diff --git a/Outputs/CRT/CRT.cpp b/Outputs/CRT/CRT.cpp
index 4db4d30e9..4bd136496 100644
--- a/Outputs/CRT/CRT.cpp
+++ b/Outputs/CRT/CRT.cpp
@@ -62,6 +62,8 @@ void CRT::set_new_timing(int cycles_per_line, int height_of_display, Outputs::Di
 	scan_target_modals_.output_scale.y = uint16_t(real_clock_scan_period / vertical_flywheel_output_divider_);
 	scan_target_modals_.expected_vertical_lines = height_of_display;
 	scan_target_modals_.composite_colour_space = colour_space;
+	scan_target_modals_.colour_cycle_numerator = colour_cycle_numerator;
+	scan_target_modals_.colour_cycle_denominator = colour_cycle_denominator;
 	scan_target_->set_modals(scan_target_modals_);
 }
 
diff --git a/Outputs/OpenGL/ScanTarget.cpp b/Outputs/OpenGL/ScanTarget.cpp
index 97a37e5f0..49d0bb93b 100644
--- a/Outputs/OpenGL/ScanTarget.cpp
+++ b/Outputs/OpenGL/ScanTarget.cpp
@@ -131,6 +131,15 @@ void ScanTarget::set_modals(Modals modals) {
 		write_pointers_.write_area = 0;
 	}
 
+	// Pick a processing width; this will be at least four times the
+	// colour subcarrier, and an integer multiple of the pixel clock and
+	// at most 2048.
+	const int colour_cycle_width = (modals.colour_cycle_numerator * 4 + modals.colour_cycle_denominator - 1) / modals.colour_cycle_denominator;
+	const int dot_clock = modals.cycles_per_line / modals.clocks_per_pixel_greatest_common_divisor;
+	const int overflow = colour_cycle_width % dot_clock;
+	processing_width_ = colour_cycle_width + (overflow ? dot_clock - overflow : 0);
+	processing_width_ = std::min(processing_width_, 2048);
+
 	// TODO: this, but not to the test shader.
 	test_shader_->set_uniform("scale", GLfloat(modals.output_scale.x), GLfloat(modals.output_scale.y));
 	test_shader_->set_uniform("rowHeight", GLfloat(1.0f / modals.expected_vertical_lines));
diff --git a/Outputs/OpenGL/ScanTarget.hpp b/Outputs/OpenGL/ScanTarget.hpp
index 578c0e71f..e4c6382a9 100644
--- a/Outputs/OpenGL/ScanTarget.hpp
+++ b/Outputs/OpenGL/ScanTarget.hpp
@@ -129,9 +129,11 @@ class ScanTarget: public Outputs::Display::ScanTarget {
 		*/
 		void enable_vertex_attributes(ShaderType type, Shader &target);
 
-		std::unique_ptr<Shader> test_shader_;
 		GLsync fence_ = nullptr;
 		std::atomic_flag is_drawing_;
+
+		int processing_width_ = 0;
+		std::unique_ptr<Shader> test_shader_;
 };
 
 }
diff --git a/Outputs/ScanTarget.hpp b/Outputs/ScanTarget.hpp
index 29fcf28a2..4f880adc5 100644
--- a/Outputs/ScanTarget.hpp
+++ b/Outputs/ScanTarget.hpp
@@ -129,6 +129,9 @@ struct ScanTarget {
 			/// the @c cycles_per_line are offered for sizing of intermediary buffers.
 			int clocks_per_pixel_greatest_common_divisor;
 
+			/// Provides the number of colour cycles in a line, as a quotient.
+			int colour_cycle_numerator, colour_cycle_denominator;
+
 			/// Provides a pre-estimate of the likely number of left-to-right scans per frame.
 			/// This isn't a guarantee, but it should provide a decent-enough estimate.
 			int expected_vertical_lines;