diff --git a/src/render.rs b/src/render.rs
index 878f3ef..d7e077c 100644
--- a/src/render.rs
+++ b/src/render.rs
@@ -50,66 +50,72 @@ fn scale_vector(x: Point, typical_mag: f64) -> Point {
     x.with_mag(new_mag)
 }
 
-struct RenderSteps(RgbImage);
+struct RenderSteps<'a> {
+    im: RgbImage, 
+    sim: &'a SimSnapshot,
+    meas: &'a [Box<dyn AbstractMeasurement>],
+}
 
-impl RenderSteps {
-    fn render(state: &SimSnapshot, measurements: &[Box<dyn AbstractMeasurement>]) -> RgbImage {
-        let w = state.width();
-        let h = state.height();
-        let mut me = RenderSteps(RgbImage::new(w, h));
-        me.render_b_field(state);
-        me.render_e_field(state);
-        me.render_measurements(state, measurements);
-        me.0
+impl<'a> RenderSteps<'a> {
+    fn render(state: &'a SimSnapshot, measurements: &'a [Box<dyn AbstractMeasurement>]) -> RgbImage {
+        let mut me = Self::new(state, measurements);
+        me.render_b_field();
+        me.render_e_field();
+        me.render_measurements();
+        me.im
     }
-    fn width(&self) -> u32 {
-        self.0.width()
+    fn new(sim: &'a SimSnapshot, meas: &'a [Box<dyn AbstractMeasurement>]) -> Self {
+        let w = sim.width();
+        let h = sim.height();
+        RenderSteps {
+            im: RgbImage::new(w, h),
+            sim,
+            meas
+        }
     }
-    fn height(&self) -> u32 {
-        self.0.height()
-    }
-    fn render_b_field(&mut self, state: &SimSnapshot) {
-        let w = self.width();
-        let h = self.height();
+
+    fn render_b_field(&mut self) {
+        let w = self.sim.width();
+        let h = self.sim.height();
         for y in 0..h {
             for x in 0..w {
-                let cell = state.get(x, y);
+                let cell = self.sim.get(x, y);
                 let r = scale_signed_to_u8(cell.mat().mz(), 100.0);
                 let b = scale_unsigned_to_u8(cell.mat().conductivity(), 10.0);
                 let g = scale_signed_to_u8(cell.bz(), 1.0e-4);
-                self.0.put_pixel(x, y, Rgb([r, g, b]));
+                self.im.put_pixel(x, y, Rgb([r, g, b]));
             }
         }
     }
-    fn render_e_field(&mut self, state: &SimSnapshot) {
-        let w = self.width();
-        let h = self.height();
+    fn render_e_field(&mut self) {
+        let w = self.sim.width();
+        let h = self.sim.height();
         let evec_spacing = 10;
         for y in 0..h {
             for x in 0..w {
                 if x % evec_spacing == 0 && y % evec_spacing == 0 {
-                    let evec = self.e_vector(state, x, y, evec_spacing);
+                    let evec = self.e_vector(x, y, evec_spacing);
                     let norm_vec = scale_vector(evec, 100.0);
                     let alpha = scale_unsigned(evec.mag_sq(), 500.0);
                     let vec = norm_vec * (evec_spacing as f64);
                     let center = Point::new(x as _, y as _) + Point::new(evec_spacing as _, evec_spacing as _)*0.5;
-                    self.0.draw_field_arrow(center, vec, Rgb([0xff, 0xff, 0xff]), alpha as f32);
+                    self.im.draw_field_arrow(center, vec, Rgb([0xff, 0xff, 0xff]), alpha as f32);
                 }
             }
         }
     }
-    fn render_measurements(&mut self, state: &SimSnapshot, measurements: &[Box<dyn AbstractMeasurement>]) {
-        for (meas_no, m) in measurements.iter().enumerate() {
-            let meas_string = m.eval(state);
+    fn render_measurements(&mut self) {
+        for (meas_no, m) in self.meas.iter().enumerate() {
+            let meas_string = m.eval(self.sim);
             for (i, c) in meas_string.chars().enumerate() {
                 let glyph = BASIC_FONTS.get(c).unwrap();
                 for (y, bmp) in glyph.iter().enumerate() {
                     for x in 0..8 {
                         if (bmp & 1 << x) != 0 {
                             let real_x = 2 + i as u32*8 + x;
-                            let real_y = y as u32 + self.height() - 10 - meas_no as u32 * 8;
-                            if real_x < self.width() {
-                                self.0.put_pixel(real_x, real_y, Rgb([0, 0, 0]));
+                            let real_y = y as u32 + self.im.height() - 10 - meas_no as u32 * 8;
+                            if real_x < self.im.width() {
+                                self.im.put_pixel(real_x, real_y, Rgb([0, 0, 0]));
                             }
                         }
                     }
@@ -118,17 +124,17 @@ impl RenderSteps {
         }
     }
 
-    fn e_vector(&self, state: &SimSnapshot, xidx: u32, yidx: u32, size: u32) -> Point {
+    fn e_vector(&self, xidx: u32, yidx: u32, size: u32) -> Point {
         let mut e = Point::default();
-        let w = self.width();
-        let h = self.height();
+        let w = self.sim.width();
+        let h = self.sim.height();
         let xstart = xidx.min(w);
         let ystart = yidx.min(h);
         let xend = (xstart + size).min(w);
         let yend = (ystart + size).min(h);
         for y in ystart..yend {
             for x in xstart..xend {
-                e += state.get(x, y).e();
+                e += self.sim.get(x, y).e();
             }
         }
         let xw = xend - xstart;