diff --git a/gtk/src/gtk_display_driver_opengl.cpp b/gtk/src/gtk_display_driver_opengl.cpp
index 8aea0bb9..276f77e0 100644
--- a/gtk/src/gtk_display_driver_opengl.cpp
+++ b/gtk/src/gtk_display_driver_opengl.cpp
@@ -14,6 +14,21 @@
 #include "shaders/shader_helpers.h"
 #include "shaders/CGLCG.h"
 
+static void S9xViewportCallback (int src_width, int src_height,
+                                 int viewport_x, int viewport_y,
+                                 int viewport_width, int viewport_height,
+                                 int *out_x, int *out_y,
+                                 int *out_width, int *out_height)
+{
+
+    S9xApplyAspect (src_width, src_height, viewport_width, viewport_height);
+    *out_x = src_width + viewport_x;
+    *out_y = src_height + viewport_y;
+    *out_width = viewport_width;
+    *out_height = viewport_height;
+    return;
+}
+
 S9xOpenGLDisplayDriver::S9xOpenGLDisplayDriver (Snes9xWindow *window,
                                                 Snes9xConfig *config)
 {
@@ -226,7 +241,7 @@ S9xOpenGLDisplayDriver::update (int width, int height, int yoffset)
 
     if (using_shaders && using_glsl_shaders)
     {
-        glsl_shader->render (texmap, width, height, w, h, x, allocation.height - y - h);
+        glsl_shader->render (texmap, width, height, x, allocation.height - y - h, w, h, S9xViewportCallback);
         gl_swap ();
         return;
     }
diff --git a/shaders/glsl.cpp b/shaders/glsl.cpp
index b4dc228f..110a43b2 100644
--- a/shaders/glsl.cpp
+++ b/shaders/glsl.cpp
@@ -552,7 +552,11 @@ bool GLSLShader::load_shader (char *filename)
     return true;
 }
 
-void GLSLShader::render(GLuint &orig, int width, int height, int viewport_width, int viewport_height, int viewport_x, int viewport_y)
+void GLSLShader::render(GLuint &orig,
+                        int width, int height,
+                        int viewport_x, int viewport_y,
+                        int viewport_width, int viewport_height,
+                        GLSLViewportCallback vpcallback)
 {
     frame_count++;
 
@@ -641,9 +645,19 @@ void GLSLShader::render(GLuint &orig, int width, int height, int viewport_width,
         }
         else
         {
+            int out_x = 0;
+            int out_y = 0;
+            int out_width = 0;
+            int out_height = 0;
+
             // output to the screen
             glBindFramebuffer(GL_FRAMEBUFFER, 0);
-            glViewport(viewport_x, viewport_y, viewport_width, viewport_height);
+            vpcallback (pass[i].width, pass[i].height,
+                        viewport_x, viewport_y,
+                        viewport_width, viewport_height,
+                        &out_x, &out_y,
+                        &out_width, &out_height);
+            glViewport(out_x, out_y, out_width, out_height);
         }
 
         // set up input texture (output of previous pass) and apply filter settings
diff --git a/shaders/glsl.h b/shaders/glsl.h
index 9f7a271e..3e5c22ff 100644
--- a/shaders/glsl.h
+++ b/shaders/glsl.h
@@ -6,6 +6,12 @@
 
 static const unsigned int glsl_max_passes = 20;
 
+typedef void (* GLSLViewportCallback) (int source_width,   int source_height,
+                                       int viewport_x,     int viewport_y,
+                                       int viewport_width, int viewport_height,
+                                       int *out_dst_x,     int *out_dst_y,
+                                       int *out_dst_width, int *out_dst_height);
+
 enum GLSLScaleType
 {
     GLSL_NONE = 0,
@@ -107,7 +113,7 @@ typedef struct
 {
     bool load_shader (char *filename);
     bool load_shader_file (char *filename);
-    void render (GLuint &orig, int width, int height, int viewport_width, int viewport_height, int viewport_x, int viewport_y);
+    void render (GLuint &orig, int width, int height, int viewport_width, int viewport_height, int viewport_x, int viewport_y, GLSLViewportCallback vpcallback);
     void set_shader_vars (unsigned int pass);
     void clear_shader_vars (void);
     void strip_parameter_pragmas(char *buffer);