爲了在GLSL中應用紋理,我們需要訪問每個頂點的紋理座標。GLSL中提供了一些屬性變量,每個紋理單元一個:
- attribute vec4 gl_MultiTexCoord0;
- attribute vec4 gl_MultiTexCoord1;
- attribute vec4 gl_MultiTexCoord2;
- attribute vec4 gl_MultiTexCoord3;
- attribute vec4 gl_MultiTexCoord4;
- attribute vec4 gl_MultiTexCoord5;
- attribute vec4 gl_MultiTexCoord6;
- attribute vec4 gl_MultiTexCoord7;
attribute vec4 gl_MultiTexCoord0;
attribute vec4 gl_MultiTexCoord1;
attribute vec4 gl_MultiTexCoord2;
attribute vec4 gl_MultiTexCoord3;
attribute vec4 gl_MultiTexCoord4;
attribute vec4 gl_MultiTexCoord5;
attribute vec4 gl_MultiTexCoord6;
attribute vec4 gl_MultiTexCoord7;
GLSL還爲訪問每個紋理的紋理矩陣提供了一個一致變量數組:- uniform mat4 gl_TextureMatrix[gl_MaxTextureCoords];
uniform mat4 gl_TextureMatrix[gl_MaxTextureCoords];
頂點shader可以通過上面所示的內容訪問OpenGL程序中指定的紋理座標。然後必須爲每個頂點計算紋理座標,並保存在預先定義的易變變量gl_TexCoord[i]中,i表示紋理單元號。下面這條語句直接複製OpenGL程序中指定的紋理座標,作爲紋理單元0的頂點紋理座標。
- gl_TexCoord[0] = gl_MultiTexCoord0;
gl_TexCoord[0] = gl_MultiTexCoord0;
下面是個簡單的例子,在頂點shader中設置紋理單元0的紋理座標。- void main()
- {
- gl_TexCoord[0] = gl_MultiTexCoord0;
- gl_Position = ftransform();
- }
void main()
{
gl_TexCoord[0] = gl_MultiTexCoord0;
gl_Position = ftransform();
}
如果你想使用紋理矩陣,可以這樣操作:- void main()
- {
- gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
- gl_Position = ftransform();
- }
void main()
{
gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;
gl_Position = ftransform();
}
前面說過,gl_TexCoord是一個易變變量,所以在片斷shder中可以訪問經過插值的紋理座標。爲了訪問紋理的數值,在片斷shader中有必要聲明一個特殊的變量,對一個2D紋理可以可以這樣寫:
- uniform sampler2D tex;
uniform sampler2D tex;
如果是1D或者3D的紋理,可以改成sampler1D和sampler3D。這個用戶定義的變量tex包含我們將會使用的紋理單元,通過texture2D函數我們可以得到一個紋素(texel),這是一個紋理圖片中的像素。函數參數分別爲simpler2D以及紋理座標:
- vec4 texture2D(sampler2D, vec2);
vec4 texture2D(sampler2D, vec2);
函數的返回值已經考慮了所有在OpenGL程序中定義的紋理設置,比如過濾、mipmap、clamp等。我們的片斷shader可以寫成如下形式:
- uniform sampler2D tex;
- void main()
- {
- vec4 color = texture2D(tex,gl_TexCoord[0].st);
- gl_FragColor = color;
- }
uniform sampler2D tex;
void main()
{
vec4 color = texture2D(tex,gl_TexCoord[0].st);
gl_FragColor = color;
}
注意訪問gl_TexCoord時選擇子st的使用。在本教程前面關於數據類型和變量的討論中說過,訪問紋理座標時可以使用如下選擇子:s、t、p、q。(r因爲和rgb選擇子衝突而沒有使用)http://www.lighthouse3d.com/wp-content/uploads/2011/03/textureSimple.zip
組合紋理與片斷
OpenGL允許我們通過多種方式將紋理顏色和片斷顏色聯合到一起。下表顯示了RGBA模式時可用的聯合方式:
GL_REPLACE | C = Ct | A = At |
GL_MODULATE | C = Ct*Cf | A = At*Af |
GL_DECAL | C = Cf * (1 – At) + Ct * At | A = Af |
上一節的例子就相當於使用了GL_REPLACE模式。下面我們我們準備在一個立方體上實現與GL_MODULATE等同的效果。兩個shader只計算使用一個白色方向光的散射以及環境光成分,關於材質的完整定義請參照光照有關的章節。
因爲使用了光照,所以頂點shader中必須處理法線信息。必須將法線變換到視圖空間然後歸一化,光線方向向量也必須歸一化(光線方向向量已經由OpenGL變換到了視圖空間)。現在新的頂點shader如下:
- varying vec3 lightDir,normal;
- void main()
- {
- normal = normalize(gl_NormalMatrix * gl_Normal);
- lightDir = normalize(vec3(gl_LightSource[0].position));
- gl_TexCoord[0] = gl_MultiTexCoord0;
- gl_Position = ftransform();
- }
varying vec3 lightDir,normal;
void main()
{
normal = normalize(gl_NormalMatrix * gl_Normal);
lightDir = normalize(vec3(gl_LightSource[0].position));
gl_TexCoord[0] = gl_MultiTexCoord0;
gl_Position = ftransform();
}
在片斷shader中,光照得到的片斷的顏色和alpha值在cf和af中分別計算。shader中剩餘代碼按照GL_MODULATE的公式計算:- varying vec3 lightDir,normal;
- uniform sampler2D tex;
- void main()
- {
- vec3 ct,cf;
- vec4 texel;
- float intensity,at,af;
- intensity = max(dot(lightDir,normalize(normal)),0.0);
- cf = intensity * (gl_FrontMaterial.diffuse).rgb +
- gl_FrontMaterial.ambient.rgb;
- af = gl_FrontMaterial.diffuse.a;
- texel = texture2D(tex,gl_TexCoord[0].st);
- ct = texel.rgb;
- at = texel.a;
- gl_FragColor = vec4(ct * cf, at * af);
- }
varying vec3 lightDir,normal;
uniform sampler2D tex;
void main()
{
vec3 ct,cf;
vec4 texel;
float intensity,at,af;
intensity = max(dot(lightDir,normalize(normal)),0.0);
cf = intensity * (gl_FrontMaterial.diffuse).rgb +
gl_FrontMaterial.ambient.rgb;
af = gl_FrontMaterial.diffuse.a;
texel = texture2D(tex,gl_TexCoord[0].st);
ct = texel.rgb;
at = texel.a;
gl_FragColor = vec4(ct * cf, at * af);
}
http://www.lighthouse3d.com/wp-content/uploads/2011/03/textureComb.zip
多重紋理
在GLSL中實現多重紋理十分容易,我們只需要訪問所有紋理即可。因爲我們打算給每個紋理使用相同的紋理座標,所以頂點shader不需要改動。片斷shader中只需要進行些許改動,加上多個紋理的顏色值。
- varying vec3 lightDir,normal;
- uniform sampler2D tex;
- void main()
- {
- vec3 ct,cf;
- vec4 texel;
- float intensity,at,af;
- intensity = max(dot(lightDir,normalize(normal)),0.0);
- cf = intensity * (gl_FrontMaterial.diffuse).rgb +
- gl_FrontMaterial.ambient.rgb;
- af = gl_FrontMaterial.diffuse.a;
- texel = texture2D(tex,gl_TexCoord[0].st) +
- texture2D(l3d,gl_TexCoord[0].st);
- ct = texel.rgb;
- at = texel.a;
- gl_FragColor = vec4(ct * cf, at * af);
- }
varying vec3 lightDir,normal;
uniform sampler2D tex;
void main()
{
vec3 ct,cf;
vec4 texel;
float intensity,at,af;
intensity = max(dot(lightDir,normalize(normal)),0.0);
cf = intensity * (gl_FrontMaterial.diffuse).rgb +
gl_FrontMaterial.ambient.rgb;
af = gl_FrontMaterial.diffuse.a;
texel = texture2D(tex,gl_TexCoord[0].st) +
texture2D(l3d,gl_TexCoord[0].st);
ct = texel.rgb;
at = texel.a;
gl_FragColor = vec4(ct * cf, at * af);
}
效果如下:如果indensity是0,第二個紋理單元取最大值,如果indensity爲1,只取第二個紋理單元顏色的10%,當indensity在0和1之間時按這兩個大小進行插值。可以使用smoothstep函數實現這個要求:
- genType smoothStep(genType edge0, genType edge1, genType x);
genType smoothStep(genType edge0, genType edge1, genType x);
如果x <= edge0結果是0,如果x >= edge1結果爲1,如果edge0 < x < edge1結果在0和1之間進行Hermite插值。在本例中我們按如下方式調用:coef = smoothStep(1.0, 0.2, intensity);
下面的片斷shader實現了需要的效果:
- varying vec3 lightDir,normal;
- uniform sampler2D tex,l3d;
- void main()
- {
- vec3 ct,cf,c;
- vec4 texel;
- float intensity,at,af,a;
- intensity = max(dot(lightDir,normalize(normal)),0.0);
- cf = intensity * (gl_FrontMaterial.diffuse).rgb +
- gl_FrontMaterial.ambient.rgb;
- af = gl_FrontMaterial.diffuse.a;
- texel = texture2D(tex,gl_TexCoord[0].st);
- ct = texel.rgb;
- at = texel.a;
- c = cf * ct;
- a = af * at;
- float coef = smoothstep(1.0,0.2,intensity);
- c += coef * vec3(texture2D(l3d,gl_TexCoord[0].st));
- gl_FragColor = vec4(c, a);
- }
varying vec3 lightDir,normal;
uniform sampler2D tex,l3d;
void main()
{
vec3 ct,cf,c;
vec4 texel;
float intensity,at,af,a;
intensity = max(dot(lightDir,normalize(normal)),0.0);
cf = intensity * (gl_FrontMaterial.diffuse).rgb +
gl_FrontMaterial.ambient.rgb;
af = gl_FrontMaterial.diffuse.a;
texel = texture2D(tex,gl_TexCoord[0].st);
ct = texel.rgb;
at = texel.a;
c = cf * ct;
a = af * at;
float coef = smoothstep(1.0,0.2,intensity);
c += coef * vec3(texture2D(l3d,gl_TexCoord[0].st));
gl_FragColor = vec4(c, a);
}
Shader Designer的工程下載地址:http://www.lighthouse3d.com/wp-content/uploads/2011/03/textureGlow.zip