现代OpenGL:对象选择(C#，OpenTK)
我试图在OpenGL中使用C#和OpenTK实现对象选择。为此，我编写了一个基于以下两个来源的类：
https://www.bfilipek.com/2012/06/select-mouse-opengl.html
目前，我的代码仅用于计算鼠标指针与(0,0,0)的任意测试坐标之间的距离，但一旦工作正常，就无需在场景中的对象中迭代以找到匹配的对象。
该方法是在鼠标指针下定义近、远剪裁平面之间的一条射线。然后找出光线上最接近被测点的点，并返回两者之间的距离。当鼠标指针直接超过(0,0,0)时，这应该是零，并随着鼠标指针向任何方向移动而增加。
有人能帮我解决这个问题吗？它执行时没有错误，但返回的距离显然是不正确的。我理解计算的原理，但不理解计算的精妙之处。
虽然我在网上发现了很多例子，但它们通常都是在另一种语言或框架中，并且/或使用了不推荐的方法和/或不完整或不起作用。
public class ObjectPicker{
public static float DistanceFromPoint(Point mouseLocation, Vector3 testPoint, Matrix4 modelView, Matrix4 projection)
{
Vector3 near = UnProject(new Vector3(mouseLocation.X, mouseLocation.Y, 0), modelView, projection); // start of ray
Vector3 far = UnProject(new Vector3(mouseLocation.X, mouseLocation.Y, 1), modelView, projection); // end of ray
Vector3 pt = ClosestPoint(near, far, testPoint); // find point on ray which is closest to test point
return Vector3.Distance(pt, testPoint); // return the distance
}
private static Vector3 ClosestPoint(Vector3 A, Vector3 B, Vector3 P)
{
Vector3 AB = B - A;
float ab_square = Vector3.Dot(AB, AB);
Vector3 AP = P - A;
float ap_dot_ab = Vector3.Dot(AP, AB);
// t is a projection param when we project vector AP onto AB
float t = ap_dot_ab / ab_square;
// calculate the closest point
Vector3 Q = A + Vector3.Multiply(AB, t);
return Q;
}
private static Vector3 UnProject(Vector3 screen, Matrix4 modelView, Matrix4 projection)
{
int[] viewport = new int[4];
OpenTK.Graphics.OpenGL.GL.GetInteger(OpenTK.Graphics.OpenGL.GetPName.Viewport, viewport);
Vector4 pos = new Vector4();
// Map x and y from window coordinates, map to range -1 to 1
pos.X = (screen.X - (float)viewport[0]) / (float)viewport[2] * 2.0f - 1.0f;
pos.Y = 1 - (screen.Y - (float)viewport[1]) / (float)viewport[3] * 2.0f;
pos.Z = screen.Z * 2.0f - 1.0f;
pos.W = 1.0f;
Vector4 pos2 = Vector4.Transform( pos, Matrix4.Invert(modelView) * projection );
Vector3 pos_out = new Vector3(pos2.X, pos2.Y, pos2.Z);
return pos_out / pos2.W;
}
}它的名称如下：
private void GlControl1_MouseMove(object sender, MouseEventArgs e)
{
float dist = ObjectPicker.DistanceFromPoint(new Point(e.X,e.Y), new Vector3(0,0,0), model, projection);
this.Text = dist.ToString(); // display in window caption for debugging
}我知道矩阵是如何传入的(按照上面的代码)。我相当肯定这些矩阵的内容必须是正确的，因为呈现很好，我可以成功地旋转/缩放。这是顶点着色器FWIW：
string vertexShaderSource =
"# version 330 core\n" +
"layout(location = 0) in vec3 aPos;" +
"layout(location = 1) in vec3 aNormal;" +
"uniform mat4 model; " +
"uniform mat4 view;" +
"uniform mat4 projection;" +
"out vec3 FragPos;" +
"out vec3 Normal;" +
"void main()" +
"{" +
"gl_Position = projection * view * model * vec4(aPos, 1.0);" +
"FragPos = vec3(model * vec4(aPos, 1.0));" +
"Normal = vec3(model * vec4(aNormal, 1.0))";
"}";我使用Arcball的实现进行旋转。缩放是使用翻译完成的，如下所示：
private void glControl1_MouseWheel(object sender, System.Windows.Forms.MouseEventArgs e)
{
zoom += (float)e.Delta / 240;
view = Matrix4.CreateTranslation(0.0f, 0.0f, zoom);
SetMatrix4(Handle, "view", view);
glControl1.Invalidate();
}回答 2
Stack Overflow用户
发布于 2020-05-30 20:20:28
通过模型视图矩阵对每个顶点坐标进行变换。这将坐标从模型空间转换为视图空间。然后利用投影矩阵对各顶点坐标进行变换。这将从视图空间转换为剪辑空间。透视分割将剪辑空间坐标转换为规范化的设备空间。
如果要从规范化设备空间转换为模型空间，则必须执行反向操作。这意味着您必须通过逆投影矩阵和逆模型视图矩阵进行转换：
Vector4 pos2 = Vector4.Transform(pos, Matrix4.Invert(projection) * Matrix4.Invert(modelView));分别
Vector4 pos2 = Vector4.Transform(pos, Matrix4.Invert(modelView * projection));注意，OpenTK矩阵必须从左到右相乘。见OpenGL 4.2 LookAt矩阵只适用于眼睛位置的-z值的答案。
Stack Overflow用户
发布于 2020-05-31 11:50:22
在这里回答我自己的问题，这样我就可以为其他用户发布工作代码，但至少一半的答案是由Rabbid76提供的，我非常感谢他的帮助。
我的原始代码中有两个错误：
Vector4 pos2 = Vector4.Transform( pos, Matrix4.Invert(modelView) * projection );
其中，两个矩阵按错误的顺序相乘，而投影矩阵没有倒置。
float dist = ObjectPicker.DistanceFromPoint(new Point(e.X,e.Y), new Vector3(0,0,0), model, projection);
其中我传递的是模型矩阵，而不是模型视图矩阵(它是模型矩阵和视图矩阵的乘积)。
这样做是可行的：
private void GlControl1_MouseMove(object sender, MouseEventArgs e)
{
float dist = ObjectPicker.DistanceFromPoint(new Point(e.X,e.Y), new Vector3(0,0,0), model * view, projection);
// do something with the result
}
public class ObjectPicker{
public static float DistanceFromPoint(Point mouseLocation, Vector3 testPoint, Matrix4 modelView, Matrix4 projection)
{
int[] viewport = new int[4];
OpenTK.Graphics.OpenGL.GL.GetInteger(OpenTK.Graphics.OpenGL.GetPName.Viewport, viewport);
Vector3 near = UnProject(new Vector3(mouseLocation.X, mouseLocation.Y, 0), modelView, projection); // start of ray (near plane)
Vector3 far = UnProject(new Vector3(mouseLocation.X, mouseLocation.Y, 1), modelView, projection); // end of ray (far plane)
Vector3 pt = ClosestPoint(near, far, testPoint); // find point on ray which is closest to test point
return Vector3.Distance(pt, testPoint); // return the distance
}
private static Vector3 ClosestPoint(Vector3 A, Vector3 B, Vector3 P)
{
Vector3 AB = B - A;
float ab_square = Vector3.Dot(AB, AB);
Vector3 AP = P - A;
float ap_dot_ab = Vector3.Dot(AP, AB);
// t is a projection param when we project vector AP onto AB
float t = ap_dot_ab / ab_square;
// calculate the closest point
Vector3 Q = A + Vector3.Multiply(AB, t);
return Q;
}
private static Vector3 UnProject(Vector3 screen, Matrix4 modelView, Matrix4 projection)
{
int[] viewport = new int[4];
OpenTK.Graphics.OpenGL.GL.GetInteger(OpenTK.Graphics.OpenGL.GetPName.Viewport, viewport);
Vector4 pos = new Vector4();
// Map x and y from window coordinates, map to range -1 to 1
pos.X = (screen.X - (float)viewport[0]) / (float)viewport[2] * 2.0f - 1.0f;
pos.Y = 1 - (screen.Y - (float)viewport[1]) / (float)viewport[3] * 2.0f;
pos.Z = screen.Z * 2.0f - 1.0f;
pos.W = 1.0f;
Vector4 pos2 = Vector4.Transform( pos, Matrix4.Invert(projection) * Matrix4.Invert(modelView) );
Vector3 pos_out = new Vector3(pos2.X, pos2.Y, pos2.Z);
return pos_out / pos2.W;
}
}在发布这个问题后，我了解到这种方法通常称为光线投射，并找到了一些很好的解释：
https://antongerdelan.net/opengl/raycasting.html作者: Anton Gerdelan
https://www.youtube.com/watch?v=DLKN0jExRIM&list=PLRIWtICgwaX0u7Rf9zkZhLoLuZVfUksDP&index=29 by ThinMatrix
https://stackoverflow.com/questions/62108117
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