Develop Reference

BitBlt failing on window rendered by OpenGL

I'm making a program to take a screenshot of a game while I'm streaming, so it take a screenshot and save it automatically, but when I set the game to use OpenGL my function fail, it keep saving the same image over and over again, it just change the image after I restart the game.
It seems it works in the first run, but on the next ones it keep saving the first image.
Here is what I'm using:
public static Bitmap PrintWindow(IntPtr hwnd) {
try {
RECT rc;
GetClientRect(hwnd, out rc);
IntPtr hdcFrom = GetDC(hwnd);
IntPtr hdcTo = CreateCompatibleDC(hdcFrom);
//X and Y coordinates of window
int Width = rc.right;
int Height = rc.bottom;
Bitmap bmp = null;
IntPtr hBitmap = CreateCompatibleBitmap(hdcFrom, Width, Height);
if (hBitmap != IntPtr.Zero) {
// adjust and copy
IntPtr hLocalBitmap = SelectObject(hdcTo, hBitmap);
BitBlt(hdcTo, 0, 0, Width, Height, hdcFrom, 0, 0, CopyPixelOperation.SourceCopy);
SelectObject(hdcTo, hLocalBitmap);
//We delete the memory device context.
DeleteDC(hdcTo);
//We release the screen device context.
ReleaseDC(hwnd, hdcFrom);
//Image is created by Image bitmap handle and assigned to Bitmap variable.
bmp = System.Drawing.Image.FromHbitmap(hBitmap);
//Delete the compatible bitmap object.
DeleteObject(hBitmap);
bmp.Save("saving.png", System.Drawing.Imaging.ImageFormat.Png);
}
return bmp;
}
catch {
}
return new Bitmap(0, 0);
}
If I change the game graphic to use DirectX it works good, it's just happening while using OpenGL, so not sude if it must be different for openGL windows or if it's impossible to capture those kind of window.

Using double buffered OpenGL is mutually exclusive with using GDI operations¹. Use glReadPixels to take a screenshot.
¹: Well, technically if you know what you're doing and take the right precautions you can mix them. But it's more trouble than it's worth doing.

After a recent windows 10 update, DirectX rendered windows now suffer from the same problem. I found a workaround, but you're not going to like it...
If you set hwnd = 0, the BitBlt now refers to the whole screen, instead of just one window. Then you can change BitBlt's source offset values to grab only the target window.
Although this works, its much slower than the original way you had it. :(
On my laptop, grabbing a 1080p window used to take 3ms, but now, using this workaround, it takes 27ms, which really ruins the streaming performance. :(
Still, its better than nothing.

How does the Graphics CopyFromScreen method copy into a bitmap?

private void startBot_Click(object sender, EventArgs e)
{
Bitmap bmpScreenshot = Screenshot();
this.BackgroundImage = bmpScreenshot;
}
private Bitmap Screenshot()
{
// This is where we will store a snapshot of the screen
Bitmap bmpScreenshot =
new Bitmap(Screen.PrimaryScreen.Bounds.Width,Screen.PrimaryScreen.Bounds.Height);
// Creates a graphic object so we can draw the screen in the bitmap (bmpScreenshot);
Graphics g = Graphics.FromImage(bmpScreenshot);
// Copy from screen into the bitmap we created
g.CopyFromScreen(0, 0, 0, 0, Screen.PrimaryScreen.Bounds.Size);
// Return the screenshot
return bmpScreenshot;
}
I've been recently playing around with C# and I'm just following some tutorial, I just don't understand how if I was to erase Graphics g it wouldn't put the image as the background, but at no point does the code assign any relation between the variables, other than Graphics g = Graphics.FromImage(bmpScreenshot), then g is given some parameters, but then we return bmpScreenshot which just doesn't make any sense, I would expect g to be returned?

Devices that can display graphics are virtualized in Windows. The concept is called a "device context" in the winapi, underlying representation is a "handle". The Graphics class wraps that handle, it does not itself store the pixels. Note the Graphics.GetHdc() method, a way to get to that handle.
The class otherwise just contains the drawing methods that produce graphics output on the device represented by that handle. Actual devices can be the screen, a printer, a metafile, a bitmap. With the big advantage in your own code that it can be used to produce output where ever you want it to go. So printing is just as easy as painting it to the screen or drawing to a bitmap that you store to a file.
So by calling Graphics.FromImage(), you associate the Graphics object to the bitmap. All of its draw methods actually set pixels in the bitmap. Like CopyFromScreen(), it simply copies pixels from the video adapter's frame buffer to the device context, in effect setting the pixels in the bitmap. So the expected return value of this code is the actual bitmap. The Graphics object should be disposed before that happens since it is no longer useful. Or in other words, the underlying handle needs to be released so the operating system de-allocates its own resources to represent the device context.
That's a bug in the code snippet. Repeated calls to this method can easily crash the program when Windows refuses to create more device contexts. And the garbage collector doesn't otherwise catch up fast enough. It should be written as:
using (var g = Graphics.FromImage(bmpScreenshot)) {
g.CopyFromScreen(0, 0, 0, 0, Screen.PrimaryScreen.Bounds.Size);
return bmpScreenshot;
}

The thing to understand is that Graphics g = Graphics.FromImage(bmpScreenshot) creates a Graphics context for drawing into the image that was passed as an argument (bmpScreenshot).
So, after you create the graphics content:
Graphics g = Graphics.FromImage(bmpScreenshot)
When you copy from the screen:
g.CopyFromScreen(0, 0, 0, 0, Screen.PrimaryScreen.Bounds.Size);
This manipulates the bmpScreenshot Bitmap which Graphics g holds a reference to.
From the documentation:
image [in]:
Type: Image*
Pointer to an Image object that will be associated with the new Graphics object.

System.ObjectDisposedException with graphics object

I have to write a game in C# using a Windows form. For my drawing method on my screen, I'm getting a weird System.ObjectDisposedException which spawns upon one of my graphics objects being instantiated.Here is the problematic code:
// Graphics variables.
Graphics graphics;
Graphics backBufferObject;
Bitmap backBuffer;
// Graphics and backBuffer are instantiated, elsewhere, in the constructor.
private void DrawScreen()
{
// Copy the BackBuffer to Graphics Object.
using (graphics = Graphics.FromImage(backBuffer))
using (backBufferObject = this.CreateGraphics())
{
// Draw BackBuffer to screen (buffer switching).
backBufferObject.DrawImage(backBuffer, 0, 0, this.Width, this.Height);
graphics.DrawImage(Properties.Resources.scary_pacman, new Rectangle(
this.Width / 2 , this.Height / 2, 32, 32));
graphics.Clear(Color.Thistle); // Clear BackBuffer for efficient rendering.
}
}
The problematic line, which throws a System.ObjectDisposedException is the line: using (backbufferObject = this.CreateGraphics()). I'm not sure why this exception is being thrown at this particular point due to the fact that this object is being reinstantiated at this point, whereas it was disposed at the end of the using brackets. So far I've tried putting those two lines in using statements due to the fact that they are affected by IDisposable.
It'd probably be noteworthy that the error is being thrown after I close the Windows form during Run time. So why is this Graphics object being disposed of in this particular instance?
Full code: http://pastebin.com/mSa6XCpP

If your form has been disabled, the this referenced in the line of code the exception happens on:
using (backbufferObject = this.CreateGraphics())
... will like already be disposed. So it isn't the Graphics object that's being disposed. It's the form.

Losing anti aliasing when sharing Graphics object between managed and unmanaged code

Passing Graphics object between native C++ and C#
I'm currently working on a Paint .NET-like application. I have mulitple types of layers which are implemented in C#. These layers are drawn into a .NET Graphics object that is provided by a WinForms user control - it is similar to the WPF canvas control. The layer base class has a Draw method that is implemented as follows:
public void Draw(IntPtr hdc)
{
using (var graphics = Graphics.FromInternalHDC(hdc)
{
// First: Setup rendering settings like SmoothingMode, TextRenderingHint, ...
// Layer specific drawing code goes here...
}
}
For performance and decompiling issues I'm doing the composition of the layers in a mixed mode assembly since I'm also applying effects like bevel or drop shadow. The wrapper, of course written in C++/CLI, gets directly called from the canvas control and forwards the metadata of each layer and the target Graphics object (Graphics object from my C# written canvas user control) to a native C++ class.
C++/CLI Wrapper:
public ref class RendererWrapper
{
public:
void Render(IEnumerable<Layer^>^ layersToDraw, Graphics^ targetGraphics)
{
// 1) For each layer get metadata (position, size AND Draw delegate)
// 2) Send layer metadata to native renderer
// 3) Call native renderer Render(targetGraphics.GetHDC()) method
// 4) Release targetGraphics HDC
};
}
Native C++ Renderer:
class NativeRenderer
{
void NativeRenderer::Render(vector<LayerMetaData> metaDataVector, HDC targetGraphicsHDC)
{
Graphics graphics(targetGraphicsHDC);
// Setup rendering settings (SmoothingMode, TextRenderingHint, ...)
for each metaData in metaDataVector
{
// Create bitmap and graphics for current layer
Bitmap* layerBitmap = new Bitmap(metaData.Width, metaData.Height, Format32bppArgb);
Graphics* layerGraphics = new Graphics(layerBitmap);
// Now the interesting interop part
// Get HDC from layerGraphics
HDC lgHDC = layerGraphics->GetHDC();
// Call metaData.Delegate and pass the layerGraphics HDC to C#
// By this call we are ending up in the Draw method of the C# Layer object
metaData.layerDrawDelegate(lgHDC);
// Releasing HDC - leaving interop...
layerGraphics->ReleaseHDC(lgHDC);
// Apply bevel/shadow effects
// Do some other fancy stuff
graphics.DrawImage(layerBitmap, metaData.X, metaData.Y, metaData.Width, metaData.Height);
}
}
}
So far so good. The above code works nearly as expected, but...
Problem
The only thing is that my current implementation is lacking of anti aliasing and semi transparency when rendering PNG with shadows for example. So I just have only 2 values for the Alpha channel: Transparent or full visible Color at 255. This side effect makes drawing PNGs with alpha channel and fonts looking very ugly. I cannot get the same smooth and nice semi transparent anti aliasing any more like before when I worked with pure C# code.
BUT: When drawing a string in the native Graphics object directly,
layerGraphics->DrawString(...);
anti aliasing and semi transparency are back for good. So the problem is only evident when passing the Graphics HDC to .NET.
Questions
Is there any solution/workaround for this problem? I've tried to create the Bitmap directly in the C# Layer class and return the IntPtr for the HBITMAP to the native code. This approach is working, but in this case I have another problem since I cannot find a perfect solution for converting HBITMAP to GDI+ Bitmap with alpha channel (white pixel noise is surrounding the edges when drawing fonts).
Thanks for your input! :)
Demo Solution
Attached you'll find a demo solution here: Sources
In this demo solution I'm testing 3 different rendering methods (all implemented in NativeRenderer.cpp), while the FIRST ONE shows the described problems:
1) RenderViaBitmapFromCSharp() - a) Creates a new bitmap in C++, creates a new Graphics object in C++, calls the C# drawing code by passing the C++ Graphics object HDC - Fails
But: b) Drawing directly from C++ works via the created bitmap too
2) RenderDirectlyFromCSharp() - Creates a new Graphics object from C# Graphics handle in C++
, calls the C# drawing code by passing the C++ Graphics object HDC - Works
3) RenderDirectlyFromCPP() - Creates a new Graphics object from C# Graphics handle in C++, draws the text directly in C++ - Works

Graphics graphics(targetGraphicsHDC);
You are creating a new Graphics object. So it won't have its properties setup like its original did. Properties like TextRenderingHint are not properties of a GDI device context, they are specific to Graphics.
Fairly ugly problem, you'll need to re-initialize the Graphics object the way it was done in the calling code. That's two chunks of code that are far removed from each other. Avoiding the conversion to HDC and back is the only really decent way to side-step the problem.

I've ended up creating the Bitmap in C# and passing the object to C++/CLI. As already mentioned by Hans and Vincent you have to avoid GetHDC. So my workaround reads in pseudo code as follows:
Layer.cs C#:
public Bitmap Draw()
{
var bitmap = new Bitmap(Width, Height, PixelFormat.Format32bppArgb);
using (var graphics = Graphics.FromBitmap(bitmap)
{
// First: Setup rendering settings like SmoothingMode, TextRenderingHint, ...
// Layer specific drawing code goes here...
}
return bitmap;
}
NativeRenderer.cs C++:
void NativeRenderer::RenderFromBitmapCSharp(System::Drawing::Bitmap^ bitmap)
{
// Create and lock empty native bitmap
Bitmap *gdiBitmap = new Bitmap(bitmap->Width, bitmap->Height, PixelFormat32bppARGB);
Rect rect(0, 0, bitmap->Width, bitmap->Height);
BitmapData bitmapData;
gdiBitmap->LockBits(&rect, Gdiplus::ImageLockModeRead | Gdiplus::ImageLockModeWrite, PixelFormat32bppARGB, &bitmapData);
// Lock managed bitmap
System::Drawing::Rectangle rectangle(0, 0, bitmap->Width, bitmap->Height);
System::Drawing::Imaging::BitmapData^ pBitmapData = bitmap->LockBits(rectangle, System::Drawing::Imaging::ImageLockMode::ReadOnly, System::Drawing::Imaging::PixelFormat::Format32bppArgb);
// Copy from managed to unmanaged bitmap
::memcpy(bitmapData.Scan0, pBitmapData->Scan0.ToPointer(), bitmap->Width * bitmap->Height * 4);
bitmap->UnlockBits(pBitmapData);
gdiBitmap->UnlockBits(&bitmapData);
// Draw it
_graphics->DrawImage(gdiBitmap, 0, 0, bitmap->Width, bitmap->Height);
}
Hope that is helpful to others - have not found any code snippet on the web which actually does converting managed to unmanaged GDI+ Bitmap.
Thank you all for your comments.
Cheers,
P

Screenshot of the main WPF window under second monitor/TV

I use my application on the second monitor and sometimes at the primary monitor of the computer.
How I can get screenshot of the second monitor?
The following code doesn't work for the second monitor...
Graphics gfx;
Bitmap bmp = new Bitmap(Screen.PrimaryScreen.Bounds.Width, Screen.PrimaryScreen.Bounds.Height);
gfx = Graphics.FromImage(bmp);
gfx.CopyFromScreen(Screen.PrimaryScreen.Bounds.X, Screen.PrimaryScreen.Bounds.Y, 0, 0, Screen.PrimaryScreen.Bounds.Size, CopyPixelOperation.SourceCopy);
MemoryStream ms = new MemoryStream();
bmp.Save(ms, ImageFormat.Jpeg);
byte[] bitmapData = ms.ToArray();

Use Screen.AllScreens[1].Bounds isntead of Screen.PrimaryScreen.Bounds.
Or more reliable to get the first non Primary Screen.
var secondScreen = Screen.AllScreens.Where(screen => !screen.Primary).FirstOrDefault();
check for secondScreen == null to know if you have a secondScreen.
Edit:
You might also be interested in Screen.FromControl that gives the screen that the application is currently running on.

That code does not work for your second screen because it is explicitly using Screen.PrimaryScreen.
If you want to pull from the second display explicitly (ignoring the case where you have 3…n displays), you can replace PrimaryScreen with AllScreens[1].
Keep in mind that this will break if you ever disconnect that second display.
It sounds like maybe you want to be capturing your application window instead of the screen, in case the application isn't taking up the whole screen or straddles two screens. WPF has this capability natively: Get System.Drawing.Bitmap of a WPF Area using VisualBrush