HI all,
I want to simulate CRT persistence in a render-to-texture effect. Essentially I'm looking to simulate an old CRT screen, like an analog oscilloscope or a radar screen. If I were using OpenGL, I figure the best way to do this would be to use an accumulation buffer, but DirectX lacks such a capability.
So then, what would be the best way to achieve this effect with hardware acceleration in D3D11?
Simulating CRT persistence?
You can make an "accumulation buffer" just by creating a new render target (texture) and accumulating values into it.
e.g. to keep 10% of the previous frame around (and 1% of the frame before that, and 0.1% of the frame before that...)
e.g. to keep 10% of the previous frame around (and 1% of the frame before that, and 0.1% of the frame before that...)
Render scene to target #1.
Blend target #1 into target #2 with 90% alpha.
Display target #2 to screen.
Author
Is there a way to blend the two targets in a blit-style approach? The only way I know to do it would require me to render two quads, one into the other, and I assume that's not best practice.
You only need one quad -- bind the "bottom" layer as the current render-target, then draw a quad textured with the "top" layer.
Rendering quads is indeed the standard way to do it - it's what the GPUs are designed to be good at. Most specialized 2D operations have been thrown out of the hardware these days.
Actually, it's often done with a single triangle that's large enough to just cover the screen, e.g. if the screen is the box:
Rendering quads is indeed the standard way to do it - it's what the GPUs are designed to be good at. Most specialized 2D operations have been thrown out of the hardware these days.
Actually, it's often done with a single triangle that's large enough to just cover the screen, e.g. if the screen is the box:
|\
| \
|__\
| |\
|__|_\but drawing quads is easier to think about ;)
Author
So basically it's just like rendering to the backbuffer without clearing it between frames?
Author
I can't quite get my blending to work right on this. The image gives a nice trail, but never quite fades out completely:
I have my blending set up as follows:
I've tried other blend settings but this is the only one that gives a trail. Others will remove the trail completely and leave me with just the dot. I'm not clearing the 2nd render target between frames (which in this case happens to be the back buffer) but I am clearing the first RTV between frames (the texture for the screen-sized quad). The dot itself is rendered as a small quad with exponential alpha fall-off from the center.
Any ideas on what I'm doing wrong?
I have my blending set up as follows:
rtbd.BlendEnable = true;
rtbd.SrcBlend = D3D11_BLEND_SRC_ALPHA;
rtbd.DestBlend = D3D11_BLEND_SRC_ALPHA;
rtbd.BlendOp = D3D11_BLEND_OP_ADD;
rtbd.SrcBlendAlpha = D3D11_BLEND_ONE;
rtbd.DestBlendAlpha = D3D11_BLEND_ONE;
rtbd.BlendOpAlpha = D3D11_BLEND_OP_ADD;
rtbd.RenderTargetWriteMask = D3D11_COLOR_WRITE_ENABLE_ALL;
I've tried other blend settings but this is the only one that gives a trail. Others will remove the trail completely and leave me with just the dot. I'm not clearing the 2nd render target between frames (which in this case happens to be the back buffer) but I am clearing the first RTV between frames (the texture for the screen-sized quad). The dot itself is rendered as a small quad with exponential alpha fall-off from the center.
Any ideas on what I'm doing wrong?
Author
I think you are not clearing the buffers after u used them.
Like I said in the post, I'm not clearing the back buffer. This is intended because it gives the accumulated trail in the first place. The problem is the trail never reaches zero.
You have 2 backBuffer, you should do something like this:
clean both buffers
loop:
render buffer1 on buffer2 with 90%
clean buffer1
render what you want on buffer 2
switch places between buffer 1 and 2
your image is now on buffer1
clean both buffers
loop:
render buffer1 on buffer2 with 90%
clean buffer1
render what you want on buffer 2
switch places between buffer 1 and 2
your image is now on buffer1
Do what Such1 said. Also your blend state description should look like this:
rtbd.BlendEnable = true;
rtbd.SrcBlend = D3D11_BLEND_SRC_ALPHA;
rtbd.DestBlend = D3D11_BLEND_INV_SRC_ALPHA;
rtbd.BlendOp = D3D11_BLEND_OP_ADD;
rtbd.SrcBlendAlpha = D3D11_BLEND_ONE;
rtbd.DestBlendAlpha = D3D11_BLEND_ZERO;
rtbd.BlendOpAlpha = D3D11_BLEND_OP_ADD;
rtbd.RenderTargetWriteMask = D3D11_COLOR_WRITE_ENABLE_ALL;
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