如何创建廉价的阴影在OpenGL？ [英] How Do I Create Cheap Shadows In OpenGL?

问题描述

我有两个型号，A和B，以及一盏灯，L。我想模型一个施展型号B的影子，我不想打扰阴影卷或适当的阴影的时刻，只是一个简单的圆形阴影就足够了。其效果是，模型A被视为一个球体阴影投射的目的。

I have two models, A and B, and one light, L. I would like model A to cast a shadow on model B. I don't want to bother with shadow volumes or proper shadows for the moment, just a simple circle shadow will suffice. The effect is that model A is treated as a sphere for shadow casting purposes.

下面是我想象的算法：

对于B型每一个三角形，绘制三角形。项目一圆到沿线由L到A三角形，增加了圆圈的大小取决于如何远离三角形。确保圆夹在三角形的边界（使用模板缓存，在某种程度上，我想）。

For each triangle in model B, draw the triangle. Project a circle onto the triangle along the line from L to A, increasing the size of the circle depending on how far away the triangle is. Ensure the circle is clipped to the triangle's boundaries (using the stencil buffer in some way, I imagine).

我正与OpenGL和普通的C。

I'm working with OpenGL and plain C.

在一些参考文献，我可以读任何指针？或者implmentation想法？

Any pointers on some reference documentation I can read? Or implmentation ideas?

推荐答案

我觉得它实际上是更容易实现正确的阴影，因为OpenGL的可以做的工作给你。

I think it is actually easier to implement correct shadows because OpenGL can do the work for you.

我找到了工作影子code，有很多在这里的文档：<一href="http://www.opengl.org/resources/$c$c/samples/mjktips/TexShadowReflectLight.html">http://www.opengl.org/resources/$c$c/samples/mjktips/TexShadowReflectLight.html

I found a working shadow code with lots of documentation here: http://www.opengl.org/resources/code/samples/mjktips/TexShadowReflectLight.html

在code以上呈现对象两次：第一次通常然后用特殊的矩阵。它做了很多不相关的东西，如鼠标和反射控制。因此，这里有一个有趣的部分。

The code above renders the object twice: first normally then with a special matrix. It does a lot of unrelated things such as control with mouse and reflections. So here are the interesting parts.

这计算的影子矩阵：

/* Create a matrix that will project the desired shadow. */
void
shadowMatrix(GLfloat shadowMat[4][4],
  GLfloat groundplane[4],
  GLfloat lightpos[4])
{
  GLfloat dot;

  /* Find dot product between light position vector and ground plane normal. */
  dot = groundplane[X] * lightpos[X] +
    groundplane[Y] * lightpos[Y] +
    groundplane[Z] * lightpos[Z] +
    groundplane[W] * lightpos[W];

  shadowMat[0][0] = dot - lightpos[X] * groundplane[X];
  shadowMat[1][0] = 0.f - lightpos[X] * groundplane[Y];
  shadowMat[2][0] = 0.f - lightpos[X] * groundplane[Z];
  shadowMat[3][0] = 0.f - lightpos[X] * groundplane[W];

  shadowMat[X][1] = 0.f - lightpos[Y] * groundplane[X];
  shadowMat[1][1] = dot - lightpos[Y] * groundplane[Y];
  shadowMat[2][1] = 0.f - lightpos[Y] * groundplane[Z];
  shadowMat[3][1] = 0.f - lightpos[Y] * groundplane[W];

  shadowMat[X][2] = 0.f - lightpos[Z] * groundplane[X];
  shadowMat[1][2] = 0.f - lightpos[Z] * groundplane[Y];
  shadowMat[2][2] = dot - lightpos[Z] * groundplane[Z];
  shadowMat[3][2] = 0.f - lightpos[Z] * groundplane[W];

  shadowMat[X][3] = 0.f - lightpos[W] * groundplane[X];
  shadowMat[1][3] = 0.f - lightpos[W] * groundplane[Y];
  shadowMat[2][3] = 0.f - lightpos[W] * groundplane[Z];
  shadowMat[3][3] = dot - lightpos[W] * groundplane[W];

}

我不知道pretend完全理解这一点。 lightpos是光源的位置。接地面的前3个坐标在地面表面的法线矢量。第四个是偏移量（多远从0,0,0是）。

I do not pretend to understand this completely. lightpos is the position of the light source. The first 3 coordinates of groundplane are the normal vector of the ground surface. The fourth is the offset (how far is it from 0,0,0).

和这部分实际上呈现的阴影：

And this part actually renders the shadow:

glPushMatrix();
/* Project the shadow. */
glMultMatrixf((GLfloat *) floorShadow);
drawDinosaur();
glPopMatrix();

有一些事情你需要过glEnable / glDisable第一次为此工作所以看的链接。

There are some things you need to glEnable/glDisable first for this to work so look at the link.

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