backend: add and implement get_shader_attributes

Used for passing back information about whether a shader needs to be
re-rendered every frame.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/backend.h

@@ -80,6 +80,12 @@ enum image_operations {
 	IMAGE_OP_APPLY_ALPHA,
 };
 
+enum shader_attributes {
+	// Whether the shader needs to be render regardless of whether the window is
+	// updated.
+	SHADER_ATTRIBUTE_ANIMATED = 1,
+};
+
 struct gaussian_blur_args {
 	int size;
 	double deviation;
@@ -203,13 +209,23 @@ struct backend_operations {
 	void (*release_image)(backend_t *backend_data, void *img_data) attr_nonnull(1, 2);
 
 	/// Create a shader object from a shader source.
+	///
+	/// Optional
 	void *(*create_shader)(backend_t *backend_data, const char *source)attr_nonnull(1, 2);
 
 	/// Free a shader object.
+	///
+	/// Required if create_shader is present.
 	void (*destroy_shader)(backend_t *backend_data, void *shader) attr_nonnull(1, 2);
 
 	// ===========        Query         ===========
 
+	/// Get the attributes of a shader.
+	///
+	/// Optional, Returns a bitmask of attributes, see `shader_attributes`.
+	uint64_t (*get_shader_attributes)(backend_t *backend_data, void *shader)
+	    attr_nonnull(1, 2);
+
 	/// Return if image is not completely opaque.
 	///
 	/// This function is needed because some backend might change the content of the

src/backend/gl/gl_common.c

@@ -1682,6 +1682,15 @@ void *gl_create_window_shader(backend_t *backend_data attr_unused, const char *s
 	return win_shader;
 }
 
+uint64_t gl_get_shader_attributes(backend_t *backend_data attr_unused, void *shader) {
+	auto win_shader = (gl_win_shader_t *)shader;
+	uint64_t ret = 0;
+	if (glGetUniformLocation(win_shader->prog, "time") >= 0) {
+		ret |= SHADER_ATTRIBUTE_ANIMATED;
+	}
+	return ret;
+}
+
 bool gl_init(struct gl_data *gd, session_t *ps) {
 	// Initialize GLX data structure
 	glDisable(GL_DEPTH_TEST);

src/backend/gl/gl_common.h

@@ -110,6 +110,7 @@ GLuint gl_create_program(const GLuint *const shaders, int nshaders);
 GLuint gl_create_program_from_str(const char *vert_shader_str, const char *frag_shader_str);
 void *gl_create_window_shader(backend_t *backend_data, const char *source);
 void gl_destroy_window_shader(backend_t *backend_data, void *shader);
+uint64_t gl_get_shader_attributes(backend_t *backend_data, void *shader);
 bool gl_set_image_property(backend_t *backend_data, enum image_properties prop,
                            void *image_data, void *args);
 

src/backend/gl/glx.c

@@ -549,6 +549,7 @@ struct backend_operations glx_ops = {
     .device_status = gl_device_status,
     .create_shader = gl_create_window_shader,
     .destroy_shader = gl_destroy_window_shader,
+    .get_shader_attributes = gl_get_shader_attributes,
     .max_buffer_age = 5,        // Why?
 };
 

src/common.h

@@ -134,6 +134,7 @@ struct shader_info {
 	char *key;
 	char *source;
 	void *backend_shader;
+	uint64_t attributes;
 	UT_hash_handle hh;
 };
 

src/picom.c

@@ -519,6 +519,14 @@ static bool initialize_backend(session_t *ps) {
 				log_warn("Failed to create shader for shader file %s, "
 				         "this shader will not be used",
 				         shader->key);
+			} else {
+				if (ps->backend_data->ops->get_shader_attributes) {
+					shader->attributes =
+					    ps->backend_data->ops->get_shader_attributes(
+					        ps->backend_data, shader->backend_shader);
+				} else {
+					shader->attributes = 0;
+				}
 			}
 		}