feat(webp): Respect the oiio:UnassociatedAlpha attribute

src/doc/builtinplugins.rst

@@ -3066,6 +3066,11 @@ attributes are supported:
      - ptr
      - Pointer to a ``Filesystem::IOProxy`` that will handle the I/O, for
        example by reading from memory rather than the file system.
+   * - ``oiio:UnassociatedAlpha``
+     - int
+     - If nonzero, will leave alpha unassociated (versus the default of
+       premultiplying color channels by alpha if the alpha channel is
+       unassociated).
 
 **Configuration settings for WebP output**
 
@@ -3088,6 +3093,12 @@ control aspects of the writing itself:
      - ptr
      - Pointer to a ``Filesystem::IOProxy`` that will handle the I/O, for
        example by writing to a memory buffer.
+   * - ``oiio:UnassociatedAlpha``
+     - int
+     - If nonzero, indicates that the data being passed is already in
+       unassociated form (non-premultiplied colors) and should stay that way
+       for output rather than being assumed to be associated and get automatic
+       un-association to store in the file.
 
 **Custom I/O Overrides**
 

src/webp.imageio/webpinput.cpp

@@ -44,8 +44,9 @@ class WebpInput final : public ImageInput {
     int m_frame_count        = 1;
     WebPDemuxer* m_demux     = nullptr;
     WebPIterator m_iter;
-    int m_subimage      = -1;  // Subimage we're pointed to
-    int m_subimage_read = -1;  // Subimage stored in decoded_image
+    int m_subimage      = -1;                // Subimage we're pointed to
+    int m_subimage_read = -1;                // Subimage stored in decoded_image
+    bool m_keep_unassociated_alpha = false;  // Do not convert unassociated alpha
 
     void init(void)
     {
@@ -204,6 +205,9 @@ WebpInput::open(const std::string& name, ImageSpec& spec,
     // Make space for the decoded image
     m_decoded_image.reset(new uint8_t[m_spec.image_bytes()]);
 
+    if (config.get_int_attribute("oiio:UnassociatedAlpha", 0) == 1)
+        m_keep_unassociated_alpha = true;
+
     seek_subimage(0, 0);
     spec = m_spec;
     return true;
@@ -306,12 +310,15 @@ WebpInput::read_current_subimage()
                                        m_decoded_image.get() + offset,
                                        m_spec.image_bytes() - offset,
                                        m_spec.scanline_bytes());
-            // WebP requires unassociated alpha, and it's sRGB.
-            // Handle this all by wrapping an IB around it.
-            ImageBuf fullbuf(m_spec,
-                             span<std::byte>((std::byte*)m_decoded_image.get(),
-                                             m_spec.image_bytes()));
-            ImageBufAlgo::premult(fullbuf, fullbuf);
+
+            // WebP is unassociated alpha and sRGB.
+            // Convert to the OIIO-native associated form if required.
+            if (!m_keep_unassociated_alpha) {
+                ImageBuf fullbuf(
+                    m_spec, span<std::byte>((std::byte*)m_decoded_image.get(),
+                                            m_spec.image_bytes()));
+                ImageBufAlgo::premult(fullbuf, fullbuf);
+            }
         }
     } else {
         // This subimage writes *atop* the prior image, we must composite
@@ -328,10 +335,14 @@ WebpInput::read_current_subimage()
                                    (uint8_t*)fragbuf.localpixels(),
                                    fragspec.image_bytes(),
                                    fragspec.scanline_bytes());
-        // WebP requires unassociated alpha, and it's sRGB.
-        // Handle this all by wrapping an IB around it.
-        ImageBufAlgo::premult(fragbuf, fragbuf);
-        ImageBufAlgo::over(fullbuf, fragbuf, fullbuf);
+
+
+        // WebP is unassociated alpha and sRGB.
+        // Convert to the OIIO-native associated form if required.
+        if (!m_keep_unassociated_alpha) {
+            ImageBufAlgo::premult(fragbuf, fragbuf);
+            ImageBufAlgo::over(fullbuf, fragbuf, fullbuf);
+        }
     }
 
     if (!okptr) {
@@ -371,7 +382,8 @@ WebpInput::close()
     }
     m_decoded_image.reset();
     m_encoded_image.reset();
-    m_subimage = -1;
+    m_subimage                = -1;
+    m_keep_unassociated_alpha = false;
     init();
     return true;
 }

src/webp.imageio/webpoutput.cpp

@@ -36,6 +36,7 @@ class WebpOutput final : public ImageOutput {
     std::string m_filename;
     imagesize_t m_scanline_size;
     unsigned int m_dither;
+    bool m_convert_alpha;  // Do we deassociate alpha?
     std::vector<uint8_t> m_uncompressed_image;
 
     void init()
@@ -113,7 +114,9 @@ WebpOutput::open(const std::string& name, const ImageSpec& spec, OpenMode mode)
 
     // forcing UINT8 format
     m_spec.set_format(TypeDesc::UINT8);
-    m_dither = m_spec.get_int_attribute("oiio:dither", 0);
+    m_dither        = m_spec.get_int_attribute("oiio:dither", 0);
+    m_convert_alpha = m_spec.alpha_channel != -1
+                      && !m_spec.get_int_attribute("oiio:UnassociatedAlpha", 0);
 
     m_scanline_size = m_spec.scanline_bytes();
     m_uncompressed_image.resize(m_spec.image_bytes(), 0);
@@ -136,20 +139,25 @@ WebpOutput::write_scanline(int y, int z, TypeDesc format, const void* data,
 
     if (y == m_spec.height - 1) {
         if (m_spec.nchannels == 4) {
-            // WebP requires unassociated alpha, and it's sRGB.
-            // Handle this all by wrapping an IB around it.
-            ImageSpec specwrap(m_spec.width, m_spec.height, 4, TypeUInt8);
-            ImageBuf bufwrap(specwrap, cspan<uint8_t>(m_uncompressed_image));
-            ROI rgbroi(0, m_spec.width, 0, m_spec.height, 0, 1, 0, 3);
-            ImageBufAlgo::pow(bufwrap, bufwrap, 2.2f, rgbroi);
-            ImageBufAlgo::unpremult(bufwrap, bufwrap);
-            ImageBufAlgo::pow(bufwrap, bufwrap, 1.0f / 2.2f, rgbroi);
+            if (m_convert_alpha) {
+                // WebP requires unassociated alpha, and it's sRGB.
+                // Handle this all by wrapping an IB around it.
+                ImageSpec specwrap(m_spec.width, m_spec.height, 4, TypeUInt8);
+                ImageBuf bufwrap(specwrap,
+                                 cspan<uint8_t>(m_uncompressed_image));
+                ROI rgbroi(0, m_spec.width, 0, m_spec.height, 0, 1, 0, 3);
+                ImageBufAlgo::pow(bufwrap, bufwrap, 2.2f, rgbroi);
+                ImageBufAlgo::unpremult(bufwrap, bufwrap);
+                ImageBufAlgo::pow(bufwrap, bufwrap, 1.0f / 2.2f, rgbroi);
+            }
+
             WebPPictureImportRGBA(&m_webp_picture, m_uncompressed_image.data(),
                                   m_scanline_size);
         } else {
             WebPPictureImportRGB(&m_webp_picture, m_uncompressed_image.data(),
                                  m_scanline_size);
         }
+
         if (!WebPEncode(&m_webp_config, &m_webp_picture)) {
             errorfmt("Failed to encode {} as WebP image", m_filename);
             close();