Computer Graphics with OpenGL ES (J. Han) Chapter 6 Fragment shader

Transcription

1 Computer Graphics with OpenGL ES (J. Han) Chapter 6 Fragment shader

2 Vertex and Fragment Shaders The inputs to the fragment shader Varyings: The per-vertex output variables produced by the vertex shader are interpolated to determine the per-fragment ones. Uniforms to be used by the fragment shader. Samplers = Textures. The fragment color stored in gl_fragcolor is passed to the output merger. Computer Graphics with OpenGL ES (J. Han) 6-2

3 Fragment Shader A simplest fragment shader uniform sampler2d s_tex0; varying vec2 v_texcoord; void main() { gl_fragcolor = texture2d(s_tex0, v_texcoord); } The above fragment shader declares a uniform variable, s_tex0, of type sampler2d, which represents a texture. Recall that our vertex shader simply output 2D texture coordinates without modification. The per-vertex texture coordinates were interpolated so that the per-fragment texture coordinates, v_texcoord, are now passed to the fragment shader and used for fetching the texture, s_tex0. The fragment shader invokes the built-in function texture2d, which returns a color. It is output to the built-in variable gl_fragcolor. Computer Graphics with OpenGL ES (J. Han) 6-3

4 Phong Lighting Model Illumination or lighting refers to the techniques handling the interaction between light sources and objects. In real-time applications, the most popular illumination method is based on the Phong model. Even though it is not physically correct, it is widely adopted in commercial games and lays foundations of various advanced lighting techniques. Computer Graphics with OpenGL ES (J. Han) 6-4

5 Phong Lighting Model - Diffuse Term Among various light sources (including point, area, spot, and directional light sources), for now let s take the simplest, the directional light source, where the light vector (l) connecting a surface point and the faraway light source is constant for the entire scene. The Phong model is composed of diffuse, specular, ambient, and emissive terms. For the sake of simplicity, let us consider the diffuse term only. The diffuse term is based on Lambert s law. Reflections from ideally diffuse surfaces (Lambertian surfaces) are scattered with equal intensity in all directions. The amount of reflection perceived by the eye is just proportional to the amount of incoming light, which is inversely proportional to the angle between l and the surface normal n. It is described as n l. In order to avoid negative reflection, it is modified into max(n l, 0). Computer Graphics with OpenGL ES (J. Han) 6-5

6 Phong Lighting Model - Diffuse Term (cont d) Suppose a white light (1,1,1). If an object lit by the light appears yellow, it means that the object reflects R and G and absorbs B. We can easily implement this kind of filtering through material parameter, i.e., if it is (1,1,0), then (1,1,1) (1,1,0)=(1,1,0) where is component-wise multiplication. R G B intensity x1=1 1x1=1 1x0=0 R G B intensity x.5=.25.5x1=.5.5x.5=.25 (1,1,0) (.5,1,.5) The diffuse term combines the intensity and color: max(n l, 0) s d m d where s d is the light source color and m d is the object material color. The texture provides m d whereas s d is given by the user. Computer Graphics with OpenGL ES (J. Han) 6-6

7 Per-fragment Lighting Let us show how to implement Phong lighting (with the diffuse term only) and texturing. The vertex shader shown below computes the clip-space vertex position, as usual, and also the world-space vertex position and normal of each vertex. In addition, it bypasses the texture coordinates. The output variables v_position, v_normal, and v_texcoord will be interpolated for fragments and they will be provided for the fragment shader. uniform mat4 viewprojmat; uniform mat4 worldmat; attribute vec3 position; attribute vec3 normal; attribute vec2 texcoord; varying vec3 v_position; varying vec3 v_normal; varying vec2 v_texcoord; void main() { gl_position = viewprojmat * worldmat * vec4(position, 1.0); v_position = (worldmat * vec4(position, 1.0)).xyz; v_normal = mat3(worldmat) * normal; v_texcoord = texcoord; } Computer Graphics with OpenGL ES (J. Han) 6-7

8 Per-fragment Lighting (cont d) The fragment shader shown below obtains the object material color (m d ) by filtering the texture and then asks the user-defined function phongdiffuse to compute the diffuse term. It uses uniform vec3 lightpos for calculating the light vector l (lightdirection) and simply implements max(n l, 0) s d m d. uniform sampler2d s_tex0; uniform vec3 lightpos; uniform vec3 Sd; varying vec3 v_position; varying vec3 v_normal; varying vec2 v_texcoord; vec3 phongdiffuse(vec3 Md) { // The diffuse term of Phong model per fragment vec3 lightdirection = normalize(lightpos - v_position); vec3 nor = normalize(v_normal); vec3 phongdiffuse(vec3 Md); // Forward declaration void main() { vec3 Md = texture2d(s_tex0, v_texcoord).xyz; vec3 phongcolor = phongdiffuse(md); } gl_fragcolor = vec4(phongcolor, 1.0); } //Diffuse term float Diffuse = max(dot(nor, lightdirection), 0.0); vec3 PhongDiffuse = Diffuse * Sd * Md; return PhongDiffuse; Computer Graphics with OpenGL ES (J. Han) 6-8