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Lowlevel Drawing

This text is intended for developers looking to provide drawing functionality that is fully independent of the Drawer class. Developers that desire to do so will essentially use the same building blocks that OPENRNDR uses internally.

A good reference for lowlevel drawing is the Filter class and it is encouraged to look at its source code as it provides a good idea of how all the lowlevel components interact and its implementation is fully independent from Drawer.

All lowlevel drawing is performed using OPENRNDR’s Driver class. This class provides a handful of functions for drawing. Below you will find a list of the essential ones.

  • Driver.setState()
  • Driver.drawVertexBuffer()
  • Driver.drawIndexedVertexBuffer()
  • Driver.drawInstances()
  • Driver.drawIndexedInstances()

Classes that belong to the lowlevel drawing API:

We discourage writing code that uses OpenGL directly; even though currently only an OpenGL 3.3 implementation exists for Driver, in the future we may add implementations that are not based on OpenGL.

Example

In the following example we show the minimum steps required for drawing a single triangle.

../media/lowlevel-drawing-001.jpg

fun main() = application {
    program {
        val geometry = vertexBuffer(vertexFormat {
            position(3) // -- this attribute is named "position"
        }, 3)
        
        geometry.put {
            for (i in 0 until geometry.vertexCount) {
                write(Vector3(2.0 * Math.random() - 1.0, 2.0 * Math.random() - 1.0, 0.0))
            }
        }
        
        // -- code for the vertex shader
        val vs = """
        #version 330                
        in vec3 a_position;  // -- driver adds a_ prefix (a for attribute)
        void main() {
            gl_Position = vec4(a_position, 1.0);                
        }
        """
        
        // -- code for the fragment shader
        val fs = """
        #version 330
        out vec4 o_output;
        void main() {
            o_output = vec4(1.0);                                                                                                                
        }                                                                
        """
        
        val shader = Shader.createFromCode(vsCode = vs, fsCode = fs, name = "custom-shader")
        
        extend {
            shader.begin()
            Driver.instance.drawVertexBuffer(shader, listOf(geometry), DrawPrimitive.TRIANGLES, 0, 3)
            shader.end()
        }
    }
}

Link to the full example

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