Hi,
I’m new in this forum and I’m new to openGL and shading languages.
I’m writing in GLSL a geometry shader that pick a line strip vertices and transform it to looks like a filled stroke, it’s like a tape; it’s a triangles strip.
I started my work from this shader https://github.com/openframeworks/openFr…aders/geom.glsl
THE PROBLEM: the problem is that now my shader scale the width of the strip processing it segment by segment; the scaling is based on the length of the segment; but this approach cause a big eye gap between small and big segment.
To solve this I thought to do a little buffer with the last segment width and start the next segment and to do a little control at the first vertex to don’t fall in error assigning it a width which doesn’t exists.
It doesn’t work.
I don’t manage why, the control point always think we are at the first vertex. Is not a simple error I think, I checked also after a walk with the dog 
.
Here is the shader:
///////////////////////////////
// GLSL shaders
// PARAMETERS PASSED TO SHADERS
// float thickness default=0.1
// float limiter default=0.2
// vec3 lightDir default= 0.0 0.04 1.0
// line_strip vertices
////////////////
// VERTEX SHADER
#version 120
uniform float limiter;
void main(void)
{
// just bit waving
vec4 wave = vec4(gl_Vertex[0],gl_Vertex[1],gl_Vertex[2]*sin(gl_Vertex[0]),gl_Vertex[3]);
gl_FrontColor = gl_Color;
gl_Position = wave;
}
//////////////////
// GEOMETRY SHADER
// input type: GL_LINES
// output type: GL_TRIANGLE_STRIP
// vertices out: 4
/***********************************************
*
* Based on:
* Geometry shader to convert lines into triangle strips
* Memo Akten
* www.memo.tv
************************************************/
#version 120
#extension GL_EXT_geometry_shader4 : enable
uniform float thickness;
uniform vec3 lightDir;
uniform float limiter;
void main(void) {
// actual working vertices
vec3 p0;
vec3 p1;
vec3 dir; // normalized direction vector from p0 to p1
vec3 thick; // thick vector
vec3 oldthick; // old thick vector
float thickM; // thick modifier
vec3 norm;
// distance calculation stuff
vec2 P;
vec2 Q;
// color stuff
float fColMult;
vec4 colMult;
// loop stuff
int i;
bool first = true;
vec3 up = vec3(0, 0, 1); // arbitrary up vector
// go trought the vertices and do the transformation
for(i=0; i<gl_VerticesIn; i++){
if (first == true) { // maybe isn't a good way do a big control like this but is for debugging
first = false; // seems this doesn't work
// set working vertices
p0 = gl_PositionIn[i].xyz;
p1 = gl_PositionIn[i+1].xyz;
// some calculations
dir = normalize(p1 - p0);
thick = normalize(cross(dir, up));
norm = cross(thick, dir);
fColMult = abs(dot(norm, lightDir)); // simple lighting
P = vec2(p0[0],p0[1]);
Q = vec2(p1[0],p1[1]);
thickM = distance(P,Q)+0.02; // the lenght of the segment influeces his thickness
if (thickM > limiter) { // clipping
thickM = limiter;
}
thick *= thickness*thickM; // set calculated thickness
gl_Position = gl_ModelViewProjectionMatrix * vec4(p0 - thick, 1.0);
gl_FrontColor = vec4(gl_FrontColorIn[i].rgb * fColMult, 1.0);
EmitVertex();
gl_Position = gl_ModelViewProjectionMatrix * vec4(p0 + thick, 1.0);
gl_FrontColor = vec4(gl_FrontColorIn[i].rgb * fColMult, 1.0);
EmitVertex();
gl_Position = gl_ModelViewProjectionMatrix * vec4(p1 - thick, 1.0);
gl_FrontColor = vec4(gl_FrontColorIn[i+1].rgb * fColMult, 1.0);
EmitVertex();
gl_Position = gl_ModelViewProjectionMatrix * vec4(p1 + thick, 1.0);
gl_FrontColor = vec4(gl_FrontColorIn[i+1].rgb * fColMult, 1.0);
EmitVertex();
oldthick = thick;
} else {
p0 = gl_PositionIn[i].xyz;
p1 = gl_PositionIn[i+1].xyz;
dir = normalize(p1 - p0);
thick = normalize(cross(dir, up));
norm = cross(thick, dir);
fColMult = abs(dot(norm, lightDir)); // simple lighting
P = vec2(p0[0],p0[1]);
Q = vec2(p1[0],p1[1]);
thickM = distance(P,Q)+0.02; // the lenght of the segment influeces his thickness
if (thickM > limiter) { // clipping
thickM = limiter;
}
thick *= thickness*thickM; // set calculated thickness
gl_Position = gl_ModelViewProjectionMatrix * vec4(p0 - oldthick, 1.0);
//gl_FrontColor = gl_FrontColorIn[i] * fColMult;
gl_FrontColor = vec4(1.0,0.0,0.0,1.0); // if the is processed i surely see this
EmitVertex();
gl_Position = gl_ModelViewProjectionMatrix * vec4(p0 + oldthick, 1.0);
//gl_FrontColor = gl_FrontColorIn[i] * fColMult;
gl_FrontColor = vec4(1.0,0.0,0.0,1.0); // if the is processed i surely see this
EmitVertex();
gl_Position = gl_ModelViewProjectionMatrix * vec4(p1 - thick, 1.0);
//gl_FrontColor = gl_FrontColorIn[i+1] * fColMult;
gl_FrontColor = vec4(1.0,0.0,0.0,1.0); // if the is processed i surely see this
EmitVertex();
gl_Position = gl_ModelViewProjectionMatrix * vec4(p1 + thick, 1.0);
//gl_FrontColor = gl_FrontColorIn[i+1] * fColMult;
gl_FrontColor = vec4(1.0,0.0,0.0,1.0); // if the is processed i surely see this
EmitVertex();
oldthick = thick; // update the starting dimension
}
}
}
//////////////////
// FRAGMENT SHADER
void main (void) // do nothing
{
gl_FragColor = gl_Color;
}
What is wrong? This is my first approach to opengl and shading and I’m learning everything via web.
You are my masters.
Thank you for the attention.