Q: (.NET/GDI/BitBlt) Problem copying form to bitmap

  • Thread starter Thread starter cranke_boy
  • Start date Start date
C

cranke_boy

Hi Folks,

I'm not sure where this post belongs since I'm using managed vc.net, but the
issue is around GDI BitBlt.

Here is a summary of the problem:
- I am trying to copy a bitmap of my main form into a picture box.
- To do this, I bitblt using the device contexts of the form and a bitmap
object.
- After blitting, the bitmap image is always blank. I don't understand what
I'm doing wrong here.

Here is my relevant code:

MainMenuForm_Shown(...)
{
Graphics ^formGraphics = this->CreateGraphics();
Bitmap ^bitmap = gcnew Bitmap(this->Width, this->Height, formGraphics);
Graphics ^bitmapGraphics = Graphics::FromImage(bitmap);
IntPtr formDC = formGraphics->GetHdc();
IntPtr bitmapDC = bitmapGraphics->GetHdc();

BitBlt(bitmapDC, 0, 0, this->Width, this->Height, formDC, 0, 0, 0xCC0020);

formGraphics->ReleaseHdc(formDC);
bitmapGraphics->ReleaseHdc(bitmapDC);

bitmap->Save(CCarthageDefs::DIR_IMAGES + "test.jpg");
}

After running the above code, the file test.jpg is tiny and basically blank
(maybe all solid grayish white). I expected it to be a copy of the form.
Any comments on why this is happening would be most appreciated.

I thought maybe the problem was with the form, so I tried an even simpler
test: copying one image to another. Here is the code:
Bitmap ^srcImage = gcnew Bitmap("input.jpg");
Graphics ^srcG = Graphics::FromImage(srcImage);
Bitmap ^toImage = gcnew Bitmap(srcImage->Width, srcImage->Height,
srcG);
Graphics ^toG = Graphics::FromImage(toImage);

IntPtr srcDc = srcG->GetHdc();
IntPtr toDc = toG->GetHdc();

BitBlt(toDc, 0, 0, srcImage->Width, srcImage->Height, srcDc, 0, 0,
0xCC0020);

srcG->ReleaseHdc(srcDc);
toG->ReleaseHdc(toDc);

srcImage->Save(CCarthageDefs::DIR_IMAGES + "test1.jpg");
toImage->Save(CCarthageDefs::DIR_IMAGES + "test2.jpg");
---
After running the above code, test1.jpg (an output of the source image) is a
correct copy of the source image. test2.jpg is *blank*?? Again, something
with the BitBlt seems to have failed ...

Also, to make sure I was actually hitting the genuine BitBlt code, I tried
using the options BLACKNESS and WHITENESS. Both worked ... the destination
turned the appropriate color.

Finally, in case anyone is interested in why I'm doing this ... I am having
some ugly flicker at the load-up of my form. I'm trying to take a copy of
the form's look, put it in a screen-sized picture box, and display that to
the user before they see the real form.

Thanks
 
I thought the following might also be helpful:
- I tried to use control->DrawToBitmap(). but it doesn't draw all the
sub-controls that I need it to.
- I am using VC++ under Visual Studio Express 2008.
- Here is my prototype for BitBlt:
[System::Runtime::InteropServices::DllImportAttribute("gdi32.dll")]
extern bool BitBlt(IntPtr hDestDC,
int x,
int y,
int nWidth,
int nHeight,
IntPtr hSrcDC,
int xSrc,
int ySrc,
int dwRop);
 
I think you're bang on. Thanks.

When I changed the source DC to be something like GetWindowDC(this->Handle),
I was able to read from the source DC successfully. As you suggested, it
looks like doing Graphics->GetHdc() results in a write-only DC.

Does anyone know how to get a **readable** DC for a Bitmap object (since it
doesn't look like I can use GetWindowDC(handle) for such an object)? Is
there some standard way to do this?

I poked around at SetDIBitsToDevice, but I don't think I fully understand
how to apply it to this situation ...
 
cranke_boy said:
I think you're bang on. Thanks.

When I changed the source DC to be something like
GetWindowDC(this->Handle), I was able to read from the source DC
successfully. As you suggested, it looks like doing
Graphics->GetHdc() results in a write-only DC.

Does anyone know how to get a **readable** DC for a Bitmap object
(since it doesn't look like I can use GetWindowDC(handle) for such an
object)? Is there some standard way to do this?
Click to expand...

The pure GDI way to do that is CreateCompatibleDC followed by SelectObject.

But since source is a .NET Bitmap object and destination is a .NET Graphics,
why not just use DrawImage or DrawImageUnscaled instead of BitBlt?
 
But since source is a .NET Bitmap object and destination is a .NET
Graphics, why not just use DrawImage or DrawImageUnscaled instead of
BitBlt?
Click to expand...

I'm not the original poster but I'm cranky enough to know the answer.

DrawImage is about 50 times slower than StretchBlt, burdens a CPU core at
100% while it's running, and causes the user to give up and look for other
programs.
 
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