Texture coordinates on a triangle strip - c++

Question

I've created a cube with a geometry shader with 14 vertex (Thanks to triangle strip). The problem now is how to place my texture coordinates. I have an image file that contains 3 textures :
Top face of the cube
Bottom face
left/right/front/back faces
I can't find a way to give a texture coordinate to each vertices (14). I always get texture displaying strangely.
Is it possible to set texture coordinates on a 14 vertex cube made with triangle strip ?

Answer 1

Is it possible to set texture coordinates on a 14 vertex cube made with triangle strip ?
At least you have to use separate vertices and attributes, for 2 edges of the cube. This means you need at least 8+2*2=12 different sets of vertex attributes.
x y z u v
0 : -1 1 1 0 0
1 : 1 1 1 1 0
2 : -1 -1 1 0 1
3 : 1 -1 1 1 1
4 : -1 -1 -1 0 0
5 : 1 -1 -1 1 0
6 : -1 1 -1 0 1
7 : 1 1 -1 1 1
8 : -1 1 -1 1 0
9 : -1 -1 -1 1 1
10 : 1 -1 -1 0 1
11 : 1 -1 1 0 0
The triangle strip may consist of 18 indices, where the triangle strip "restarts at the points 3 and 6. This is achieved by adding the indices to the list twice. This causes a triangles which are "invisible", because 2 points are equal:
[8, 9, 0, 2, 1, 3,
3, 2, 5, 4, 7, 6,
6, 0, 7, 1, 10, 11]
Note, to improve the pervormance, it is possibel to enable Face Culling. For the above indices, front faces are counterclockwise:
glCullFace(GL_BACK);
glFrontFace(GL_CCW);
glEnable(GL_CULL_FACE);
See the WebGL example, which demonstrates the mesh:
var readInput = true;
function changeEventHandler(event){
readInput = true;
}
(function loadscene() {
var gl, progDraw, vp_size;
var bufCube = {};
var tex_unit = 1;
function render(delteMS){
if ( readInput ) {
readInput = false;
// ...
}
Camera.create();
Camera.vp = vp_size;
gl.viewport( 0, 0, vp_size[0], vp_size[1] );
gl.enable( gl.DEPTH_TEST );
gl.clearColor( 0.0, 0.0, 0.0, 1.0 );
gl.clear( gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT );
gl.frontFace(gl.CCW)
gl.cullFace(gl.BACK)
gl.enable(gl.CULL_FACE)
// set up draw shader
ShaderProgram.Use( progDraw );
ShaderProgram.SetUniformI1( progDraw, "u_texture", tex_unit );
ShaderProgram.SetUniformM44( progDraw, "u_projectionMat44", Camera.Perspective() );
ShaderProgram.SetUniformM44( progDraw, "u_viewMat44", Camera.LookAt() );
var modelMat = IdentityMat44()
modelMat = RotateAxis( modelMat, CalcAng( delteMS, 13.0 ), 0 );
modelMat = RotateAxis( modelMat, CalcAng( delteMS, 17.0 ), 1 );
ShaderProgram.SetUniformM44( progDraw, "u_modelMat44", modelMat );
// draw scene
VertexBuffer.Draw( bufCube );
requestAnimationFrame(render);
}
function resize() {
//vp_size = [gl.drawingBufferWidth, gl.drawingBufferHeight];
vp_size = [window.innerWidth, window.innerHeight]
canvas.width = vp_size[0];
canvas.height = vp_size[1];
}
function initScene() {
canvas = document.getElementById( "canvas");
gl = canvas.getContext( "experimental-webgl" );
//gl = canvas.getContext( "webgl2" );
if ( !gl )
return null;
progDraw = ShaderProgram.Create(
[ { source : "draw-shader-vs", stage : gl.VERTEX_SHADER },
{ source : "draw-shader-fs", stage : gl.FRAGMENT_SHADER }
] );
if ( !progDraw.progObj )
return null;
progDraw.inPos = ShaderProgram.AttributeIndex( progDraw, "inPos" );
progDraw.inUV = ShaderProgram.AttributeIndex( progDraw, "inUV" );
var cubePosData = [
-1, 1, 1,
1, 1, 1,
-1, -1, 1,
1, -1, 1,
-1, -1, -1,
1, -1, -1,
-1, 1, -1,
1, 1, -1,
-1, 1, -1,
-1, -1, -1,
1, -1, -1,
1, -1, 1 ];
var cubeTexData = [
0, 0,
1, 0,
0, 1,
1, 1,
0, 0,
1, 0,
0, 1,
1, 1,
1, 0,
1, 1,
0, 1,
0, 0 ];
cubeInxData = [
8, 9, 0, 2, 1, 3,
3, 2, 5, 4, 7, 6,
6, 0, 7, 1, 10, 11];
bufCube = VertexBuffer.Create(
[ { data : cubePosData, attrSize : 3, attrLoc : progDraw.inPos },
{ data : cubeTexData, attrSize : 2, attrLoc : progDraw.inUV } ],
cubeInxData );
bufCube.primitve_type = gl.TRIANGLE_STRIP;
Texture.LoadTexture2D( tex_unit, "https://raw.githubusercontent.com/Rabbid76/graphics-snippets/master/resource/texture/test1_texture.bmp" );
window.onresize = resize;
resize();
requestAnimationFrame(render);
}
function Fract( val ) {
return val - Math.trunc( val );
}
function CalcAng( deltaTime, intervall ) {
return Fract( deltaTime / (1000*intervall) ) * 2.0 * Math.PI;
}
function CalcMove( deltaTime, intervall, range ) {
var pos = self.Fract( deltaTime / (1000*intervall) ) * 2.0
var pos = pos < 1.0 ? pos : (2.0-pos)
return range[0] + (range[1] - range[0]) * pos;
}
function EllipticalPosition( a, b, angRag ) {
var a_b = a * a - b * b
var ea = (a_b <= 0) ? 0 : Math.sqrt( a_b );
var eb = (a_b >= 0) ? 0 : Math.sqrt( -a_b );
return [ a * Math.sin( angRag ) - ea, b * Math.cos( angRag ) - eb, 0 ];
}
glArrayType = typeof Float32Array !="undefined" ? Float32Array : ( typeof WebGLFloatArray != "undefined" ? WebGLFloatArray : Array );
function IdentityMat44() {
var m = new glArrayType(16);
m[0] = 1; m[1] = 0; m[2] = 0; m[3] = 0;
m[4] = 0; m[5] = 1; m[6] = 0; m[7] = 0;
m[8] = 0; m[9] = 0; m[10] = 1; m[11] = 0;
m[12] = 0; m[13] = 0; m[14] = 0; m[15] = 1;
return m;
};
function RotateAxis(matA, angRad, axis) {
var aMap = [ [1, 2], [2, 0], [0, 1] ];
var a0 = aMap[axis][0], a1 = aMap[axis][1];
var sinAng = Math.sin(angRad), cosAng = Math.cos(angRad);
var matB = new glArrayType(16);
for ( var i = 0; i < 16; ++ i ) matB[i] = matA[i];
for ( var i = 0; i < 3; ++ i ) {
matB[a0*4+i] = matA[a0*4+i] * cosAng + matA[a1*4+i] * sinAng;
matB[a1*4+i] = matA[a0*4+i] * -sinAng + matA[a1*4+i] * cosAng;
}
return matB;
}
function Cross( a, b ) { return [ a[1] * b[2] - a[2] * b[1], a[2] * b[0] - a[0] * b[2], a[0] * b[1] - a[1] * b[0], 0.0 ]; }
function Dot( a, b ) { return a[0]*b[0] + a[1]*b[1] + a[2]*b[2]; }
function Normalize( v ) {
var len = Math.sqrt( v[0] * v[0] + v[1] * v[1] + v[2] * v[2] );
return [ v[0] / len, v[1] / len, v[2] / len ];
}
var Camera = {};
Camera.create = function() {
this.pos = [0, 3, 0.0];
this.target = [0, 0, 0];
this.up = [0, 0, 1];
this.fov_y = 90;
this.vp = [800, 600];
this.near = 0.5;
this.far = 100.0;
}
Camera.Perspective = function() {
var fn = this.far + this.near;
var f_n = this.far - this.near;
var r = this.vp[0] / this.vp[1];
var t = 1 / Math.tan( Math.PI * this.fov_y / 360 );
var m = IdentityMat44();
m[0] = t/r; m[1] = 0; m[2] = 0; m[3] = 0;
m[4] = 0; m[5] = t; m[6] = 0; m[7] = 0;
m[8] = 0; m[9] = 0; m[10] = -fn / f_n; m[11] = -1;
m[12] = 0; m[13] = 0; m[14] = -2 * this.far * this.near / f_n; m[15] = 0;
return m;
}
Camera.LookAt = function() {
var mz = Normalize( [ this.pos[0]-this.target[0], this.pos[1]-this.target[1], this.pos[2]-this.target[2] ] );
var mx = Normalize( Cross( this.up, mz ) );
var my = Normalize( Cross( mz, mx ) );
var tx = Dot( mx, this.pos );
var ty = Dot( my, this.pos );
var tz = Dot( [-mz[0], -mz[1], -mz[2]], this.pos );
var m = IdentityMat44();
m[0] = mx[0]; m[1] = my[0]; m[2] = mz[0]; m[3] = 0;
m[4] = mx[1]; m[5] = my[1]; m[6] = mz[1]; m[7] = 0;
m[8] = mx[2]; m[9] = my[2]; m[10] = mz[2]; m[11] = 0;
m[12] = tx; m[13] = ty; m[14] = tz; m[15] = 1;
return m;
}
var Texture = {};
Texture.HandleLoadedTexture2D = function( texture, flipY ) {
gl.activeTexture( gl.TEXTURE0 + texture.unit );
gl.bindTexture( gl.TEXTURE_2D, texture.obj );
gl.texImage2D( gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, texture.image );
if ( flipY != undefined && flipY == true )
gl.pixelStorei( gl.UNPACK_FLIP_Y_WEBGL, true );
gl.texParameteri( gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR );
gl.texParameteri( gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR );
gl.texParameteri( gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.REPEAT );
gl.texParameteri( gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.REPEAT );
return texture;
}
Texture.LoadTexture2D = function( unit, name ) {
var texture = {};
texture.obj = gl.createTexture();
texture.unit = unit;
texture.image = new Image();
texture.image.setAttribute('crossorigin', 'anonymous');
texture.image.onload = function () {
Texture.HandleLoadedTexture2D( texture, true )
}
texture.image.src = name;
return texture;
}
var ShaderProgram = {};
ShaderProgram.Create = function( shaderList ) {
var shaderObjs = [];
for ( var i_sh = 0; i_sh < shaderList.length; ++ i_sh ) {
var shderObj = this.CompileShader( shaderList[i_sh].source, shaderList[i_sh].stage );
if ( shderObj == 0 )
return 0;
shaderObjs.push( shderObj );
}
var prog = {}
prog.progObj = this.LinkProgram( shaderObjs )
if ( prog.progObj ) {
prog.attribIndex = {};
var noOfAttributes = gl.getProgramParameter( prog.progObj, gl.ACTIVE_ATTRIBUTES );
for ( var i_n = 0; i_n < noOfAttributes; ++ i_n ) {
var name = gl.getActiveAttrib( prog.progObj, i_n ).name;
prog.attribIndex[name] = gl.getAttribLocation( prog.progObj, name );
}
prog.unifomLocation = {};
var noOfUniforms = gl.getProgramParameter( prog.progObj, gl.ACTIVE_UNIFORMS );
for ( var i_n = 0; i_n < noOfUniforms; ++ i_n ) {
var name = gl.getActiveUniform( prog.progObj, i_n ).name;
prog.unifomLocation[name] = gl.getUniformLocation( prog.progObj, name );
}
}
return prog;
}
ShaderProgram.AttributeIndex = function( prog, name ) { return prog.attribIndex[name]; }
ShaderProgram.UniformLocation = function( prog, name ) { return prog.unifomLocation[name]; }
ShaderProgram.Use = function( prog ) { gl.useProgram( prog.progObj ); }
ShaderProgram.SetUniformI1 = function( prog, name, val ) { if(prog.unifomLocation[name]) gl.uniform1i( prog.unifomLocation[name], val ); }
ShaderProgram.SetUniformF1 = function( prog, name, val ) { if(prog.unifomLocation[name]) gl.uniform1f( prog.unifomLocation[name], val ); }
ShaderProgram.SetUniformF2 = function( prog, name, arr ) { if(prog.unifomLocation[name]) gl.uniform2fv( prog.unifomLocation[name], arr ); }
ShaderProgram.SetUniformF3 = function( prog, name, arr ) { if(prog.unifomLocation[name]) gl.uniform3fv( prog.unifomLocation[name], arr ); }
ShaderProgram.SetUniformF4 = function( prog, name, arr ) { if(prog.unifomLocation[name]) gl.uniform4fv( prog.unifomLocation[name], arr ); }
ShaderProgram.SetUniformM33 = function( prog, name, mat ) { if(prog.unifomLocation[name]) gl.uniformMatrix3fv( prog.unifomLocation[name], false, mat ); }
ShaderProgram.SetUniformM44 = function( prog, name, mat ) { if(prog.unifomLocation[name]) gl.uniformMatrix4fv( prog.unifomLocation[name], false, mat ); }
ShaderProgram.CompileShader = function( source, shaderStage ) {
var shaderScript = document.getElementById(source);
if (shaderScript)
source = shaderScript.text;
var shaderObj = gl.createShader( shaderStage );
gl.shaderSource( shaderObj, source );
gl.compileShader( shaderObj );
var status = gl.getShaderParameter( shaderObj, gl.COMPILE_STATUS );
if ( !status ) alert(gl.getShaderInfoLog(shaderObj));
return status ? shaderObj : null;
}
ShaderProgram.LinkProgram = function( shaderObjs ) {
var prog = gl.createProgram();
for ( var i_sh = 0; i_sh < shaderObjs.length; ++ i_sh )
gl.attachShader( prog, shaderObjs[i_sh] );
gl.linkProgram( prog );
status = gl.getProgramParameter( prog, gl.LINK_STATUS );
if ( !status ) alert("Could not initialise shaders");
gl.useProgram( null );
return status ? prog : null;
}
var VertexBuffer = {};
VertexBuffer.Create = function( attributes, indices ) {
var buffer = {};
buffer.buf = [];
buffer.attr = []
for ( var i = 0; i < attributes.length; ++ i ) {
buffer.buf.push( gl.createBuffer() );
buffer.attr.push( { size : attributes[i].attrSize, loc : attributes[i].attrLoc } );
gl.bindBuffer( gl.ARRAY_BUFFER, buffer.buf[i] );
gl.bufferData( gl.ARRAY_BUFFER, new Float32Array( attributes[i].data ), gl.STATIC_DRAW );
}
buffer.inx = gl.createBuffer();
gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, buffer.inx );
gl.bufferData( gl.ELEMENT_ARRAY_BUFFER, new Uint16Array( indices ), gl.STATIC_DRAW );
buffer.inxLen = indices.length;
gl.bindBuffer( gl.ARRAY_BUFFER, null );
gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, null );
return buffer;
}
VertexBuffer.Draw = function( bufObj ) {
for ( var i = 0; i < bufObj.buf.length; ++ i ) {
gl.bindBuffer( gl.ARRAY_BUFFER, bufObj.buf[i] );
gl.vertexAttribPointer( bufObj.attr[i].loc, bufObj.attr[i].size, gl.FLOAT, false, 0, 0 );
gl.enableVertexAttribArray( bufObj.attr[i].loc );
}
gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, bufObj.inx );
gl.drawElements( bufObj.primitve_type, bufObj.inxLen, gl.UNSIGNED_SHORT, 0 );
for ( var i = 0; i < bufObj.buf.length; ++ i )
gl.disableVertexAttribArray( bufObj.attr[i].loc );
gl.bindBuffer( gl.ARRAY_BUFFER, null );
gl.bindBuffer( gl.ELEMENT_ARRAY_BUFFER, null );
}
initScene();
})();
<script id="draw-shader-vs" type="x-shader/x-vertex">
precision highp float;
attribute vec3 inPos;
attribute vec2 inUV;
varying vec3 vertPos;
varying vec2 vertUV;
uniform mat4 u_projectionMat44;
uniform mat4 u_viewMat44;
uniform mat4 u_modelMat44;
void main()
{
mat4 mv = u_viewMat44 * u_modelMat44;
vertUV = inUV;
vec4 viewPos = mv * vec4( inPos, 1.0 );
vertPos = viewPos.xyz;
gl_Position = u_projectionMat44 * viewPos;
}
</script>
<script id="draw-shader-fs" type="x-shader/x-fragment">
precision mediump float;
varying vec3 vertPos;
varying vec2 vertUV;
uniform sampler2D u_texture;
void main()
{
vec3 color = texture2D( u_texture, vertUV.st ).rgb;
gl_FragColor = vec4( color.rgb+0.1, 1.0 );
}
</script>
<canvas id="canvas"></canvas>

Answer 2

So you do have to unwrap it in a similar fashion that Rabbid76 mentions, however you don't need invisible triangles or an index buffer for the triangle strip mode drawing.
Here are the values I got:
position UV
(-1f, 1f, 1f), (0, 0)
( 1f, 1f, 1f), (1, 0)
(-1f, -1f, 1f), (0, 1)
( 1f, -1f, 1f), (1, 1)
( 1f, -1f, -1f), (1, 0)
( 1f, 1f, 1f), (0, 1)
( 1f, 1f, -1f), (0, 0)
(-1f, 1f, 1f), (1, 1)
(-1f, 1f, -1f), (1, 0)
(-1f, -1f, 1f), (0, 1)
(-1f, -1f, -1f), (0, 0)
( 1f, -1f, -1f), (1, 0)
(-1f, 1f, -1f), (0, 1)
( 1f, 1f, -1f), (1, 1)
It's also worth noting that there are 14 vertices listed here and 12 triangles