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I'm having difficulties rendering just a single pixel which is on the defined coords of viewport. I'm using D3DXMatrixPerspectiveOffCenter function, but the result is not what I expected.

Here is how I calculate the parameters for the function:

// pCamera is the actual camera used to render full screen picture

// what is the width and height of camera near plane in Camera Space
double dHeight = 2 * pCamera->m_dNear * (tan(pCamera->m_dFOVy/2.0)); 
double dWidth  = dHeight * pCamera->m_dRatioXY;

// what is the pixel size in Camera space
double dHeightReduced = (dHeight / pCamera->m_dViewportHeight) * CAMERA_1x1px_SIZE;
double dWidthReduced = (dWidth / pCamera->m_dViewportWidth) * CAMERA_1x1px_SIZE;

// mouse pixel position in Camera space
double dCamSpaceX = -0.5*dWidth  + double(uiMousePosX) / pCamera->m_dViewportWidth    * dWidth;
double dCamSpaceY = -0.5*dHeight + double(uiMousePosY) / pCamera->m_dViewportHeight   * dHeight;

// create matrix
D3DXMatrixPerspectiveOffCenterRH( &m_mProjection,
         (float)(dCamSpaceX),
         (float)(dCamSpaceX+dWidthReduced),
         (float)(dCamSpaceY),
         (float)(dCamSpaceY+dHeightReduced),
         (float)m_dNear,
         (float)m_dFar );

(I set CAMERA_1x1px_SIZE = 100 to see a part of the scene, but I see the scene distorted and it's definitely NOT just the cut-off of the original)

How do I need to calc the params to make it work?

Thanks.

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1 Answer 1

I am more into openGL, but I think its simpler to use Scissor test to clip part of the Framebuffer/render target plus I don't see any benefits from modifying the projection matrix (unless its a requirement to modify the projection matrix).

Keep in mind that scissor test and projection+viewport work on a different part of the pipeline, scissor test for instance is a fragment operation while projection+viewport matrix is not. You can also use stencil test which can be used if the area you want to mask is not rectangular. Performance wise I don't think the two methods are equivalent but you need to measure and decide.

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1  
+1 Use a scissor, or a viewport, this is a screen space operation. D3DXMatrixPerspectiveOffCenterRH is to create non-symmetric projections, it has nothing to do with limiting the output to a certain rectangle. –  Laurent Couvidou Apr 18 '13 at 9:51
    
I've found some older code, that produces hi-res images via render target tiling. I though my problem is similar in some way. I've tried to adjust the older code to serve my purpose. So there must be a solution. –  GPUquant Apr 18 '13 at 12:09
3  
Using an off center projection matrix combined with a smaller viewport is a valid way to achive what you want and may result in a higher peformance as non-visible geometry gets clipped before the fragment shader is executed but scissor rects are far simpler to use. –  Archy Apr 18 '13 at 13:56
    
Would somebody be so kind and explain the usage and setting of off center projection matrix in combination with viewport to me? –  GPUquant May 3 '13 at 6:57

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