diff -r a4055f67b9c7 -r cb81ecb5fb23 src/gl/partrenderer.cpp
--- a/src/gl/partrenderer.cpp	Thu Feb 13 15:25:01 2020 +0200
+++ b/src/gl/partrenderer.cpp	Wed Feb 26 02:21:07 2020 +0200
@@ -22,6 +22,7 @@
 #include <QMouseEvent>
 #include <QMessageBox>
 #include <QAbstractButton>
+#include "geometry.h"
 #include "partrenderer.h"
 
 PartRenderer::PartRenderer(
@@ -62,6 +63,8 @@
 	this->initialized = true;
 	this->modelQuaternion = glm::angleAxis(glm::radians(30.0f), glm::vec3{-1, 0, 0});
 	this->modelQuaternion *= glm::angleAxis(glm::radians(225.0f), glm::vec3{-0, 1, 0});
+	this->setupBackgroundColor();
+	this->updateModelMatrix();
 	this->updateViewMatrix();
 	this->update();
 }
@@ -69,6 +72,11 @@
 void PartRenderer::resizeGL(int width, int height)
 {
 	glViewport(0, 0, width, height);
+	this->viewportMatrix = {
+		{1, 0, 0},
+		{0, 1, 0},
+		{width * 0.5f, height * 0.5f, 1}
+	};
 	this->projectionMatrix = glm::perspective(
 		glm::radians(45.0f),
 		static_cast<float>(width) / static_cast<float>(height),
@@ -194,6 +202,23 @@
 	this->gridProgram->setViewMatrix(this->viewMatrix);
 }
 
+void PartRenderer::updateModelMatrix()
+{
+	const glm::mat4 modelMatrix = glm::mat4_cast(this->modelQuaternion);
+	this->compiler->setUniformMatrix("modelMatrix", modelMatrix);
+	this->gridProgram->setModelMatrix(modelMatrix);
+	this->update();
+}
+
+void PartRenderer::setupBackgroundColor()
+{
+	if (this->gridProgram.has_value())
+	{
+		const bool isDark = luma(this->renderPreferences.backgroundColor) < 0.25;
+		this->gridProgram->setGridColor(isDark ? Qt::white : Qt::black);
+	}
+}
+
 void PartRenderer::renderVao(const gl::ArrayClass arrayClass)
 {
 	this->compiler->bindVertexArray(arrayClass);
@@ -246,10 +271,7 @@
 		const glm::quat q_x = glm::angleAxis(scalar * move_x, glm::vec3{0, -1, 0});
 		const glm::quat q_y = glm::angleAxis(scalar * move_y, glm::vec3{-1, 0, 0});
 		this->modelQuaternion = q_x * q_y * this->modelQuaternion;
-		const glm::mat4 modelMatrix = glm::mat4_cast(this->modelQuaternion);
-		this->compiler->setUniformMatrix("modelMatrix", modelMatrix);
-		this->gridProgram->setModelMatrix(modelMatrix);
-		this->update();
+		this->updateModelMatrix();
 	}
 	this->lastMousePosition = pointToPointF(event->pos());
 }
@@ -263,6 +285,33 @@
 	this->update();
 }
 
+std::optional<glm::vec3> PartRenderer::cameraToGrid(const QPoint& point)
+{
+	glm::vec3 pp = {point.x(), this->height() - point.y(), 1};
+	pp = glm::inverse(this->viewportMatrix) * pp;
+	glm::mat4 matrix;
+	matrix = this->projectionMatrix * this->viewMatrix * glm::mat4_cast(this->modelQuaternion);
+	matrix = glm::transpose(glm::inverse(matrix));
+	auto p1 = matrix * glm::vec4{pp.x, pp.y, 0, 1};
+	auto p2 = matrix * glm::vec4{pp.x, pp.y, 1, 1};
+	geom::Line line = geom::lineFromPoints(p1, p2);
+	return geom::linePlaneIntersection(line, geom::XY);
+}
+
+QPointF PartRenderer::worldToCamera(const glm::vec3& point)
+{
+	glm::vec4 p = {point, 1};
+	p = glm::mat4_cast(this->modelQuaternion) * p;
+	p = this->viewMatrix * p;
+	p = this->projectionMatrix * p;
+	glm::vec2 pp = this->viewportMatrix * glm::vec3{p.x, p.y, 1};
+	return toQPointF(pp);
+	/*
+	const glm::mat4 matrix = this->projectionMatrix * this->viewMatrix * glm::mat4_cast(this->modelQuaternion);
+	return toQPointF(matrix * glm::vec4{point, 1});
+	*/
+}
+
 ldraw::Id PartRenderer::pick(const QPoint& where)
 {
 	const gl::RenderStyle oldRenderStyle = this->renderPreferences.style;
@@ -299,6 +348,8 @@
 	if (mainColorChanged or backgroundColorChanged)
 	{
 		this->compiler->build(this->model, this->documents, this->renderPreferences);
+		this->setupBackgroundColor();
+
 	}
 	this->update();
 }