Writing a CDC to a jpeg file

Hi all
I've been trying all day to sort this out so I was hoping someone could help!!

Basically, I am trying to write the contents of a CDC to a JPEG file and I am trying to use libjpeg.lib. Below is a simplified snippet of the code I am using at present. What I am intending the code to do is to create a picture with a square and a circle and then write this to a JPEG file. It manages to create a JPEG file with the right size however it is completely black so obviously I am doing something wrong! Any help would be greatly appreciated.

Thanks very much for reading,

Robbie

(ps. I am using Visual C++ v6 and am trying to do this within a MFC dialog based project)

Code:

---

void CNucPlot::SaveFile()
{
        CDC myDC;
        HBITMAP myBMP;
        myDC.CreateCompatibleDC(NULL);

        //  Set pictures size;
        int Width = 500;
        int Height = 500;

        // Draw rectangle and an ellipse to myDC ..
        CBrush newColour(RGB(10,10,10));
        myDC.SelectObject(newColour);
        CPen lColourPen (PS_SOLID, 1,RGB(50,100,150));
        myDC.SelectObject(&lColourPen); // 100

        myDC.SetBkColor(RGB(255,255,255));
        myDC.Rectangle(10,20,100,90);
        CRect Rect;
        Rect.left  = 10;
        Rect.top    = 20;
        Rect.right  = 100;
        Rect.bottom = 90;
        myDC.Ellipse(Rect);

        myBMP = CreateCompatibleBitmap(myDC,Width,Height);
        SelectObject(myDC,myBMP);

        struct jpeg_compress_struct cinfo;
        struct jpeg_error_mgr jerr;
        FILE * outfile;
        JSAMPLE* scanline;
        COLORREF pixel;

        cinfo.err = jpeg_std_error(&jerr);
        jpeg_create_compress(&cinfo);

        outfile = fopen("robbie.jpg", "wb");
        if(outfile == NULL) return;

        jpeg_stdio_dest(&cinfo, outfile);
        cinfo.image_width = Width;
        cinfo.image_height = Height;
        cinfo.input_components = 3;
        cinfo.in_color_space = JCS_RGB;       
        jpeg_set_defaults(&cinfo);

        int quality = 90;
        if(quality < 0) quality = 0;
        if(quality > 100) quality = 100;

        jpeg_set_quality(&cinfo, quality, FALSE);
        jpeg_start_compress(&cinfo, TRUE);
        scanline = new JSAMPLE[Width*3];

        for(int posy = 0; posy < Height; posy++)
        {
                for(int posx = 0; posx < Width; posx++)
                {
                        pixel = GetPixel(myDC, posx, posy);
                        scanline[posx*3+0] = GetRValue(pixel);
                        scanline[posx*3+1] = GetGValue(pixel);
                        scanline[posx*3+2] = GetBValue(pixel);

                }
                jpeg_write_scanlines(&cinfo, &scanline, 1);
        }
        jpeg_finish_compress(&cinfo);
        jpeg_destroy_compress(&cinfo);

        delete scanline;
        fclose(outfile);
}

---

Please put [code][/code] tags around your code and indent it properly to make it more readable.

Try painting the entire DC white before you paint something else on it.

Hi, I've edited my original post with the tags. I've also added the line myDC.SetBkColor(RGB(255,255,255)); but the JPEG image created is still black!

Any other ideas anyone?

Thanks

You have a lot of reading to do about GDI…
First, ignore the JPEG stuff for now.
A newly created DC has a 1x1 monochrome bitmap selected into it.
All your drawings have no effect on that DC,
CreateCompatibleBitmap() creates bitmap *compatible* to that DC (also monochrome), so colored output to that makes a little sense anyway.
The steps should be:

• create DC compatible to a screen;
• create bitmap with a required size, also compatible to a screen;
• select that bitmap into DC;
• NOW draw whatever you want on that DC;
• and don’t forget to clean up after yourself.

I would like to suggest a great GDI debugging tool that let you examine content of your GDI objects (memory DC, bitmap, etc.) while you debug your code, line-by-line.

Thanks VladimirF

I've taken your advice and re-wrote the code as shown below:

Code:

---

void CNucPlot::SaveFile()
{
        CPaintDC dc(this); // device context for painting
        //  Set pictures size;
        int Width = 700;
        int Height = 700;

  //  get the area to draw in.
        CRect rcClient;
        rcClient.top = 0;
        rcClient.left = 0;
        rcClient.bottom = Height;
        rcClient.right = Width;

  //  create a memory dc for double buffering.
        CMemDC myDC(&dc, &rcClient);

        myDC.SetBkColor(RGB(255,255,255));
        int nFontHeight = Height / 80.0f;
        CBrush newColour(RGB(255,199,10));
        myDC.SelectObject(newColour);
        CPen lColourPen (PS_SOLID, 5,RGB(50,100,150));
        myDC.SelectObject(&lColourPen); // 100

        myDC.Rectangle(10,10,Width-10,Height-10);
        myDC.Ellipse(10,10,Width-10,Height-10);

        // JPEG Saving ..
        struct jpeg_compress_struct cinfo;
        struct jpeg_error_mgr jerr;
        FILE * outfile;
        JSAMPLE* scanline;
        COLORREF pixel;

        cinfo.err = jpeg_std_error(&jerr);
        jpeg_create_compress(&cinfo);

        outfile = fopen("robbie.jpg", "wb");
        if(outfile == NULL) return;

        jpeg_stdio_dest(&cinfo, outfile);
        cinfo.image_width = Width;
        cinfo.image_height = Height;
        cinfo.input_components = 3;
        cinfo.in_color_space = JCS_RGB;       
        jpeg_set_defaults(&cinfo);

        int quality = 90;
        if(quality < 0) quality = 0;
        if(quality > 100) quality = 100;

        jpeg_set_quality(&cinfo, quality, FALSE);
        jpeg_start_compress(&cinfo, TRUE);
        scanline = new JSAMPLE[Width*3];
//        MessageBox("Finished setting up JPEG - now go through each JPEG line\n");

        for(int posy = 0; posy < Height; posy++)
        {
                for(int posx = 0; posx < Width; posx++)
                {
                        pixel = GetPixel(myDC, posx, posy);
                        scanline[posx*3+0] = GetRValue(pixel);
                        scanline[posx*3+1] = GetGValue(pixel);
                        scanline[posx*3+2] = GetBValue(pixel);

                }
                jpeg_write_scanlines(&cinfo, &scanline, 1);
        }
        jpeg_finish_compress(&cinfo);
        jpeg_destroy_compress(&cinfo);

        delete scanline;
        fclose(outfile);
}

---

Now it writes correctly to a JPEG file. My problem with this code is that the picture displays on screen as well. Does anyone know how I can modify the code so that it only generates the JPEG file?

Also, when I try to generate a really big JPEG (say 2000 x 2000), it takes along time to complete. It might just be that it is a CPU intensive task to create a big JPEG. However does anyone know a better way of reading the CDC and dumping it to a JPEG file?

Thanks

Robbie

From my experience, GetPixel() is probably the bottleneck in your code. It is best to grab a pointer to the buffer directly and iterate through it directly than calling GetPixel for every pixel. Use GetDIBits() to grab a pointer to the raw buffer, but keep in mind that the buffer is a bottom-up bitmap so you have to iterate backwards.

what about this

#include //For Image
using namespace Gdiplus; //For Image

void BitmapUtils::SaveBitmapToFile(HDC hdc,CRect rect)
{
HDC hMemDC = CreateCompatibleDC(hdc);
HBITMAP hBitmap = CreateCompatibleBitmap(hdc,rect.right, rect.Height() + rect.bottom);
if (hBitmap)
{
HBITMAP hOld = (HBITMAP) SelectObject(hMemDC, hBitmap);
BitBlt(hMemDC, 0, 0, rect.right, rect.Height() + rect.bottom, hdc, 0, 0, SRCCOPY);
SelectObject(hMemDC, hOld);
DeleteDC(hMemDC);
::ReleaseDC(NULL, hdc);
}
CImage im;
im.Attach(hBitmap);
im.Save( "c:\\image.jpg", ImageFormatJPEG);
}