pixel-formats: Add hsub and vsub helpers

We already had these with effective width and height, but they're useful
externally as well. Pull them out to a helper.

Signed-off-by: Daniel Stone <daniels@collabora.com>

libweston/pixel-formats.c

@@ -654,16 +654,34 @@ pixel_format_get_info_by_opaque_substitute(uint32_t format)
 	return NULL;
 }
 
+WL_EXPORT unsigned int
+pixel_format_hsub(const struct pixel_format_info *info,
+		  unsigned int plane)
+{
+	/* We don't support any formats where the first plane is subsampled. */
+	if (plane == 0 || info->hsub == 0)
+		return 1;
+
+	return info->hsub;
+}
+
+WL_EXPORT unsigned int
+pixel_format_vsub(const struct pixel_format_info *info,
+		  unsigned int plane)
+{
+	/* We don't support any formats where the first plane is subsampled. */
+	if (plane == 0 || info->vsub == 0)
+		return 1;
+
+	return info->vsub;
+}
+
 WL_EXPORT unsigned int
 pixel_format_width_for_plane(const struct pixel_format_info *info,
 			     unsigned int plane,
 			     unsigned int width)
 {
-	/* We don't support any formats where the first plane is subsampled. */
-	if (plane == 0 || !info->hsub)
-		return width;
-
-	return width / info->hsub;
+	return width / pixel_format_hsub(info, plane);
 }
 
 WL_EXPORT unsigned int
@@ -671,11 +689,7 @@ pixel_format_height_for_plane(const struct pixel_format_info *info,
 			      unsigned int plane,
 			      unsigned int height)
 {
-	/* We don't support any formats where the first plane is subsampled. */
-	if (plane == 0 || !info->vsub)
-		return height;
-
-	return height / info->vsub;
+	return height / pixel_format_vsub(info, plane);
 }
 
 #ifdef HAVE_HUMAN_FORMAT_MODIFIER

libweston/pixel-formats.h

@@ -248,6 +248,36 @@ pixel_format_get_opaque_substitute(const struct pixel_format_info *format);
 const struct pixel_format_info *
 pixel_format_get_info_by_opaque_substitute(uint32_t format);
 
+/**
+ * Return the horizontal subsampling factor for a given plane
+ *
+ * When horizontal subsampling is effective, a sampler bound to a secondary
+ * plane must bind the sampler with a smaller effective width. This function
+ * returns the subsampling factor to use for the given plane.
+ *
+ * @param format Pixel format info structure
+ * @param plane Zero-indexed plane number
+ * @returns Horizontal subsampling factor for the given plane
+ */
+unsigned int
+pixel_format_hsub(const struct pixel_format_info *format,
+		  unsigned int plane);
+
+/**
+ * Return the vertical subsampling factor for a given plane
+ *
+ * When vertical subsampling is effective, a sampler bound to a secondary
+ * plane must bind the sampler with a smaller effective height. This function
+ * returns the subsampling factor to use for the given plane.
+ *
+ * @param format Pixel format info structure
+ * @param plane Zero-indexed plane number
+ * @returns Vertical subsampling factor for the given plane
+ */
+unsigned int
+pixel_format_vsub(const struct pixel_format_info *format,
+		  unsigned int plane);
+
 /**
  * Return the effective sampling width for a given plane
  *