How to work with pixels using Direct2D

Question

Could somebody provide an example of an efficient way to work with pixels using Direct2D?

For example, how can I swap all green pixels (RGB = 0x00FF00) with red pixels (RGB = 0xFF0000) on a render target? What is the standard approach? Is it possible to use ID2D1HwndRenderTarget for that? Here I assume using some kind of hardware acceleration. Should I create a different object for direct pixels manipulations?

Using DirectDraw I would use BltFast method on the IDirectDrawSurface7 with logical operation. Is there something similar with Direct2D?

Another task is to generate complex images dynamically where each point location and color is a result of a mathematical function. For the sake of an example let's simplify everything and draw Y = X ^ 2. How to do that with Direct2D? Ultimately I'm going to need to draw complex functions but if somebody could give me a simple example for Y = X ^ 2.

Answer 1

The specific issue of swapping all green pixels with red pixels can be addressed via ID2D1Effect as of Windows 8 and Platform Update for Windows 7.

More specifically, Color matrix effect.

Answer 2

Rick's answer talks about the methods you can use if you don't care about losing hardware acceleration. I'm focusing on how to accomplish this using a substantial amount of GPU acceleration.

In order to keep your rendering hardware accelerated and to get the best performance, you are going to want to switch from ID2DHwndRenderTarget to using the newer ID2DDevice and ID2DDeviceContext interfaces. It honestly doesn't add that much more logic to your code and the performance benefits are substantial. It also works on Windows 7 with the Platform Update. To summarize the process:

1. Create a DXGI factory when you create your D2D factory.
2. Create a D3D11 device and a D2D device to match.
3. Create a swap chain using your DXGI factory and the D3D device.
4. Ask the swap chain for its back buffer and wrap it in a D2D bitmap.
5. Render like before, between calls to BeginDraw() and EndDraw(). Remember to unbind the back buffer and destroy the D2D bitmap wrapping it!
6. Call Present() on the swap chain to see the results.
7. Repeat from 4.

Once you've done that, you have unlocked a number of possible solutions. Probably the simplest and most performant way to solve your exact problem (swapping color channels) is to use the color matrix effect as one of the other answers mentioned. It's important to recognize that you need to use the newer ID2DDeviceContext interface rather than the ID2DHwndRenderTarget to get this however. There are lots of other effects that can do more complicated operations if you so choose. Here are some of the most useful ones for simple pixel manipulation:

• Color matrix effect
• Arithmetic operation
• Blend operation

For generally solving the problem of manipulating the pixels directly without dropping hardware acceleration or doing tons of copying, there are two options. The first is to write a pixel shader and wrap it in a completely custom D2D effect. It's more work than just getting the pixel buffer on the CPU and doing old-fashioned bit mashing, but doing it all on the GPU is substantially faster. The D2D effects framework also makes it super simple to reuse your effect for other purposes, combine it with other effects, etc.

For those times when you absolutely have to do CPU pixel manipulation but still want a substantial degree of acceleration, you can manage your own mappable D3D11 textures. For example, you can use staging textures if you want to asynchronously manipulate your texture resources from the CPU. There is another answer that goes into more detail. See ID3D11Texture2D for more information.

Answer 3

First, it helps to think of ID2D1Bitmap as a "device bitmap". It may or may not live in local, CPU-addressable memory, and it doesn't give you any convenient (or at least fast) way to read/write the pixels from the CPU side of the bus. So approaching from that angle is probably the wrong approach.

What I think you want is a regular WIC bitmap, IWICBitmap, which you can create with IWICImagingFactory::CreateBitmap(). From there you can call Lock() to get at the buffer, and then read/write using pointers and do whatever you want. Then, when you need to draw it on-screen with Direct2D, use ID2D1RenderTarget::CreateBitmap() to create a new device bitmap, or ID2D1Bitmap::CopyFromMemory() to update an existing device bitmap. You can also render into an IWICBitmap by making use of ID2D1Factory::CreateWicBitmapRenderTarget() (not hardware accelerated).

You will not get hardware acceleration for these types of operations. The updated Direct2D in Win8 (should also be available for Win7 eventually) has some spiffy stuff for this but it's rather complex looking.