fix(webp): Missing oiio:UnassociatedAlpha on input (AcademySoftwareFoundation#5020)

Like other file formats, the returned ImageSpec should indicate if the
image contains unassociated alpha.

This was an oversight in AcademySoftwareFoundation#4770.

Test added.

Signed-off-by: Brecht Van Lommel <[email protected]>

Author: brechtvl

src/webp.imageio/webpinput.cpp

@@ -214,8 +214,11 @@ 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)
+    if (config.get_int_attribute("oiio:UnassociatedAlpha", 0) == 1) {
         m_keep_unassociated_alpha = true;
+        if (m_spec.alpha_channel != -1)
+            m_spec.attribute("oiio:UnassociatedAlpha", 1);
+    }
 
     seek_subimage(0, 0);
     spec = m_spec;

testsuite/webp/ref/out-webp1.1.txt

@@ -18,3 +18,9 @@ Reading ../oiio-images/webp/4.webp
     SHA-1: 8F42E3DCCE6FE15146BA06C440C15B7831F60572
     channel list: R, G, B
     oiio:ColorSpace: "srgb_rec709_scene"
+Reading rgba.webp
+rgba.webp            :   64 x   64, 4 channel, uint8 webp
+    SHA-1: 897256B6709E1A4DA9DABA92B6BDE39CCFCCD8C1
+    channel list: R, G, B, A
+    oiio:ColorSpace: "srgb_rec709_scene"
+    oiio:UnassociatedAlpha: 1

testsuite/webp/run.py

@@ -11,3 +11,6 @@
     # a lossy format and is not stable under the round trip
     # command += rw_command (OIIO_TESTSUITE_IMAGEDIR, f,
     #                        extraargs='-attrib compression lossless')
+
+command += oiiotool ("--create 64x64 4 -o rgba.webp")
+command += info_command ("rgba.webp", "--iconfig oiio:UnassociatedAlpha 1", safematch=True)