Use double precision

frag_shader.glsl

@@ -4,15 +4,16 @@
  */
 
 #version 450
+#extension GL_ARB_gpu_shader_fp64 : enable
 
 #define MAXITER 1000
 
 layout(location = 0) out vec4 out_colour;
 
 layout(push_constant) uniform Constants {
-	vec2 img_size;
-	vec2 shift;
-	float zoom;
+	dvec2 img_size;
+	dvec2 shift;
+	double zoom;
 } params;
 
 vec2 ss_offsets[4] = vec2[](
@@ -22,28 +23,31 @@ vec2 ss_offsets[4] = vec2[](
 	vec2( 0.25,  0.25)
 );
 
-vec2 maptoz(vec2 xy)
+dvec2 maptoz(vec2 xy)
 {
-	return params.zoom * ((xy / params.img_size.xy) * 4.0 - 2.0) + params.shift;
+	dvec2 dxy = dvec2(xy);
+	dvec2 uv = ((dxy / params.img_size.xy) * 4.0LF - 2.0LF);
+	return params.zoom * uv + params.shift;
 }
 
-float mandelbrot(vec2 c) {
-	vec2 z = vec2(0.0, 0.0);
+double mandelbrot(dvec2 c) {
+	dvec2 z = dvec2(0.0LF, 0.0LF);
 	for (int i = 0; i < MAXITER; ++i) {
-		z = vec2(z.x * z.x - z.y * z.y, 2.0 * z.x * z.y) + c;
-		if (length(z) > 2.0)
-			return float(i) / float(MAXITER);
+		z = dvec2(z.x * z.x - z.y * z.y, 2.0LF * z.x * z.y) + c;
+		if (length(z) > 2.0LF)
+			return double(i) / double(MAXITER);
 	}
 	return 0.0;
 }
 
-vec3 colour(float i) {
-	return vec3(i, i * i, i * i * i);
+vec3 colour(double i) {
+	float fi = float(i);
+	return vec3(fi, fi * fi, fi * fi * fi);
 }
 
 void main()
 {
-	vec2 c;
+	dvec2 c;
 	vec3 col = vec3(0.0, 0.0, 0.0);
 	for (int i = 0; i < 4; ++i) {
 		c = maptoz(gl_FragCoord.xy + ss_offsets[i]);

main.c

@@ -24,11 +24,11 @@
 #define MAX_SHADER_SIZE (4 * 1024)
 
 typedef struct {
-	float width, height;
+	double width, height;
 	struct {
-		float x, y;
+		double x, y;
 	} shift;
-	float zoom;
+	double zoom;
 } params_t;
 
 static const char *layers[] = { "VK_LAYER_KHRONOS_validation" };
@@ -101,6 +101,11 @@ int main(void)
 	result = vkEnumeratePhysicalDevices(inst, &dev_count, &physical_dev);
 	assert(result == VK_SUCCESS || result == VK_INCOMPLETE);
 
+	// Check that the device supports double precision
+	VkPhysicalDeviceFeatures features;
+	vkGetPhysicalDeviceFeatures(physical_dev, &features);
+	assert(features.shaderFloat64);
+
 	// Select queue family
 	uint32_t queue_family_count = QUEUE_FAMILY_PROP_BUFFER_SIZE;
 	VkQueueFamilyProperties
@@ -131,12 +136,16 @@ int main(void)
 		.queueCount = 1,
 		.pQueuePriorities = &queue_priority,
 	};
+	const VkPhysicalDeviceFeatures enabled_features = {
+		.shaderFloat64 = VK_TRUE,
+	};
 	const VkDeviceCreateInfo dev_create_info = {
 		.sType = VK_STRUCTURE_TYPE_DEVICE_CREATE_INFO,
 		.pQueueCreateInfos = &queue_create_info,
 		.queueCreateInfoCount = 1,
 		.enabledExtensionCount = 1,
 		.ppEnabledExtensionNames = &swapchain_ext_name,
+		.pEnabledFeatures = &enabled_features,
 	};
 	VkDevice dev;
 	result = vkCreateDevice(physical_dev, &dev_create_info, NULL, &dev);
@@ -462,8 +471,8 @@ int main(void)
 	assert(result == VK_SUCCESS);
 
 	params_t params = {
-		.width = (float)swapchain_extent.width,
-		.height = (float)swapchain_extent.height,
+		.width = (double)swapchain_extent.width,
+		.height = (double)swapchain_extent.height,
 		.shift = {
 			.x = -0.743643887037158704752191506114774,
 			.y = 0.131825904205311970493132056385139,