How to use CImage class to resize a bitmap

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I want to resize a bit picture saved in a file with the following code

C++

CImage img;
CImage sm_img;

img.Load(FileName);

sm_img.Create(80,60,img.GetBPP(),
HDC sm_hdc = sm_img.GetDC();

img.StretchBlt(sm_hdc,0,0,80,60,SRCCOPY);
sm_img.Save(sjpgFileName);

img.Detach();

The result is bad as colors are lost. I guest we need to set som attributes for the variable sm_img but don't know how. Can you help me, please !

Posted 4-Jun-12 23:58pm

Lam Giang

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2 solutions

Solution 1

See, for instance, here: "Scaling CImage"[^].

Posted 5-Jun-12 0:03am

CPallini

Comments

Lam Giang 5-Jun-12 8:46am

In the first version, I got a bad image, but in the version using your solution
I've got a black one. Still do not understand

CPallini 5-Jun-12 8:55am

Did you pass the screen DC to CreateCompatibleDC, as suggested?

Lam Giang 5-Jun-12 9:44am

This is the code snippet I tested
<pre lang="c++">

CImage img;
CImage sm_img;
img.Load(jpgFileName);

CDC *screenDC = GetDC();
CDC *pMDC = new CDC;
pMDC->CreateCompatibleDC(screenDC);

CBitmap *pb = new CBitmap;

int iNewWidth = 80;
int iNewHeight = 60;

pb->CreateCompatibleBitmap(pMDC, iNewWidth, iNewHeight);
//SetStretchBltMode(pMDC->m_hDC,COLORONCOLOR);
img.StretchBlt(pMDC->m_hDC,0, 0, iNewWidth, iNewHeight, SRCCOPY);

// The next two lines test the image on a picture control.
// The output has wrong color palette, like my initial version

HDC hdcpic = ::GetDC(m_ClipPreview.m_hWnd);
img.StretchBlt( hdcpic,0, 0, iNewWidth, iNewHeight, SRCCOPY);

CBitmap *pob = pMDC->SelectObject(pb);

// The next lines create a black image

sm_img.Attach((HBITMAP)(*pb));
sm_img.Save(sjpgFileName);
img.Detach();
ReleaseDC(screenDC);

</pre>

I am running VS2005 on Windows 7, screen resolution 1280x800, 32 color bit

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ErnestoNet · Accepted Answer · 2012-06-05T06:57:00

Solution 3

You don't need to create a DC (pMDC). Just use the DC from sm_img (sm_img.GetDC()):

    CImage img;
CImage sm_img;
HRESULT res = img.Load(jpgFileName);

CDC *screenDC = GetDC();

int iNewWidth = 80;
int iNewHeight = 60;

sm_img.Create(iNewWidth, iNewHeight,32);

SetStretchBltMode(sm_img.GetDC(),COLORONCOLOR);
img.StretchBlt(sm_img.GetDC(),0, 0, iNewWidth, iNewHeight, SRCCOPY);
// The next two lines test the image on a picture control.
// The output has wrong color palette, like my initial version
HDC hdcpic = ::GetDC(m_ClipPreview.m_hWnd);

img.StretchBlt( hdcpic,0, 0, iNewWidth, iNewHeight, SRCCOPY);

sm_img.Save(jpgFileName);
ReleaseDC(screenDC);

Hope it helps

I added one line

C++

sm_img.ReleaseDC();

in the end to avoid error when exiting the function

Posted 5-Jun-12 6:57am

ErnestoNet

Updated 5-Jun-12 18:30pm

v2

Comments

Lam Giang 5-Jun-12 23:52pm

Yes, it really helps.

ErnestoNet 6-Jun-12 9:49am

sm_img.ReleaseDC();
You're right, I forgot to release the DC.
With the line you added it should work.

Member 15786969 23-Mar-23 2:08am

CImage img;
CImage sm_img;
img.Load(L"C://Users//mi//Downloads//drishg.bmp");

CDC* screenDC = GetDC();

int iNewWidth = 384;
int iNewHeight = 384;

sm_img.Create(iNewWidth, iNewHeight, 24);

SetStretchBltMode(sm_img.GetDC(), COLORONCOLOR);
img.StretchBlt(sm_img.GetDC(), 0, 0, iNewWidth, iNewHeight, SRCCOPY);

// The next two lines test the image on a picture control.
//HDC hdcpic = sm_img.GetDC();

//img.StretchBlt(hdcpic, 0, 0, iNewWidth, iNewHeight, SRCCOPY);

sm_img.Save(L"C://Users//mi//Downloads//Resizedrish.bmp");

sm_img.Destroy();
ReleaseDC(screenDC);
sm_img.ReleaseDC();

I'm using this but still giving me asserrtion