Opengl tessellation example3/16/2024 ![]() Not very impressive yet, but it's interesting to consider that this output was generated using just the following render call: If we were to render this it looks something like this: This geometry shader takes a point primitive as its input and creates a horizontal line primitive with the input point at its center. Now that you (sort of) know how geometry shaders work you can probably guess what this geometry shader does. This particular case emits two vertices that were translated by a small offset from the original vertex position and then calls EndPrimitive, combining the two vertices into a single line strip of 2 vertices. By repeatedly calling EndPrimitive, after one or more EmitVertex calls, multiple primitives can be generated. Whenever EndPrimitive is called, all emitted vertices for this primitive are combined into the specified output render primitive. In our case we want to at least generate one line strip primitive.Įach time we call EmitVertex, the vector currently set to gl_Position is added to the output primitive. The geometry shader expects you to generate/output at least one of the primitives you specified as output. Using the vertex data from the vertex shader stage we can generate new data with 2 geometry shader functions called EmitVertex and EndPrimitive. The geometry shader receives all vertices of a primitive as its input. Note that it is declared as an array, because most render primitives contain more than 1 vertex. Here it is declared as an interface block (as discussed in the previous chapter) that contains a few interesting variables of which the most interesting one is gl_Position that contains the vector we set as the vertex shader's output. GLSL gives us a built-in variable called gl_in that internally (probably) looks something like this: To generate meaningful results we need some way to retrieve the output from the previous shader stage. Each extra point results in a new line between the new point and the previous point as you can see in the following image with 5 point vertices: In case you're wondering what a line strip is: a line strip binds together a set of points to form one continuous line between them with a minimum of 2 points. ![]() In this particular case we're going to output a line_strip with a maximum number of 2 vertices. The geometry shader also expects us to set a maximum number of vertices it outputs (if you exceed this number, OpenGL won't draw the extra vertices) which we can also do within the layout qualifier of the out keyword. To generate a single triangle for example we'd specify triangle_strip as the output and output 3 vertices. With just these 3 output specifiers we can create almost any shape we want from the input primitives. Like the input layout qualifier, the output layout qualifier can take several primitive values: We also need to specify a primitive type that the geometry shader will output and we do this via a layout specifier in front of the out keyword. The number within the parenthesis represents the minimal number of vertices a single primitive contains. If we'd chosen to draw vertices as GL_TRIANGLES we should set the input qualifier to triangles. These are almost all the rendering primitives we're able to give to rendering calls like glDrawArrays. triangles_adjacency : GL_TRIANGLES_ADJACENCY or GL_TRIANGLE_STRIP_ADJACENCY ( 6).triangles: GL_TRIANGLES, GL_TRIANGLE_STRIP or GL_TRIANGLE_FAN ( 3).lines_adjacency: GL_LINES_ADJACENCY or GL_LINE_STRIP_ADJACENCY ( 4). ![]() lines: when drawing GL_LINES or GL_LINE_STRIP ( 2).points: when drawing GL_POINTS primitives ( 1).This input layout qualifier can take any of the following primitive values: We do this by declaring a layout specifier in front of the in keyword. Gl_Position = gl_in.gl_Position + vec4( 0.1, 0.0, 0.0, 0.0) Īt the start of a geometry shader we need to declare the type of primitive input we're receiving from the vertex shader. Layout (line_strip, max_vertices = 2) out ![]() We're going to throw you right into the deep by showing you an example of a geometry shader: What makes the geometry shader interesting is that it is able to convert the original primitive (set of vertices) to completely different primitives, possibly generating more vertices than were initially given. The geometry shader can then transform these vertices as it sees fit before sending them to the next shader stage. A geometry shader takes as input a set of vertices that form a single primitive e.g. Geometry Shader Advanced-OpenGL/Geometry-Shaderīetween the vertex and the fragment shader there is an optional shader stage called the geometry shader.
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