old text because I dont know what lies underneath. CPallini - I hope I am not coming across as stupid, but I still cannot get your suggestion to work. Please correct my thinking if it is wrong (see below); Thank you for your time. MS. // // A gloabal rectangle. CRect rTextMain; // // In my timer; CRect rMoveTo; InvalidateRect(&rTextMain); // Your steps said to do this after, I tried both. InvalidateRect(&rMoveTo); rMoveTo.top = val1; rMoveTo.left....... rTextMain = rMoveTo; UpdateWindow(); // // In OnPaint; CFont newFont; newFont.CreateFont(15, 0, 0, 0, 400, 0, 0, 0, 0, 0, 0, 0, 0, _T("MS Sans Serif")); CFont *oldFont = dc.SelectObject(&newFont); CString sWords = "test"; dc.DrawText(sWords, &rTextMain, DT_TOP | DT_LEFT)

Mark Salsbery

You've used rMoveTo before it's initialized. You shouldn't need the second (rMoveTo) rect. This should be enough:

//
// A gloabal rectangle.
CRect rTextMain;

//
// In my timer;
InvalidateRect(&rTextMain); // Your steps said to do this after, I tried both.

rTextMain.top = val1; rTextMain.left.......

UpdateWindow();

//
// In OnPaint;
CFont newFont;
newFont.CreateFont(15, 0, 0, 0, 400, 0, 0, 0, 0, 0, 0, 0, 0, _T("MS Sans Serif"));
CFont *oldFont = dc.SelectObject(&newFont);

CString sWords = "test";
dc.DrawText(sWords, &rTextMain, DT_TOP | DT_LEFT)

The WM_ERASEBKGND should take care of the invalidated rect. Then the WM_PAINT handler should draw the text at the new location. I don't see anything related to transparency or images. Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

led mike

Mark Salsbery wrote:

you don't need to preserve a DC between WM_PAINT messages.

ummm.... don't you mean you should not?

Mark Salsbery

led mike wrote:

don't you mean you should not?

For the sake of this thread - yes. Discussing owned DCs is beyond the scope of the problem/OP/thread, IMO :) Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

Mr Simple

Hi, Unfortunately I could not get this to work either. Do you have any other ideas? This is being run on Win CE by the way. I didn't put the transparency stuff in the last post, but it comes just before the text is printed. A global region is made in the constructor with bits missing (missing bits are therefore transparent). The images are loaded into a CBitmap array. In OnPaint; To display the BMP; Create CDC from DC CDC select object (global/main region) CDC select object (bitmap) TransparentBlt to DC; TransparentBlt(dc.GetSafeHdc(), 0, 0, floatingBMP.bmWidth, floatingBMP.bmHeight, myCDC->GetSafeHdc(), 0, 0, floatingBMP.bmWidth, floatingBMP.bmHeight, 0x00ff00);// Transparency = lime green, altho this doesnt matter as the region has already been created to take this into account; After this is the DrawText code which has already been show. Any ideas? Thanks MS.

Mark Salsbery

Without knowing what's not working...

Mr Simple wrote:

Create CDC from DC

How are you doing this? It should be a memory DC created with CDC::CreateCompatibleDC().

Mr Simple wrote:

CDC select object (global/main region)

What's the purpose of the region - for clipping? Why select it into the source DC? That looks suspicious...I would try commenting the region stuff out and see what happens. Are you handling WM_ERASEBKGND? If so, what are you doing there? What do you want to see and what is happening that is wrong? Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

Mr Simple

Hi, I am using CreateCompatibleDC; CPaintDC dc(this); // device context for painting CDC *floatCDC = new CDC; floatCDC->CreateCompatibleDC(&dc); I have removed the region code, and can report having it there made no difference to my application. I had read about it in a 'funky shaped window' article and assumed I needed it, obviously I was wrong :-) I am not handling WM_ERASEBKGND. What I want to see is text moving on a transparent background. I have manged this to a degree, however when the text is moved, the old text remains underneath the new text. I think part of the problem is that I am drawing over the old text with a transparent image, so the old text is still visible. Any other ideas? Thank you for your time! MS

Mark Salsbery

Here's a working example, moving text with a timer across a bitmap background. This is implemented on a simple dialog with just an ok button. The same code would work in a CWnd-derived class - In that case, WM_CREATE/OnCreate would be used instead of OnInitDialog()...

#pragma once

#include <atlimage.h>

// CMovingTextDialog dialog

class CMovingTextDialog : public CDialog
{
DECLARE_DYNAMIC(CMovingTextDialog)

public:
CMovingTextDialog(CWnd* pParent = NULL); // standard constructor
virtual ~CMovingTextDialog();

// Dialog Data
enum { IDD = IDD_DIALOG2 }; //<-- Use the correct dialog resource ID here!

protected:
virtual void DoDataExchange(CDataExchange* pDX); // DDX/DDV support

DECLARE_MESSAGE_MAP()
public:
    virtual BOOL OnInitDialog();
protected:
    virtual void OnOK();
public:
    CImage BkImage;
    UINT_PTR MoveTimer;
    CString TextStr;
    CSize TextSize;
    CRect TextRect;

afx_msg void OnPaint();
afx_msg BOOL OnEraseBkgnd(CDC* pDC);
afx_msg void OnTimer(UINT_PTR nIDEvent);

};

// MovingTextDialog.cpp : implementation file
//

#include "stdafx.h"
#include "MFCTester.h"
#include "MovingTextDialog.h"

// CMovingTextDialog dialog

IMPLEMENT_DYNAMIC(CMovingTextDialog, CDialog)

CMovingTextDialog::CMovingTextDialog(CWnd* pParent /*=NULL*/)
    : CDialog(CMovingTextDialog::IDD, pParent)
{
    BkImage.Load(_T("e:\\test.bmp")); //<-- load your own bmp/jpg/tiff here!
    MoveTimer = 0;
    TextStr = _T("Moving Text");
    TextSize.cx = TextSize.cy = 0;
    TextRect.SetRect(0,0,0,0);
}

CMovingTextDialog::~CMovingTextDialog()
{
}

void CMovingTextDialog::DoDataExchange(CDataExchange* pDX)
{
CDialog::DoDataExchange(pDX);
}

BEGIN_MESSAGE_MAP(CMovingTextDialog, CDialog)
    ON_WM_PAINT()
    ON_WM_ERASEBKGND()
    ON_WM_TIMER()
END_MESSAGE_MAP()

// CMovingTextDialog message handlers

BOOL CMovingTextDialog::OnInitDialog()
{
CDialog::OnInitDialog();

MoveTimer = ::SetTimer(*this, (UINT_PTR)12345, 10, NULL);

return TRUE;&n

Mr Simple

Hello, The only major difference is that I am not using CImage, as I need to be able to TransparentBlt which I have been told there are problems with on WinCE. I converted your code closer to my existing (using CBitmap, transparency, removing erase bkground) and it worked great. Then I made my existing code practically idential, and it still didn't work :-( The only thing I can find different is the way I create the window. I have a method in the class called "CreateFloaterWnd" (parent as parameter) which loads the bitmap, gets it's size, then calls CWnd::CreateEx, positioning itself relative to the parent's X/Y location. I will try to convert it to use a dialog and see what happens. I still dont understand how this could be the problem though. Thank you very much for your effort!! MS.

Mark Salsbery

Hmmm I don't know about WinCE unfortunately. CImage wraps a little GDI+ for loading and saving, and also wraps a HBITMAP/DIBSection, which works with TransparentBlt() just fine (in fact, the class has a TransparentBlt() method). Again, I'm not sure about WinCE compatibility though. If you have a little project that doesn't work that I can test on regular Windows I'll check it out. Good luck :) Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

Mr Simple

Hi, Everything in normal windows can be done in CE, sometimes you just need to go about it a bit differently. I have cut out everything from my project and managed to write a non-working class if you are interested. I wont have access to CE again until monday so wont be able to test the dialog thing yet. Below is the code, it is very simple as you can probably see. Just call the .CreateTransparent(this); and you should get a nice streak down the form. Once again, thanks for your help! MS. The .H; class CNotWorking : public CWnd { public: CNotWorking(void); ~CNotWorking(void); void CreateTransparent(CWnd *pParent); DECLARE_MESSAGE_MAP() afx_msg void OnPaint(); afx_msg void OnTimer(UINT nIDEvent); private: CRect rTextRect; UINT tTicker; }; ////////// And the .CPP; BEGIN_MESSAGE_MAP(CNotWorking, CWnd) ON_WM_PAINT() ON_WM_TIMER() END_MESSAGE_MAP() CNotWorking::CNotWorking(void) { rTextRect.SetRect(0, 0, 40, 20); tTicker = 0; } CNotWorking::~CNotWorking(void) { } void CNotWorking::CreateTransparent(CWnd *pParent) { // Create & flag as existing. CWnd::CreateEx(NULL, NULL, _T("Transparent"), WS_CHILD, 20, 20, 500, 500, pParent->GetSafeHwnd(), NULL); tTicker = ::SetTimer(*this, (UINT_PTR)12345, 20, NULL); ShowWindow(SW_SHOW); } void CNotWorking::OnPaint() { CPaintDC dc(this); // device context for painting dc.SetBkMode(TRANSPARENT); CString textStr = "test"; dc.DrawText(textStr, &rTextRect, DT_LEFT | DT_TOP); } void CNotWorking::OnTimer(UINT nIDEvent) { InvalidateRect(rTextRect); rTextRect.OffsetRect(0, 2); UpdateWindow(); CWnd::OnTimer(nIDEvent); }

Mark Salsbery

Your code works for me. In OnTimer(), I had to change InvalidateRect(rTextRect); to InvalidateRect(&rTextRect); Mark

Mark Salsbery Microsoft MVP - Visual C++ :java: