Databases
StackOverflow2013
Postsdbo
POI need to calculate the edges of visible space from current OpenGL state
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POI need to calculate the edges of visible space from current OpenGL state
    primarykey
    data
    text
    <p>In OpenGL in Delphi, I am attempting to calculate how much of a scene is viewable by finding the left, right, top and bottom extents of the viewing area in an orthographic projection. This will make some mouse interactions seem to be 1-to-1. I don't have much access to the code that sets up the orthographic projection matrix, but I know it calls glOrtho(...) to set up the projection matrix.</p> <p>I need to be able to take a cartesian plane (not a rendered polygonal plane), then calculate the left, right, top and bottom edges of the viewing frustum from whatever information opengl can provide (I don't care about the front and back clipping planes). I don't have access to the code which sets up the viewing volume (using glScene library for Delphi), so the only information I can use to find this information (as far as I know) is </p> <pre><code>glGet(GL_PROJECTION_MATRIX, @projectionMatrix); </code></pre> <p>Based on the documentation in <a href="http://www.talisman.org/opengl-1.1/Reference/glOrtho.html" rel="nofollow noreferrer">http://www.talisman.org/opengl-1.1/Reference/glOrtho.html</a>, I decided I can do some math to calculate what the left, right, top and bottom extents were for the near plane (which in ortho mode is enough) (see my code below). However, when I actually look at the values I retrieve from glGet, I see a matrix with values that don't fit the description in the documentation link I gave above (see matrix data below). With such disappointing numbers, I have doubts about whether I am using glGet properly, and whether I can actually rely on those numbers to give me the left/right/top/bottom extents of the visible volume. I suppose I could also be making a mistake in Delphi, but I don't know what it would be.</p> <p>In this particular instance, the plane is oriented to face the camera directly, and the camera has an orthogonal projection. I am not in complete control of how the projection matrix is set up, so I had hoped I could calculate the left edge as</p> <pre><code> left = 2 / (A * (tx - 1)/(tx+1) - 1) </code></pre> <p>Where A and tx are retrieved from the projection matrix, as shown at <a href="http://www.talisman.org/opengl-1.1/Reference/glOrtho.html" rel="nofollow noreferrer">http://www.talisman.org/opengl-1.1/Reference/glOrtho.html</a> (I am not showing how I derived the formula above, so it could be incorrect--right now it gives an answer of -INF). The code I was using to retrieve the projection matrix values is as follows:</p> <pre><code>var projectionMatrix: TMatrix; Left, Right, Top, Bottom: Single; A, B, C, tx, ty, tz: Single; begin A := projectionMatrix[0][0]; B := projectionMatrix[1][1]; C := projectionMatrix[2][2]; tx := projectionMatrix[0][3]; ty := projectionMatrix[1][3]; tz := projectionMatrix[2][3]; Left := 2 / (A * ((tx - 1) / (tx + 1) - 1)); ShowMessage(FloatToStr(A)); //etc... </code></pre> <p>The matrix values I see are about like this:</p> <pre><code>[0] (0, 1.7436188636e-39, 6.4310754487e+37, 1.3822464104e-39) [1] (3.8535707769e-43, 1.4012984643e-45, 0, 2.8086960027e+32) [2] (-4.20345941e+17, 0, 1.7437029415e-39, 2.8086960027e+32) [3] (1.7437645986e-39, 6.4311900443e+37, +NAN, 1.7437645986e-39) </code></pre> <p>[Edit] glScene has the following declarations:</p> <pre><code>TMatrix = THomogeneousFltMatrix; THomogeneousFltMatrix = TMatrix4f; TMatrix4f = array[0..3] of TVector4f; TVector4f = array[0..3] of single; </code></pre> <p>[Edit] If I change the projection matrix to a 16-value array, here is what happens:</p> <pre><code>var projectionMatrix, modelviewMatrix: array[0..16] of single; begin ... glGetFloatv(GL_PROJECTION_MATRIX, @projectionMatrix); ... </code></pre> <p>the resulting array is elements 0 through 16, which still doesn't look right.</p> <blockquote> <p>projectionMatrix (5.9208279286e-39, -1536643072, 1.2429653406, -1.2128044229e-11, 2.8763115406, 0, 4.728515625, 1.7430022923e-39, 6.5138089186e+37, -3.7955583126e+30, -2.4000201225, -3.7955583126e+30, 2.4000201225, -1.2128044229e-11, 2.6263115406, -1.2128044229e-11, -2.6263115406)</p> </blockquote>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USKimball Robinson
    1. USKimball Robinson
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POI need to calculate the edges of visible space from current OpenGL state
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POI need to calculate the edges of visible space from current OpenGL state
      1. This table or related slice is empty.
      1. VTUpMod
    1. COCan you please show the declaration of TMatrix? Can you please show the exact code for your call to glGet()?
      singulars
      1. POI need to calculate the edges of visible space from current OpenGL state
      1. USAlistair Ward
    2. COas Alistar already said, check your TMatrix declaration. Essentially it hjas to be 16 consecutive single precision floats in memory, not 4 pointers to 4 floats.
      singulars
      1. POI need to calculate the edges of visible space from current OpenGL state
      1. USheeen
    3. COThese are all from the glScene library: TMatrix = THomogeneousFltMatrix; THomogeneousFltMatrix = TMatrix4f; TMatrix4f = array[0..3] of TVector4f; TVector4f = array[0..3] of single; It seems Heeen is on the right track.
      singulars
      1. POI need to calculate the edges of visible space from current OpenGL state
      1. USKimball Robinson
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload