GDI+ Load a jpg and save as 24bit png problem

Question

Problem

Hello all!

I have this code which takes my jpg image loops through altering pixels and finally saving it as a png type. The problem is that the resulting image has a bit depth of 32 bits. I need it to be 24 bit, can any one shiny some light on the correct method of setting it? Am I along the right tracks looking at setting the pixel format to PixelFormat24bppRGB?

Code

static inline void Brighten(Gdiplus::Bitmap* img)
{
    int width = img->GetWidth()/8,height = img->GetHeight(), max = (width*height),r,g,b;
    Gdiplus::Color pixel;   
    for(int a = 0,x = 0, y = -1; a < max; ++a)
    {
        x = a%width;    
        if(x == 0)
            ++y;
        img->GetPixel(x,y,&pixel);
        r = pixel.GetR();
        g = pixel.GetG();
        b = pixel.GetB();
        if (r > 245) r = 245; 
        if (g > 245) g = 245;  
        if (b > 245) b = 245; 
        r += 10;
        g += 10;
        b += 10;
        pixel = Gdiplus::Color(r,g,b);
        img->SetPixel(x,y,pixel);;
    }
}

ULONG_PTR m_dwToken = 0;
Gdiplus::GdiplusStartupInput input;
Gdiplus::GdiplusStartupOutput output;
Gdiplus::GdiplusStartup( &m_dwToken, &input, &output );
USES_CONVERSION_EX;
Gdiplus::ImageCodecInfo* pEncoders = static_cast< Gdiplus::ImageCodecInfo* >( _ATL_SAFE_ALLOCA(1040, _ATL_SAFE_ALLOCA_DEF_THRESHOLD));
Gdiplus::DllExports::GdipGetImageEncoders(5, 1040, pEncoders );
CLSID clsidEncoder = pEncoders[4].Clsid;

Gdiplus::Bitmap img1((CT2W)L"IMG_1.JPG");
Brighten(&img1);
img1.Save((CT2W)L"IMG_1_R3.PNG",&clsidEncoder,NULL);

Thanks in advance!

Answer 1

Ok I actually fixed it, I've used the LockBits() to access the bits directly and modify them, this gives me a much needed performance boost and keeps it as a 24bit image.

static inline void Brighten(Gdiplus::Bitmap* img)
{
    int width = img->GetWidth(),height = img->GetHeight(),r,g,b;
    Gdiplus::Rect rect(0,0,width,height);
    Gdiplus::BitmapData data;
    img->LockBits(&rect,Gdiplus::ImageLockModeWrite,PixelFormat24bppRGB,&data);
    int stride = data.Stride,offset = stride - width*3;
    byte * p1 = (byte *)(void *)data.Scan0;
    for (int y = 0; y < height; y++)
    {
        for (int x = 0; x < width; x++)
        {
            r = p1[0];
            g = p1[1];
            b = p1[2];
            if (r > 245) r = 245; 
            if (g > 245) g = 245;  
            if (b > 245) b = 245; 
            r += 10;
            g += 10;
            b += 10;    
            p1[0] = r;
            p1[1] = g;
            p1[2] = b;
            p1 += 3;
        }
        p1 += offset;
    }
    img->UnlockBits(&data);     
}

Thanks for all of your help!