diff --git a/src/consts.cu b/src/consts.cu
index 7ed979a..30e201f 100644
--- a/src/consts.cu
+++ b/src/consts.cu
@@ -28,9 +28,9 @@ const ColorStop h_stopsBluePurleRed[] = {
 
 __device__ Point3 d_cameraPos;
 __device__ Vec3 d_cameraDir;
-__device__ Vec3 d_cameraUp;
 __device__ Point3 d_lightPos;
 __device__ Color3 d_backgroundColor;
+__device__ Vec3 d_cameraUp;
 
 Vec3 h_cameraUp = Vec3::init(0.0, 1.0, 0.0).normalize();
 
@@ -45,3 +45,9 @@ void copyConstantsToDevice() {
     // ----------------------- Camera and Light -----------------------
     cudaMemcpyToSymbol(d_cameraUp, &h_cameraUp, sizeof(Vec3));
 }
+
+
+// ----------------------- TransferFunction -----------------------
+__device__ float d_opacityK;
+__device__ float d_sigmoidOne;
+__device__ float d_sigmoidTwo;
diff --git a/src/consts.h b/src/consts.h
index 70d6ccb..4790915 100644
--- a/src/consts.h
+++ b/src/consts.h
@@ -11,9 +11,8 @@ const int VOLUME_WIDTH  = 97;  // lon
 const int VOLUME_HEIGHT = 71;  // lat
 const int VOLUME_DEPTH  = 42;  // lev
 
-// TODO: replace with window->w and window->h
-const int IMAGE_WIDTH   = 800;
-const int IMAGE_HEIGHT  = 600;
+const int INITIAL_WINDOW_WIDTH   = 1200;
+const int INITIAL_WINDOW_HEIGHT  = 900;
 
 const double epsilon = 1e-10f;
 const double infty   = 1e15f;  // This value is used to represent missing values in data
@@ -52,12 +51,19 @@ extern __device__ Point3 d_lightPos;
 // Background color
 extern __device__ Color3 d_backgroundColor;
 
+
 // --------------------------- Transfer Function Constants ---------------------------
 struct ColorStop {
     float pos;       // in [0,1]  
     Color3 color;
 };
 
+// factor for the opacity function
+extern __device__ float d_opacityK;
+// sigmoid function variables
+extern __device__ float d_sigmoidOne;
+extern __device__ float d_sigmoidTwo;
+
 const int lenStopsPythonLike = 5;
 const int lenStopsGrayscale = 2;
 const int lenStopsBluePurpleRed = 3;
diff --git a/src/gui/input/Widget.cpp b/src/gui/input/Widget.cpp
index 8684db3..e57d9a1 100644
--- a/src/gui/input/Widget.cpp
+++ b/src/gui/input/Widget.cpp
@@ -13,9 +13,9 @@ Widget::Widget(GLFWwindow* window) {
   ImGui_ImplGlfw_InitForOpenGL(window, true);
   ImGui_ImplOpenGL3_Init();
 
-  // this->cameraPos = Point3::init(50.0f, -50.0f, -75.0f);  // Camera for partially trimmed data set
-  this->cameraPos = Point3::init(300.0f, 200.0f, -700.0f);  // Camera for full data set
-  //
+  this->cameraPos = Point3::init(50.0f, -50.0f, -75.0f);  // Camera for partially trimmed data set
+  // this->cameraPos = Point3::init(300.0f, 200.0f, -700.0f);  // Camera for full data set
+  
   Vec3 h_center = Vec3::init((float)VOLUME_WIDTH/2.0f, (float)VOLUME_HEIGHT/2.0f, (float)VOLUME_DEPTH/2.0f);
   this->cameraDir = (h_center - this->cameraPos).normalize();
 
@@ -25,8 +25,13 @@ Widget::Widget(GLFWwindow* window) {
   this->fps = (char*)malloc(512*sizeof(char));
   this->paused = true;
   this->renderOnce = false;
+
+  this->opacityK = 0;
+  this->sigmoidOne = 0.5f;
+  this->sigmoidTwo = -250.0f;
 };
 
+// TODO: can be marginally improvement by only copying changed values to device - however we're dealing with individual floats here so i dont think the benefit would be all that obvious.
 void Widget::tick(double fps) {
   if (this->renderOnce) {
     this->renderOnce = false;
@@ -40,16 +45,22 @@ void Widget::tick(double fps) {
   // input widgets
   float min = -1, max = 1;
 
+  ImGui::Begin("Transfer Function Controls");
+  ImGui::DragInt("k (log [1e-10, 1])", &this->opacityK, 1, 0, 100, "%d%%", ImGuiSliderFlags_AlwaysClamp);
+  ImGui::DragFloat("sigmoidOne", &this->sigmoidOne, 0.01f, 0.0f, 1.0f, "%.2f");
+  ImGui::InputFloat("sigmoidTwo", &this->sigmoidTwo, 10.0f, 100.0f, "%.0f");
+  ImGui::End();
+
   ImGui::Begin("Light Controls");
-  ImGui::DragScalar("X coordinate", ImGuiDataType_Double, &this->lightPos.x, 0.005f, &min, &max, "%.3f");
-  ImGui::DragScalar("Y coordinate", ImGuiDataType_Double, &this->lightPos.y, 0.005f, &min, &max, "%.3f");
-  ImGui::DragScalar("Z coordinate", ImGuiDataType_Double, &this->lightPos.z, 0.005f, &min, &max, "%.3f");
+  ImGui::DragScalar("X coordinate", ImGuiDataType_Double, &this->lightPos.x, 0.5f, &min, &max, "%.3f");
+  ImGui::DragScalar("Y coordinate", ImGuiDataType_Double, &this->lightPos.z, 0.5f, &min, &max, "%.3f");
+  ImGui::DragScalar("Z coordinate", ImGuiDataType_Double, &this->lightPos.y, 0.5f, &min, &max, "%.3f");
   ImGui::End();
 
   ImGui::Begin("Miscellaneous");
   if (ImGui::Button(this->paused ? "Unpause" : "Pause")) this->paused = !this->paused;
   ImGui::SameLine();
-  if (ImGui::Button("render Once")) {
+  if (ImGui::Button("Render once")) {
     this->paused = !this->paused;
     this->renderOnce = true;
   }
@@ -58,12 +69,12 @@ void Widget::tick(double fps) {
   ImGui::End();
 
   ImGui::Begin("Camera Controls");
-  ImGui::DragScalar("X position", ImGuiDataType_Double, &this->cameraPos.x, 0.005f, &min, &max, "%.3f");
-  ImGui::DragScalar("Y position", ImGuiDataType_Double, &this->cameraPos.y, 0.005f, &min, &max, "%.3f");
-  ImGui::DragScalar("Z position", ImGuiDataType_Double, &this->cameraPos.z, 0.005f, &min, &max, "%.3f");
+  ImGui::DragScalar("X position", ImGuiDataType_Double, &this->cameraPos.x, 0.5f, &min, &max, "%.3f");
+  ImGui::DragScalar("Y position", ImGuiDataType_Double, &this->cameraPos.z, 0.5f, &min, &max, "%.3f");
+  ImGui::DragScalar("Z position", ImGuiDataType_Double, &this->cameraPos.y, 0.5f, &min, &max, "%.3f");
   ImGui::DragScalar("X direction", ImGuiDataType_Double, &this->cameraDir.x, 0.005f, &min, &max, "%.3f");
-  ImGui::DragScalar("Y direction", ImGuiDataType_Double, &this->cameraDir.y, 0.005f, &min, &max, "%.3f");
-  ImGui::DragScalar("Z direction", ImGuiDataType_Double, &this->cameraDir.z, 0.005f, &min, &max, "%.3f");
+  ImGui::DragScalar("Y direction", ImGuiDataType_Double, &this->cameraDir.z, 0.005f, &min, &max, "%.3f");
+  ImGui::DragScalar("Z direction", ImGuiDataType_Double, &this->cameraDir.y, 0.005f, &min, &max, "%.3f");
   ImGui::End();
   
   copyToDevice();
@@ -79,6 +90,13 @@ void Widget::copyToDevice() {
   cudaMemcpyToSymbol(&d_cameraDir, &this->cameraDir, sizeof(Vec3));
   cudaMemcpyToSymbol(&d_lightPos, &this->lightPos, sizeof(Point3));
   cudaMemcpyToSymbol(&d_backgroundColor, &this->bgColor, sizeof(Color3));
+  
+  // cudaMemcpyToSymbol(&d_opacityK, &this->opacityK, sizeof(float));
+  this->opacityKReal = std::pow(10.0f, (-10 + 0.1 * this->opacityK));
+  cudaMemcpyToSymbol(&d_opacityK, &this->opacityKReal, sizeof(float));
+
+  cudaMemcpyToSymbol(&d_sigmoidOne, &this->sigmoidOne, sizeof(float));
+  cudaMemcpyToSymbol(&d_sigmoidTwo, &this->sigmoidTwo, sizeof(float));
 }
 
 Widget::~Widget() {
diff --git a/src/gui/input/Widget.h b/src/gui/input/Widget.h
index 2fb7813..21440f9 100644
--- a/src/gui/input/Widget.h
+++ b/src/gui/input/Widget.h
@@ -12,12 +12,17 @@ public:
   Point3 cameraDir;
   Vec3 cameraPos;
   Point3 lightPos;
-  Color3 bgColor;
+  Color3 bgColor; // TODO: widget
 
   bool paused;
   bool renderOnce;
   char* fps;
 
+  int opacityK;
+  float opacityKReal;
+  float sigmoidOne;
+  float sigmoidTwo;
+
   ImGuiIO io;
 
   void tick(double fps);
diff --git a/src/illumination/Raycaster.cu b/src/illumination/Raycaster.cu
index 58837ad..400cce1 100644
--- a/src/illumination/Raycaster.cu
+++ b/src/illumination/Raycaster.cu
@@ -12,10 +12,10 @@
 #include <curand_kernel.h>
 
 // TODO: instead of IMAGEWIDTH and IMAGEHEIGHT this should reflect the windowSize;
-__global__ void raycastKernel(float* volumeData, FrameBuffer framebuffer) {
+__global__ void raycastKernel(float* volumeData, FrameBuffer framebuffer, const int width, const int height) {
     int px = blockIdx.x * blockDim.x + threadIdx.x;
     int py = blockIdx.y * blockDim.y + threadIdx.y;
-    if (px >= IMAGE_WIDTH || py >= IMAGE_HEIGHT) return;
+    if (px >= width || py >= height) return;
 
     float accumR = 0.0f;
     float accumG = 0.0f;
@@ -24,15 +24,15 @@ __global__ void raycastKernel(float* volumeData, FrameBuffer framebuffer) {
 
     // Initialize random state for ray scattering
     curandState randState;
-    curand_init(1234, px + py * IMAGE_WIDTH, 0, &randState);
+    curand_init(1234, px + py * width, 0, &randState);
 
     // Multiple samples per pixel
     for (int s = 0; s < SAMPLES_PER_PIXEL; s++) {
         // Map to [-1, 1]
-        float jitterU = (curand_uniform(&randState) - 0.5f) / IMAGE_WIDTH;
-        float jitterV = (curand_uniform(&randState) - 0.5f) / IMAGE_HEIGHT;
-        float u = ((px + 0.5f + jitterU) / IMAGE_WIDTH ) * 2.0f - 1.0f;
-        float v = ((py + 0.5f + jitterV) / IMAGE_HEIGHT) * 2.0f - 1.0f;
+        float jitterU = (curand_uniform(&randState) - 0.5f) / width;
+        float jitterV = (curand_uniform(&randState) - 0.5f) / height;
+        float u = ((px + 0.5f + jitterU) / width ) * 2.0f - 1.0f;
+        float v = ((py + 0.5f + jitterV) / height) * 2.0f - 1.0f;
 
         float tanHalfFov = tanf(fov * 0.5f);
         u *= tanHalfFov;
@@ -167,7 +167,7 @@ void Raycaster::render() {
 
   // TODO: pass camera info at some point
 	// frame buffer is implicitly copied to the device each frame
-  raycastKernel<<<threadSize, blockSize>>> (this->data, *this->fb);
+  raycastKernel<<<threadSize, blockSize>>> (this->data, *this->fb, this->w, this->h);
 
   check_cuda_errors(cudaGetLastError());
   check_cuda_errors(cudaDeviceSynchronize());
diff --git a/src/illumination/transferFunction.cu b/src/illumination/transferFunction.cu
index fd8db0c..c33e8e5 100644
--- a/src/illumination/transferFunction.cu
+++ b/src/illumination/transferFunction.cu
@@ -1,4 +1,7 @@
 #include "transferFunction.h"
+#include "consts.h"
+
+#include <stdio.h>
 
 
 // Samples the voxel nearest to the given coordinates.
@@ -53,13 +56,12 @@ __device__ float sampleVolumeTrilinear(float* volumeData, const int volW, const
 
 __device__ float opacityFromGradient(const Vec3 &grad) {
     float gradMag = grad.length();
-    float k = 1e-6f;               // tweak (the smaller the value, the less opacity)  // TODO: Add a slider for this
-    float alpha = 1.0f - expf(-k * gradMag);
+    float alpha = 1.0f - expf(-d_opacityK * gradMag);
     return alpha;
 }
 
 __device__ float opacitySigmoid(float val) {
-    return 1.0f / (1.0f + expf(-250.f * (val - 0.5f)));  // TODO: Parametrize and add sliders
+    return 1.0f / (1.0f + expf(d_sigmoidTwo * (val - d_sigmoidOne)));
 }
 
 __device__ Color3 colorMap(float normalizedValues, const ColorStop stops[], int N) {
@@ -96,6 +98,7 @@ __device__ float4 transferFunction(float density, const Vec3& grad, const Point3
   // TODO: Add a way to pick stops here
   Color3 baseColor = colorMap(normDensity, d_stopsPythonLike, lenStopsPythonLike);
 
+  // TODO: This is a Gui select element
   // TODO: Add a way to pick different function for alpha
   float alpha = opacityFromGradient(grad);
   // alpha = 0.1f;
diff --git a/src/main.cu b/src/main.cu
index d74f264..39e5c49 100644
--- a/src/main.cu
+++ b/src/main.cu
@@ -15,7 +15,7 @@
 
 static float* d_volume = nullptr;
 
-// FIXME: segfaults on window resize - the raycasting function should work with window->w and window-h instead of constants.
+// FIXME: segfaults on window resize - the problem is _not_ in the raycasting function
 
 // TODO: general
 // * very similarly - actual code for loading new data as the simulation progresses - right now its effectively a static image loader * pause button once that dataloading is implemented 
@@ -116,7 +116,7 @@ int main() {
 
   copyConstantsToDevice();
   // Create the GUI
-  Window window(IMAGE_WIDTH, IMAGE_HEIGHT);
+  Window window(INITIAL_WINDOW_WIDTH, INITIAL_WINDOW_HEIGHT);
   int out = window.init(d_volume);
 
   // memory management