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_Sauce_

perspective divide by w and clipping

3 posts in this topic

I'm learning DirectX 11 and I'm having trouble with the perspective divide. Specifically, the perspective transform is resulting in all my vertices getting clipped before rasterisation.
I'm testing with a 200x200 cube centered at the origin. You can see an image of the problem in PIX below


[img]http://img849.imageshack.us/img849/4861/prevs.jpg[/img] [img]http://img837.imageshack.us/img837/7189/postvs.jpg[/img]

After the the w-divide my vertices lie wholly outside of clip space, but I can't see anything wrong with the matrices I'm passing to my shaders.

Setting up the viewport
[source lang="cpp"]const int windowWidth = 800, windowHeight = 600;
D3D11_VIEWPORT vp;
ZeroMemory(&vp, sizeof(vp));
vp.TopLeftX = D3D11_VIEWPORT_BOUNDS_MIN;
vp.TopLeftY = D3D11_VIEWPORT_BOUNDS_MIN;
vp.Width = (float)windowWidth;
vp.Height = (float)windowHeight;
vp.MinDepth = 0.0f;
vp.MaxDepth = 1.0f;
g_pImmediateDeviceContext->RSSetViewports(1, &vp);[/source]

Building the matrices
[source lang="cpp"]
VSConstants VSConstData;
D3DXMatrixIdentity(&VSConstData.world);
D3DXMatrixLookAtLH(&VSConstData.view, &eye, &lookAt, &up);
D3DXMatrixPerspectiveFovLH(&VSConstData.projection, toRadians(90.0f), (float)windowWidth / (float)windowHeight, 0.01f, 1000.0f);
VSConstData.worldViewProjection = VSConstData.world * VSConstData.view * VSConstData.projection;
D3DXMatrixInverse(&VSConstData.invWorldViewProjection, 0, &VSConstData.worldViewProjection);
g_pImmediateDeviceContext->UpdateSubresource(g_pVSConstants, 0, 0, &VSConstData, 0, 0);[/source]

and here is the contents of the constants buffer as reported by PIX
[img]http://img163.imageshack.us/img163/1633/constantsbuffer.jpg[/img]

Vertex & pixel shaders
[source lang="cpp"]float4x4 world;
float4x4 view;
float4x4 projection;
float4x4 worldViewProjection;
float4x4 invWorldViewProjection;
Texture2D g_texture;
SamplerState defaultSampler
{
MipFilter = LINEAR;
MinFilter = LINEAR;
MagFilter = LINEAR;
};

struct VSInput
{
float4 position : POSITION;
float3 normal : NORMAL;
float2 texcoord : TEXCOORD0;
};

struct PSInput
{
float4 position : SV_POSITION;
float3 normal : NORMAL;
float2 texcoord : TEXCOORD0;
};

PSInput VSDefault(VSInput input)
{
PSInput result;
result.position = mul(input.position, world);
result.position = mul(result.position, view);
result.position = mul(result.position, projection);
result.normal = input.normal;//mul(input.normal, invWorldViewProjection);
result.texcoord = input.texcoord;

return result;
}

float4 PSDefault(PSInput input) : SV_TARGET
{
return g_texture.Sample(defaultSampler, input.texcoord);
}[/source]

I have a feeling the problem is to do with my viewport settings as the post-vs output looks fine. It is only after clipping that I lose everything. Unfortunately I can't find much information on the appropriate values for the D3D11_VIEWPORT structure and trial and error has proven unsuccessful. Any idea what I'm doing wrong?
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I held off for 4 days on creating this thread, exhausting every option I could think of, and not even 10 minutes after I posted did I find the solution.

[source lang="cpp"]const int windowWidth = 800, windowHeight = 600;
D3D11_VIEWPORT vp;
ZeroMemory(&vp, sizeof(vp));
vp.TopLeftX = 0; //Change these two lines
vp.TopLeftY = 0; //
vp.Width = (float)windowWidth;
vp.Height = (float)windowHeight;
vp.MinDepth = 0.0f;
vp.MaxDepth = 1.0f;
g_pImmediateDeviceContext->RSSetViewports(1, &vp);[/source]

MSDN says what the possible values for the D3D11_VIEWPORT structure are but it doesn't clearly explain what those values mean. I wrongly assumed the top left corner would be the minimum possible value given the conventions used for texture addressing and window layouts.

Can anyone offer an in-depth explanation of the interpretation of these values? Edited by _Sauce_
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Your vertex shader outputs homogeneous coordinates in clip space. When you divide XYZ by W, you get "normalized device coordinates" where X = -1 is the left side of the viewport, x = 1 is the right side, Y = -1 is the bottom, Y = 1 is the top, Z = 0 is the near clip plane, and z = 1 is the far clip plane. Any coordinates outside of these bounds will be clipped.

After that comes the viewport transform, which looks like this:
[code]
X = (X + 1) * Viewport.Width * 0.5 + Viewport.TopLeftX
Y = (1 - Y) * Viewport.Height * 0.5 + Viewport.TopLeftY
Z = Viewport.MinDepth + Z * (Viewport.MaxDepth - Viewport.MinDepth)
[/code]

At this point the Y-axis is flipped such that (0, 0) is the top left, and (Viewport.Width, Viewport.Height) is the bottom right.
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[quote name='MJP' timestamp='1340648112' post='4952726']
Your vertex shader outputs homogeneous coordinates in clip space. When you divide XYZ by W, you get "normalized device coordinates" where X = -1 is the left side of the viewport, x = 1 is the right side, Y = -1 is the bottom, Y = 1 is the top, Z = 0 is the near clip plane, and z = 1 is the far clip plane. Any coordinates outside of these bounds will be clipped.

After that comes the viewport transform, which looks like this:
[code]
X = (X + 1) * Viewport.Width * 0.5 + Viewport.TopLeftX
Y = (1 - Y) * Viewport.Height * 0.5 + Viewport.TopLeftY
Z = Viewport.MinDepth + Z * (Viewport.MaxDepth - Viewport.MinDepth)
[/code]

At this point the Y-axis is flipped such that (0, 0) is the top left, and (Viewport.Width, Viewport.Height) is the bottom right.
[/quote]
Thanks MJP, that helps a lot.
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