Fix T64625: Eevee image textures with alpha have dark edges

Now texture storage of images is defined by the alpha mode of the image. The
downside of this is that there can be artifacts near alpha edges where pixels
with zero alpha bleed in. It also adds more code complexity since image textures
are no longer all stored the same way.

This changes allows us to keep using sRGB texture formats, which have edge
darkening when stored with premultiplied alpha. Game engines seems to generally
do the same thing, and we want to be compatible with them.

2 files changed, 38 insertions, 9 deletions

diff --git a/source/blender/nodes/shader/nodes/node_shader_tex_environment.c b/source/blender/nodes/shader/nodes/node_shader_tex_environment.c
index 615f55e4350..bd8355ec885 100644
--- a/source/blender/nodes/shader/nodes/node_shader_tex_environment.c
+++ b/source/blender/nodes/shader/nodes/node_shader_tex_environment.c
@@ -121,7 +121,20 @@ static int node_shader_gpu_tex_environment(GPUMaterial *mat,
   }
 
   if (out[0].hasoutput) {
-    GPU_link(mat, "tex_color_alpha_clear", out[0].link, &out[0].link);
+    if (ELEM(ima->alpha_mode, IMA_ALPHA_IGNORE, IMA_ALPHA_CHANNEL_PACKED) ||
+        IMB_colormanagement_space_name_is_data(ima->colorspace_settings.name)) {
+      /* Don't let alpha affect color output in these cases. */
+      GPU_link(mat, "tex_color_alpha_clear", out[0].link, &out[0].link);
+    }
+    else {
+      /* Always output with premultiplied alpha. */
+      if (ima->alpha_mode == IMA_ALPHA_PREMUL) {
+        GPU_link(mat, "tex_color_alpha_clear", out[0].link, &out[0].link);
+      }
+      else {
+        GPU_link(mat, "tex_color_alpha_premultiply", out[0].link, &out[0].link);
+      }
+    }
   }
 
   return true;
diff --git a/source/blender/nodes/shader/nodes/node_shader_tex_image.c b/source/blender/nodes/shader/nodes/node_shader_tex_image.c
index 786386bb63e..6f3614e357d 100644
--- a/source/blender/nodes/shader/nodes/node_shader_tex_image.c
+++ b/source/blender/nodes/shader/nodes/node_shader_tex_image.c
@@ -180,16 +180,32 @@ static int node_shader_gpu_tex_image(GPUMaterial *mat,
   }
 
   if (out[0].hasoutput) {
-    /* When the alpha socket is used, unpremultiply alpha. This makes it so
-     * that if we blend the color with a transparent shader using alpha as
-     * a factor, we don't multiply alpha into the color twice. */
-    if (out[1].hasoutput &&
-        !(ELEM(ima->alpha_mode, IMA_ALPHA_IGNORE, IMA_ALPHA_CHANNEL_PACKED) ||
-          IMB_colormanagement_space_name_is_data(ima->colorspace_settings.name))) {
-      GPU_link(mat, "tex_color_alpha_unpremultiply", out[0].link, &out[0].link);
+    if (ELEM(ima->alpha_mode, IMA_ALPHA_IGNORE, IMA_ALPHA_CHANNEL_PACKED) ||
+        IMB_colormanagement_space_name_is_data(ima->colorspace_settings.name)) {
+      /* Don't let alpha affect color output in these cases. */
+      GPU_link(mat, "tex_color_alpha_clear", out[0].link, &out[0].link);
     }
     else {
-      GPU_link(mat, "tex_color_alpha_clear", out[0].link, &out[0].link);
+      /* Output premultiplied alpha depending on alpha socket usage. This makes
+       * it so that if we blend the color with a transparent shader using alpha as
+       * a factor, we don't multiply alpha into the color twice. And if we do
+       * not, then there will be no artifacts from zero alpha areas. */
+      if (ima->alpha_mode == IMA_ALPHA_PREMUL) {
+        if (out[1].hasoutput) {
+          GPU_link(mat, "tex_color_alpha_unpremultiply", out[0].link, &out[0].link);
+        }
+        else {
+          GPU_link(mat, "tex_color_alpha_clear", out[0].link, &out[0].link);
+        }
+      }
+      else {
+        if (out[1].hasoutput) {
+          GPU_link(mat, "tex_color_alpha_clear", out[0].link, &out[0].link);
+        }
+        else {
+          GPU_link(mat, "tex_color_alpha_premultiply", out[0].link, &out[0].link);
+        }
+      }
     }
   }