Drawing a triangle in OpenGL fragment shader

Question

I\'m trying to draw a triangle using an OpenGL fragment shader.

I succeeded in drawing a circle but I have a problem with handling the equation/logic or the code to draw

Answer 1

Hope you are rendering QUAD covering the view/screen...

The fragment shader friendly way of rendering triangle is to:

1. compute barycentric s,t coordinates of fragment

   go for the matrix approach as you got mat3,vec3 in GLSL ...

2. decide if it is inside or outside

   simply by testing s+t<=1.0

3. then set output color or discard;

   however discard is not an option for you as you got more shapes...

So compute:

--------------------------------------------------------
| s |           | (p1.a - p0.a) , (p2.a - p0.a) , p0.a |   | p.a |
| t | = inverse | (p1.b - p0.b) , (p2.b - p0.b) , p0.b | * | p.b |
| 1 |           |       0       ,       0       ,   1  |   |  1  |
------------------------------------------------------------------
if (s+t<=1.0) set output color

You can also use the s,t for texturing (even procedural one).