[librsvg: 10/14] Optimize srgb::* for alpha_only

[Federico Mena Quintero <federico src gnome org>]

commit 5b637bee0ff5ba9c0757786d69751b426ee259bb
Author: Ivan Molodetskikh <yalterz gmail com>
Date:   Sun Jul 15 14:19:23 2018 +0300

    Optimize srgb::* for alpha_only

 rsvg_internals/src/filters/context.rs |  8 +-------
 rsvg_internals/src/srgb.rs            | 19 +++++++++----------
 2 files changed, 10 insertions(+), 17 deletions(-)
---
diff --git a/rsvg_internals/src/filters/context.rs b/rsvg_internals/src/filters/context.rs
index a41216fc..14d385f1 100644
--- a/rsvg_internals/src/filters/context.rs
+++ b/rsvg_internals/src/filters/context.rs
@@ -407,10 +407,7 @@ impl FilterContext {
         if self.processing_linear_rgb {
             result.output.surface =
                 unlinearize_surface(&result.output.surface, result.output.bounds)
-                    .map_err(FilterError::CairoError)
-                    .and_then(|surface| {
-                        SharedImageSurface::new(surface).map_err(FilterError::CairoError)
-                    })?;
+                    .map_err(FilterError::CairoError)?;
         }
 
         if let Some(name) = result.name {
@@ -575,9 +572,6 @@ impl FilterContext {
 
             linearize_surface(surface, bounds)
                 .map_err(FilterError::CairoError)
-                .and_then(|surface| {
-                    SharedImageSurface::new(surface).map_err(FilterError::CairoError)
-                })
                 .map(|surface| match raw {
                     FilterInput::StandardInput(_) => FilterInput::StandardInput(surface),
                     FilterInput::PrimitiveOutput(ref output) => {
diff --git a/rsvg_internals/src/srgb.rs b/rsvg_internals/src/srgb.rs
index 136cafd7..0726314a 100644
--- a/rsvg_internals/src/srgb.rs
+++ b/rsvg_internals/src/srgb.rs
@@ -36,16 +36,19 @@ pub fn unlinearize(c: f64) -> f64 {
 }
 
 /// Applies the function to each pixel component after unpremultiplying.
-///
-/// The returned surface is transparent everywhere except the rectangle defined by `bounds`.
 fn map_unpremultiplied_components<F>(
     surface: &SharedImageSurface,
     bounds: IRect,
     f: F,
-) -> Result<cairo::ImageSurface, cairo::Status>
+) -> Result<SharedImageSurface, cairo::Status>
 where
     F: Fn(f64) -> f64,
 {
+    // This function doesn't affect the alpha channel.
+    if surface.is_alpha_only() {
+        return Ok(surface.clone());
+    }
+
     let width = surface.width();
     let height = surface.height();
 
@@ -77,27 +80,23 @@ where
         }
     }
 
-    Ok(output_surface)
+    SharedImageSurface::new(output_surface)
 }
 
 /// Converts an sRGB surface to a linear sRGB surface (undoes the gamma correction).
-///
-/// The returned surface is transparent everywhere except the rectangle defined by `bounds`.
 #[inline]
 pub fn linearize_surface(
     surface: &SharedImageSurface,
     bounds: IRect,
-) -> Result<cairo::ImageSurface, cairo::Status> {
+) -> Result<SharedImageSurface, cairo::Status> {
     map_unpremultiplied_components(surface, bounds, linearize)
 }
 
 /// Converts a linear sRGB surface to a normal sRGB surface (applies the gamma correction).
-///
-/// The returned surface is transparent everywhere except the rectangle defined by `bounds`.
 #[inline]
 pub fn unlinearize_surface(
     surface: &SharedImageSurface,
     bounds: IRect,
-) -> Result<cairo::ImageSurface, cairo::Status> {
+) -> Result<SharedImageSurface, cairo::Status> {
     map_unpremultiplied_components(surface, bounds, unlinearize)
 }