OpenGL Shader in OpenCASCADE

OpenGL Shader in OpenCASCADE

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Abstract. As implementation of one of the strategic steps in OpenCASCADE visualization component development road-map, support for GLSL shader programs has been added in OpenCASCADE Technology 6.7.0. node

Key Words. OpenCASCADE, GLSL, Shader, Gooch Shaderide

1. Introduction 函數

OpenCASCADE從6.7.0以後,Shader程序也成了源碼的一部分。程序開發者能夠爲實現各類特效提供本身的Shader程序。在TKOpenGl庫中就有Shader相關的類,以下圖所示:post

wps_clip_image-16881wps_clip_image-26090

OpenCASCADE中的Shader管理是徹底自動完成的,和其餘OpenGL的資源同樣。仔細看看這幾個類相關的函數,會發現與Qt中的Shader比較相似。對比使用,會加快對OpenGL Shader程序的理解。學習

在 OCCT6.7.0版本中的Shader程序還有一些侷限性,如不能使用GLSL1.40版本或更新版本的功能。爲了克服這個侷限性,OCCT爲了實現一 些shader的操做定義了一些uniform的變量,這樣Shader程序就不依賴於GLSL1.30定義的一些變量了,使程序的兼容性更好。在源文件 的Shaders文件夾中的Declarations.glsl中定義了這些變量,摘抄部分以下所示:測試

// Vertex attributes
#ifdef VERTEX_SHADER
  attribute vec4 occVertex;
  attribute vec3 occNormal;
  attribute vec4 occTexCoord;
  attribute vec4 occVertColor;
#endif

// Matrix state
uniform mat4 occWorldViewMatrix;  //!< World-view  matrix
uniform mat4 occProjectionMatrix; //!< Projection  matrix
uniform mat4 occModelWorldMatrix; //!< Model-world matrix

uniform mat4 occWorldViewMatrixInverse;    //!< Inverse of the world-view  matrix
uniform mat4 occProjectionMatrixInverse;   //!< Inverse of the projection  matrix
uniform mat4 occModelWorldMatrixInverse;   //!< Inverse of the model-world matrix

uniform mat4 occWorldViewMatrixTranspose;  //!< Transpose of the world-view  matrix
uniform mat4 occProjectionMatrixTranspose; //!< Transpose of the projection  matrix
uniform mat4 occModelWorldMatrixTranspose; //!< Transpose of the model-world matrix

uniform mat4 occWorldViewMatrixInverseTranspose;  //!< Transpose of the inverse of the world-view  matrix
uniform mat4 occProjectionMatrixInverseTranspose; //!< Transpose of the inverse of the projection  matrix
uniform mat4 occModelWorldMatrixInverseTranspose; //!< Transpose of the inverse of the model-world matrix

與OpenGL的內置uniform變量對比會發現,主要內容都是相似的:spa

//
uniform mat4  gl_ModelViewMatrix;
uniform mat4  gl_ProjectionMatrix;
uniform mat4  gl_ModelViewProjectionMatrix;
uniform mat4  gl_TextureMatrix[gl_MaxTextureCoords];

//
// Derived matrix state that provides inverse and transposed versions
// of the matrices above. Poorly conditioned matrices may result
// in unpredictable values in their inverse forms.
//
uniform mat3  gl_NormalMatrix; // transpose of the inverse of the upper
                               // leftmost 3x3 of gl_ModelViewMatrix

uniform mat4  gl_ModelViewMatrixInverse;
uniform mat4  gl_ProjectionMatrixInverse;
uniform mat4  gl_ModelViewProjectionMatrixInverse;
uniform mat4  gl_TextureMatrixInverse[gl_MaxTextureCoords];

uniform mat4  gl_ModelViewMatrixTranspose;
uniform mat4  gl_ProjectionMatrixTranspose;
uniform mat4  gl_ModelViewProjectionMatrixTranspose;
uniform mat4  gl_TextureMatrixTranspose[gl_MaxTextureCoords]

uniform mat4  gl_ModelViewMatrixInverseTranspose;
uniform mat4  gl_ProjectionMatrixInverseTranspose;
uniform mat4  gl_ModelViewProjectionMatrixInverseTranspose;
uniform mat4  gl_TextureMatrixInverseTranspose[gl_MaxTextureCoords]

看樣子OpenCASCADE爲了GLSL版本兼容性也是動了些腦筋啊!3d

2.Test Shader code

爲 了方便在OpenCASCADE中測試自定義的Shader程序,OpenCASCADE在Draw Test Harness中提供了一個vshaderprog命令。以下圖所示爲在Draw Test Harness中使用命令vshaderprog產生的Gouraud Shading和Phong Shading效果:

wps_clip_image-4915

Figure 2.1 Gouraud Shading(Left) and Phong Shading(right) by OCCT

在Draw Test Harness中使用vshaderprog命令很簡單,以下圖所示:

wps_clip_image-1384

Figure 2.2 vshaderprog usage

爲指定的模型指定頂點着色器和片斷着色器便可。

3.Gooch Shader

Gooch着色屬於非真實性圖像着色,主要用於繪製各類手冊、技術圖書上的示意圖。1998年,Bruce和Amy Gooch、Peter Shirley以及Elaine Cohen對示意圖進行了調查並提出了使用噴槍和畫筆繪製的彩色示意圖特徵:

v 表面邊界、輪廓邊緣及對象表面的不連續處一般是使用黑色曲線繪製;

v 使用一個單獨的光源,它會在對象上產生白色的高光;

v 光源一般位於對象的上方,這樣在對象的可見區域上,漫反射部分會在[0,1]中變化;

v ……

wps_clip_image-12173

Figure 3.1 Gooch Shader Effect

上圖所示爲Gooch着色器渲染出來的效果,看上去是否是頗有感受啊!以上圖片來自Amy Gooch, Bruce Gooch, Peter Shirley和Elaine Cohen的論文:

A Non-Photorealistic Lighting Model For Automatic Technical Illustration.

Department of Computer Science University of Utah. Utah大學還有著名的Utah Teapot。

在《OpenGL Shading Language》一書中發現了Gooch着色器的相關實現代碼,以下所示:

頂點着色器Gooch.vs:

//
// Vertex shader for Gooch shading
//
// Author: Randi Rost
//
// Copyright (c) 2002-2005 3Dlabs Inc. Ltd. 
//
// See 3Dlabs-License.txt for license information
//

uniform vec3  LightPosition;  // (0.0, 10.0, 4.0) 

varying float NdotL;
varying vec3  ReflectVec;
varying vec3  ViewVec;

void main()
{
    LightPosition = vec3(0.0, 10.0, 4.0);
    
    vec3 ecPos      = vec3(gl_ModelViewMatrix * gl_Vertex);
    vec3 tnorm      = normalize(gl_NormalMatrix * gl_Normal);
    vec3 lightVec   = normalize(LightPosition - ecPos);
    ReflectVec      = normalize(reflect(-lightVec, tnorm));
    ViewVec         = normalize(-ecPos);
    NdotL           = (dot(lightVec, tnorm) + 1.0) * 0.5;
    gl_Position     = ftransform();
}

 

片斷着色器Gooch.fs:

//
// Fragment shader for Gooch shading
//
// Author: Randi Rost
//
// Copyright (c) 2002-2005 3Dlabs Inc. Ltd. 
//
// See 3Dlabs-License.txt for license information
//

uniform vec3  SurfaceColor; // (0.75, 0.75, 0.75)
uniform vec3  WarmColor;    // (0.6, 0.6, 0.0)
uniform vec3  CoolColor;    // (0.0, 0.0, 0.6)
uniform float DiffuseWarm;  // 0.45
uniform float DiffuseCool;  // 0.45

varying float NdotL;
varying vec3  ReflectVec;
varying vec3  ViewVec;

void main()
{
    SurfaceColor = vec3(0.75, 0.75, 0.75);
    WarmColor = vec3(0.6, 0.6, 0.0);
    CoolColor = vec3(0.0, 0.0, 0.6);
    DiffuseWarm = 0.45;
    DiffuseCool = 0.45;
    
    vec3 kcool    = min(CoolColor + DiffuseCool * SurfaceColor, 1.0);
    vec3 kwarm    = min(WarmColor + DiffuseWarm * SurfaceColor, 1.0); 
    vec3 kfinal   = mix(kcool, kwarm, NdotL);

    vec3 nreflect = normalize(ReflectVec);
    vec3 nview    = normalize(ViewVec);

    float spec    = max(dot(nreflect, nview), 0.0);
    spec          = pow(spec, 32.0);

    gl_FragColor = vec4(min(kfinal + spec, 1.0), 1.0);
}

將它們加載到OpenCASCADE中,顯示效果以下圖所示:

wps_clip_image-8706

Figure 3.2 A Main Engine in Draw Test Harness

wps_clip_image-24159

Figure 3.3 Use Gooch Shader Program

由上圖可知,Shader有必定的做用,但效果不是很理想。還須要學習相關的知識,才能完善。若是在OpenCASCADE中只修改下shader就能夠獲得各類特效,豈不快哉!

4.Conclusion

綜上所述,OpenGL的Shader是個很好玩的東西,因此OpenCASCADE中引入了這個。爲了保證GLSL的兼容性,OpenCASCADE也定義了一些變量。

在不改變程序源碼的狀況下,只換上不一樣的shader,就能夠獲得各類炫麗的特效,這些特效主要是利用GPU的資源完成,不佔用CPU。看OpenGL最新的規格書中,GLSL已經愈來愈Fashion了!

5. References

1. Randi J. Rost. OpenGL Shading Language. Addison Wesley. 2006

2. Amy Gooch, Bruce Gooch, Peter Shirley, Elaine Cohen. A Non-Photorealistic Lighting Model For Automatic Technical Illustration. Department of Computer Science University of Utah.

3. San, Shader support in OCCT6.7.0. http://dev.opencascade.org/index.php?q=node/902

 

PDF version and Gooch Shader OpenGL Shader in OpenCASCADE

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