diff --git a/Outputs/CRT/Internals/CRTOpenGL.cpp b/Outputs/CRT/Internals/CRTOpenGL.cpp
index cfe48fc67..19f24884e 100644
--- a/Outputs/CRT/Internals/CRTOpenGL.cpp
+++ b/Outputs/CRT/Internals/CRTOpenGL.cpp
@@ -238,14 +238,15 @@ void OpenGLOutputBuilder::draw_frame(unsigned int output_width, unsigned int out
 //	glTextureBarrierNV();
 #endif
 
-	// copy framebuffer to the intended place
+	// Copy framebuffer to the intended place; apply a threshold so that any persistent errors in
+	// the lower part of the colour channels are invisible.
 	glDisable(GL_BLEND);
 	glBindFramebuffer(GL_FRAMEBUFFER, static_cast<GLuint>(target_framebuffer_));
 	glViewport(0, 0, (GLsizei)output_width, (GLsizei)output_height);
 
 	glActiveTexture(pixel_accumulation_texture_unit);
 	framebuffer_->bind_texture();
-	framebuffer_->draw(static_cast<float>(output_width) / static_cast<float>(output_height));
+	framebuffer_->draw(static_cast<float>(output_width) / static_cast<float>(output_height), 4.0f / 255.0f);
 
 	fence_ = glFenceSync(GL_SYNC_GPU_COMMANDS_COMPLETE, 0);
 	draw_mutex_.unlock();
diff --git a/Outputs/CRT/Internals/TextureTarget.cpp b/Outputs/CRT/Internals/TextureTarget.cpp
index b9652d8eb..c2d6f8f57 100644
--- a/Outputs/CRT/Internals/TextureTarget.cpp
+++ b/Outputs/CRT/Internals/TextureTarget.cpp
@@ -56,7 +56,7 @@ void TextureTarget::bind_texture() {
 	glBindTexture(GL_TEXTURE_2D, texture_);
 }
 
-void TextureTarget::draw(float aspect_ratio) {
+void TextureTarget::draw(float aspect_ratio, float colour_threshold) {
 	if(!pixel_shader_) {
 		const char *vertex_shader =
 			"#version 150\n"
@@ -75,12 +75,15 @@ void TextureTarget::draw(float aspect_ratio) {
 			"#version 150\n"
 
 			"in vec2 texCoordVarying;"
+
 			"uniform sampler2D texID;"
+			"uniform float threshold;"
+
 			"out vec4 fragColour;"
 
 			"void main(void)"
 			"{"
-				"fragColour = texture(texID, texCoordVarying);"
+				"fragColour = clamp(texture(texID, texCoordVarying), threshold, 1.0);"
 			"}";
 		pixel_shader_.reset(new Shader(vertex_shader, fragment_shader));
 		pixel_shader_->bind();
@@ -103,6 +106,8 @@ void TextureTarget::draw(float aspect_ratio) {
 
 		GLint texIDUniform = pixel_shader_->get_uniform_location("texID");
 		glUniform1i(texIDUniform, static_cast<GLint>(texture_unit_ - GL_TEXTURE0));
+
+		threshold_uniform_ = pixel_shader_->get_uniform_location("threshold");
 	}
 
 	if(set_aspect_ratio_ != aspect_ratio) {
@@ -134,6 +139,8 @@ void TextureTarget::draw(float aspect_ratio) {
 	}
 
 	pixel_shader_->bind();
+	glUniform1f(threshold_uniform_, colour_threshold);
+
 	glBindVertexArray(drawing_vertex_array_);
 	glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
 }
diff --git a/Outputs/CRT/Internals/TextureTarget.hpp b/Outputs/CRT/Internals/TextureTarget.hpp
index 6c7b04b99..9a69efa47 100644
--- a/Outputs/CRT/Internals/TextureTarget.hpp
+++ b/Outputs/CRT/Internals/TextureTarget.hpp
@@ -58,9 +58,17 @@ class TextureTarget {
 		}
 
 		/*!
+			Draws this texture to the currently-bound framebuffer, which has the aspect ratio
+			@c aspect_ratio. This texture will fill the height of the frame buffer, and pick
+			an appropriate width based o the aspect ratio.
 
+			@c colour_threshold sets a threshold test that each colour must satisfy to be
+			output. A threshold of 0.0f means that all colours will pass through. A threshold
+			of 0.5f means that only colour components above 0.5f will pass through, with
+			0.5f being substituted elsewhere. This provides a way to ensure that the sort of
+			persistent low-value errors that can result from an IIR are hidden.
 		*/
-		void draw(float aspect_ratio);
+		void draw(float aspect_ratio, float colour_threshold = 0.0f);
 
 		enum {
 			ErrorFramebufferIncomplete
@@ -75,6 +83,8 @@ class TextureTarget {
 		std::unique_ptr<Shader> pixel_shader_;
 		GLuint drawing_vertex_array_ = 0, drawing_array_buffer_ = 0;
 		float set_aspect_ratio_ = 0.0f;
+
+		GLint threshold_uniform_;
 };
 
 }