Q2 mdl to obj converter

mdl2obj.kt

package shaders

fun convertToObj(model: Md2Model, frameIndex: Int): String {
    val result = StringBuilder()
    val frame = model.frames[frameIndex] ?: return ""
    result.appendLine("mtllib sphere.mtl")
    result.appendLine("o testModel")
    // v vec3_position
    frame.points.forEach {
        result.appendLine("v ${it.x * frame.scale[0]} ${it.z * frame.scale[2]} ${it.y * frame.scale[1]}")
    }
    // vn vec3_normal
    // optional
//    result.appendLine("# test texture normals")
//    result.appendLine("vn 0.0 1.0 0.0")
//    // vt vec2_textureCoords
//    result.appendLine("# test texture coords")
//    result.appendLine("vt 0.0 0.0")
//    result.appendLine("vt 1.0 0.0")
//    result.appendLine("vt 0.0 1.0")
    result.appendLine("# smoothing groop")
    result.appendLine("s 0")
    result.appendLine("usemtl IcosphereMaterial")
    // f position/normal/texture
    var glCmdIndex = 0 // todo: use queue to pop elements instead of using mutable index?
    while (true) {
        val numOfPoints = model.glCmds[glCmdIndex]
        glCmdIndex++
        glCmdIndex += if (numOfPoints == 0) {
            break
        } else if (numOfPoints >= 0) {
            // triangle strip
            val vertices = mutableListOf<Int>()
            for (i in glCmdIndex until (glCmdIndex + numOfPoints * 3) step 3) {
                vertices.add(model.glCmds[i + 2])
            }
            // obj indices are is 1 based, converting strips into separate triangles
            vertices.map { it + 1 }.windowed(3).forEach {
                result.appendLine("f ${it[0]} ${it[1]} ${it[2]}")
            }
            numOfPoints * 3
        } else {
            // triangle fan
            val vertices = mutableListOf<Int>()
            for (i in glCmdIndex until (glCmdIndex - numOfPoints * 3) step 3) {
                vertices.add(model.glCmds[i + 2])
            }
            // obj indices are is 1 based
            convertStripToTriangles(vertices).map { it + 1 }.windowed(3).forEach {
                result.appendLine("f ${it[0]} ${it[1]} ${it[2]}")
            }


            (-numOfPoints * 3)
        }

    }
    return result.toString()
}

private fun convertStripToTriangles(vertices: List<Int>): List<Int> {
    val result = mutableListOf<Int>()
    vertices.drop(1).windowed(2).forEach {
        result.add(vertices.first())
        result.add(it.first())
        result.add(it.last())
    }
    return result
}