Adds a right gutter to clip persistence errors.

Also uncovers and corrects a long-standing centring error.

Outputs/CRT/Internals/CRTOpenGL.cpp

@@ -227,11 +227,18 @@ void OpenGLOutputBuilder::draw_frame(unsigned int output_width, unsigned int out
 			output_shader_program_->set_output_size(output_width, output_height, visible_area_);
 			last_output_width_ = output_width;
 			last_output_height_ = output_height;
+
+			// Configure a right gutter to crop the right-hand 2% of the display.
+			right_overlay_.reset(new OpenGL::Rectangle(output_shader_program_->get_right_extent() * 0.98, -1.0, 1.0, 2.0));
 		}
 		output_shader_program_->bind();
 
 		// draw
 		glDrawArraysInstanced(GL_TRIANGLE_STRIP, 0, 4, (GLsizei)array_submission.output_size / OutputVertexSize);
+
+		// mask off the gutter
+		glDisable(GL_BLEND);
+		right_overlay_->draw(0.0, 0.0, 0.0);
 	}
 
 #ifdef GL_NV_texture_barrier

Outputs/CRT/Internals/CRTOpenGL.hpp

@@ -20,6 +20,7 @@
 
 #include "Shaders/OutputShader.hpp"
 #include "Shaders/IntermediateShader.hpp"
+#include "Rectangle.hpp"
 
 #include <mutex>
 #include <vector>
@@ -106,6 +107,18 @@ class OpenGLOutputBuilder {
 
 		float integer_coordinate_multiplier_ = 1.0f;
 
+		// Maintain a couple of rectangles for masking off the extreme edge of the display;
+		// this is a bit of a cheat: there's some tolerance in when a sync pulse will be
+		// generated. So it might be slightly later than expected. Which might cause a scan
+		// that is slightly longer than expected. Which means that from then on, those scans
+		// might have touched parts of the extreme edge of the display which are not rescanned.
+		// Which because I've implemented persistence-of-vision as an in-buffer effect will
+		// cause perpetual persistence.
+		//
+		// The fix: just always treat that area as invisible. This is acceptable thanks to
+		// the concept of overscan. One is allowed not to display extreme ends of the image.
+		std::unique_ptr<OpenGL::Rectangle> right_overlay_;
+
 	public:
 		// These two are protected by output_mutex_.
 		TextureBuilder texture_builder;

Outputs/CRT/Internals/Shaders/OutputShader.cpp

@@ -94,15 +94,21 @@ std::unique_ptr<OutputShader> OutputShader::make_shader(const char *fragment_met
 
 void OutputShader::set_output_size(unsigned int output_width, unsigned int output_height, Outputs::CRT::Rect visible_area) {
 	GLfloat outputAspectRatioMultiplier = (static_cast<float>(output_width) / static_cast<float>(output_height)) / (4.0f / 3.0f);
-
 	GLfloat bonusWidth = (outputAspectRatioMultiplier - 1.0f) * visible_area.size.width;
-	visible_area.origin.x -= bonusWidth * 0.5f * visible_area.size.width;
+
+	right_extent_ = (1.0f / outputAspectRatioMultiplier) / visible_area.size.width;
+
+	visible_area.origin.x -= bonusWidth * 0.5f;
 	visible_area.size.width *= outputAspectRatioMultiplier;
 
 	set_uniform("boundsOrigin", (GLfloat)visible_area.origin.x, (GLfloat)visible_area.origin.y);
 	set_uniform("boundsSize", (GLfloat)visible_area.size.width, (GLfloat)visible_area.size.height);
 }
 
+float OutputShader::get_right_extent() {
+	return right_extent_;
+}
+
 void OutputShader::set_source_texture_unit(GLenum unit) {
 	set_uniform("texID", (GLint)(unit - GL_TEXTURE0));
 }

Outputs/CRT/Internals/Shaders/OutputShader.hpp

@@ -87,6 +87,14 @@ public:
 		space, 0.5 means use half, etc.
 	*/
 	void set_input_width_scaler(float input_scaler);
+
+	/*!
+		@returns The location, in eye coordinates, of the right edge of the output area.
+	*/
+	float get_right_extent();
+
+private:
+	float right_extent_ = 0.0f;
 };
 
 }