Direct3D
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Why do
my pre-lit vertices appear black under DirectX 7?
Why
are my Indexed Primitive calls so slow?
My
existing application successfully renders with the DrawPrimitive method, but
when I started using the Direct3DDevice3 Interface, the primitives are not
rendered properly. Why?
Why
are some or all of my triangles are not visible when I render them?
My
world matrix scales an item, and the normals are scaled as well. How can
I prevent this from affecting my lighting?
After installing the DX 8 SDK, I
recieve the error LNK1104: Cannot Open File "d3dim.lib" when
attempting o compile my program.
Q. Why are some or all of my triangles
are not visible when I render them?
A common reason for this is that the winding order of the
triangles is incorrect. By default, any triangle that would be drawn
counterclockwise, after transformation to the camera view, is ignored.
This increases the performance of the rasterizer and prevents artifacts from
backfaces by eliminating any triangles that are facing away from the viewer.
If this is the problem, you will have to swap two of the to reverse the winding
order. For a quick test to see if this is the problem, try turning off
culling prior to rendering:
lpDevice->SetRenderState(D3DRENDERSTATE_CULLMODE,D3DCULL_NONE);
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Q. Why do my pre-lit vertices appear black under
DirectX 7?
When the lighting engine is enabled, which is the default, the
color values of processed vertices will be set according to the normal provided
for each vertex. In the case of a D3DLVERTEX, there is no normal
provided, since you have already calculated your own lighting values.
However, DirectX will try to light them anyway, and in the absence of a normal
vector will set the color to black.
To prevent this, you must explicitly turn off lighting prior to
rendering pre-lit primitives. Unlike previous versions of DirectX, where a
flag could be provided during rendering, there is now a render state which
enables or disables lighting operations:
lpDevice->SetRenderState(D3DRENDERSTATE_LIGHTING,FALSE);
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Q. Why are my Indexed Primitive
calls so slow?
Note: The following information is based on D3DIM
7. Under DirectX 8, the DrawIndexedPrimitive command allows you to specify
a starting vertex and number of vertices use. By restricting this to the
range used in a rendering call, only those vertices get processed.
In most postings I see of this nature, there is a common thread
: A) There are a large number of vertexes overall, but B) each call to
DrawIndexedPrimitive() is only drawing a few polygons.
The reason that these operations appear to have huge performance
penalties is because each call to DrawIndexedPrimitive() will transform all of
the vertices in the list, regardless of which are actually utilized. For
example, take a moment to consider the following code :
for (int i=0;i<1000;i+=4)
lpDevice->DrawIndexedPrimitive(D3DPT_TRIANGLESTRIP,D3DFVF_VERTEX,
verts,1600,index+i,4,0)
That code will render a total of 500 polygons, which is
quite reasonable... However, in doing so, it will transform all 1600 vertexes in
the "verts" array 250 times - that means 400,000 transformations per
frame! To be able to sustain 30 FPS at that kind of load would mean the
CPU has to transform 12 Million transformations each second.
To avoid this, only pass the vertexes you will be using, and
batch your primitives together if possible - approximately 100 triangles per
call is optimal. The benefit of batching is significant, and is worth
breaking your object into a single triangle list if it consists of many small
triangle strips.
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Q. My existing application successfully
renders with the DrawPrimitive method, but when I started using the
Direct3DDevice3 Interface, the primitives are not rendered properly. Why?
The vertex description parameter of the DrawPrimitive functions
has changed with the introduction of the flexible vertex format in DirectX 6.0
and above. For example, if you previously specified a vertex type flag of
D3DVT_LVERTEX, you will now need to use a value of D3DFVF_LVERTEX instead.
For a complete listing of the flexible vertex format flags, refer to the DirectX
documentation for the DrawPrimitive method.
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Q. My world matrix scales an item, and
the normals are scaled as well. How can I prevent this from affecting my
lighting?
This can be achieved in one of two ways:
1. Scale the length of your normals by the reciprocal of the
scaling factor. For example, if an object is scaled to 5 times it's size,
then multiply the normals by 1/5, or 0.2.
2. Set the D3DRENDERSTATE_NORMALIZENORMALS to TRUE. This
will cause normals to be normalized (set to a length of 1.0) after
transformation into camera space. Note that this can be computationally
expensive.
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