opengl_boilerplate.go

package main

import (
	"fmt"
	"image"
	"log"
	"runtime"
	"strings"

	"github.com/go-gl/gl/v4.1-core/gl" // OR: github.com/go-gl/gl/v2.1/gl
	"github.com/go-gl/glfw/v3.2/glfw"
)

const (
	vertexShaderSource = `
	    #version 410
	    in vec3 vp;
			in vec2 vertTexCoord;
			out vec2 fragTexCoord;
	    void main() {
					fragTexCoord = vertTexCoord;
	        gl_Position = vec4(vp, 1.0);
	    }
	` + "\x00"

	fragmentShaderSource = `
		#version 330
		uniform sampler2D tex;
		in vec2 fragTexCoord;
		out vec4 outputColor;
		void main() {
				outputColor = texture(tex, fragTexCoord);
		}
	` + "\x00"
)

var fullscreenQuad = []float32{
	//  X, Y, Z, U, V
	-1, 1, 0, 0, 1,
	-1, -1, 0, 0, 0,
	1, -1, 0, 1, 0,
	-1, 1, 0, 0, 1,
	1, 1, 0, 1, 1,
	1, -1, 0, 1, 0,
}

// basic OpenGL based display application copy and pasted from
// https://kylewbanks.com/blog/tutorial-opengl-with-golang-part-1-hello-opengl
func run_opengl_app(getImage func() *image.RGBA) {
	runtime.LockOSThread()

	window := initGlfw()
	defer glfw.Terminate()

	program := initOpenGL()

	for !window.ShouldClose() {
		texture, err := newTexture(getImage())
		if err != nil {
			panic(err)
		}

		draw(makeVertexArrayObject(fullscreenQuad, program), texture, window, program)
	}
}

func draw(vertexArrayObject uint32, texture uint32, window *glfw.Window, program uint32) {
	gl.Clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT)
	gl.UseProgram(program)

	gl.BindVertexArray(vertexArrayObject)

	gl.ActiveTexture(gl.TEXTURE0)
	gl.BindTexture(gl.TEXTURE_2D, texture)

	gl.DrawArrays(gl.TRIANGLES, 0, int32(len(fullscreenQuad)/3))

	glfw.PollEvents()
	window.SwapBuffers()
}

func initOpenGL() uint32 {
	if err := gl.Init(); err != nil {
		panic(err)
	}
	version := gl.GoStr(gl.GetString(gl.VERSION))
	log.Println("OpenGL version", version)

	vertexShader, err := compileShader(vertexShaderSource, gl.VERTEX_SHADER)
	if err != nil {
		panic(err)
	}
	fragmentShader, err := compileShader(fragmentShaderSource, gl.FRAGMENT_SHADER)
	if err != nil {
		panic(err)
	}

	prog := gl.CreateProgram()
	gl.AttachShader(prog, vertexShader)
	gl.AttachShader(prog, fragmentShader)
	gl.LinkProgram(prog)
	return prog
}

// initGlfw initializes glfw and returns a Window to use.
func initGlfw() *glfw.Window {
	if err := glfw.Init(); err != nil {
		panic(err)
	}

	glfw.WindowHint(glfw.Resizable, glfw.False)
	glfw.WindowHint(glfw.ContextVersionMajor, 4) // OR 2
	glfw.WindowHint(glfw.ContextVersionMinor, 1)
	glfw.WindowHint(glfw.OpenGLProfile, glfw.OpenGLCoreProfile)
	glfw.WindowHint(glfw.OpenGLForwardCompatible, glfw.True)

	window, err := glfw.CreateWindow(dim, dim, "hilbert test", nil, nil)
	if err != nil {
		panic(err)
	}
	window.MakeContextCurrent()

	return window
}

// texure coordinate stuff sourced from https://github.com/go-gl/example/blob/master/gl41core-cube/cube.go
func makeVertexArrayObject(points []float32, program uint32) uint32 {

	var vao uint32
	gl.GenVertexArrays(1, &vao)
	gl.BindVertexArray(vao)

	var vbo uint32
	gl.GenBuffers(1, &vbo)
	gl.BindBuffer(gl.ARRAY_BUFFER, vbo)
	gl.BufferData(gl.ARRAY_BUFFER, len(points)*4, gl.Ptr(points), gl.STATIC_DRAW)

	vertAttrib := uint32(gl.GetAttribLocation(program, gl.Str("vp\x00")))
	gl.EnableVertexAttribArray(vertAttrib)
	gl.VertexAttribPointerWithOffset(vertAttrib, 3, gl.FLOAT, false, 5*4, 0)

	texCoordAttrib := uint32(gl.GetAttribLocation(program, gl.Str("vertTexCoord\x00")))
	gl.EnableVertexAttribArray(texCoordAttrib)
	gl.VertexAttribPointerWithOffset(texCoordAttrib, 2, gl.FLOAT, false, 5*4, 3*4)

	return vao
}

func compileShader(source string, shaderType uint32) (uint32, error) {
	shader := gl.CreateShader(shaderType)

	csources, free := gl.Strs(source)
	gl.ShaderSource(shader, 1, csources, nil)
	free()
	gl.CompileShader(shader)

	var status int32
	gl.GetShaderiv(shader, gl.COMPILE_STATUS, &status)
	if status == gl.FALSE {
		var logLength int32
		gl.GetShaderiv(shader, gl.INFO_LOG_LENGTH, &logLength)

		log := strings.Repeat("\x00", int(logLength+1))
		gl.GetShaderInfoLog(shader, logLength, nil, gl.Str(log))

		return 0, fmt.Errorf("failed to compile %v: %v", source, log)
	}

	return shader, nil
}

func newTexture(rgba *image.RGBA) (uint32, error) {

	if rgba.Stride != rgba.Rect.Size().X*4 {
		return 0, fmt.Errorf("unsupported stride")
	}

	var texture uint32
	gl.GenTextures(1, &texture)
	gl.ActiveTexture(gl.TEXTURE0)
	gl.BindTexture(gl.TEXTURE_2D, texture)
	gl.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR)
	gl.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR)
	gl.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE)
	gl.TexParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE)
	gl.TexImage2D(
		gl.TEXTURE_2D,
		0,
		gl.RGBA,
		int32(rgba.Rect.Size().X),
		int32(rgba.Rect.Size().Y),
		0,
		gl.RGBA,
		gl.UNSIGNED_BYTE,
		gl.Ptr(rgba.Pix))

	return texture, nil
}