Fix T78956: banding artifacts of vertex colors in Cycles

Byte colors must be encoded in sRGB and converted to linear on lookup,
to avoid precision loss.

4 files changed, 25 insertions, 18 deletions

diff --git a/intern/cycles/blender/blender_mesh.cpp b/intern/cycles/blender/blender_mesh.cpp
index e40e1f5f001..b78a2c30b5e 100644
--- a/intern/cycles/blender/blender_mesh.cpp
+++ b/intern/cycles/blender/blender_mesh.cpp
@@ -346,7 +346,7 @@ static void attr_create_vertex_color(Scene *scene, Mesh *mesh, BL::Mesh &b_mesh,
         for (int i = 0; i < n; i++) {
           float4 color = get_float4(l->data[p->loop_start() + i].color());
           /* Compress/encode vertex color using the sRGB curve. */
-          *(cdata++) = color_float4_to_uchar4(color_srgb_to_linear_v4(color));
+          *(cdata++) = color_float4_to_uchar4(color);
         }
       }
     }
@@ -368,9 +368,10 @@ static void attr_create_vertex_color(Scene *scene, Mesh *mesh, BL::Mesh &b_mesh,
         float4 c3 = get_float4(l->data[li[2]].color());
 
         /* Compress/encode vertex color using the sRGB curve. */
-        cdata[0] = color_float4_to_uchar4(color_srgb_to_linear_v4(c1));
-        cdata[1] = color_float4_to_uchar4(color_srgb_to_linear_v4(c2));
-        cdata[2] = color_float4_to_uchar4(color_srgb_to_linear_v4(c3));
+        cdata[0] = color_float4_to_uchar4(c1);
+        cdata[1] = color_float4_to_uchar4(c2);
+        cdata[2] = color_float4_to_uchar4(c3);
+
         cdata += 3;
       }
     }
diff --git a/intern/cycles/kernel/geom/geom_patch.h b/intern/cycles/kernel/geom/geom_patch.h
index 8b4b91b96c8..b996294c51f 100644
--- a/intern/cycles/kernel/geom/geom_patch.h
+++ b/intern/cycles/kernel/geom/geom_patch.h
@@ -405,7 +405,8 @@ ccl_device float4 patch_eval_uchar4(KernelGlobals *kg,
     *dv = make_float4(0.0f, 0.0f, 0.0f, 0.0f);
 
   for (int i = 0; i < num_control; i++) {
-    float4 v = color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, offset + indices[i]));
+    float4 v = color_srgb_to_linear_v4(
+        color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, offset + indices[i])));
 
     val += v * weights[i];
     if (du)
diff --git a/intern/cycles/kernel/geom/geom_subd_triangle.h b/intern/cycles/kernel/geom/geom_subd_triangle.h
index 3eef9857ae3..317f3984a9a 100644
--- a/intern/cycles/kernel/geom/geom_subd_triangle.h
+++ b/intern/cycles/kernel/geom/geom_subd_triangle.h
@@ -581,14 +581,14 @@ ccl_device_noinline float4 subd_triangle_attribute_float4(KernelGlobals *kg,
     int corners[4];
     subd_triangle_patch_corners(kg, patch, corners);
 
-    float4 f0 = color_uchar4_to_float4(
-        kernel_tex_fetch(__attributes_uchar4, corners[0] + desc.offset));
-    float4 f1 = color_uchar4_to_float4(
-        kernel_tex_fetch(__attributes_uchar4, corners[1] + desc.offset));
-    float4 f2 = color_uchar4_to_float4(
-        kernel_tex_fetch(__attributes_uchar4, corners[2] + desc.offset));
-    float4 f3 = color_uchar4_to_float4(
-        kernel_tex_fetch(__attributes_uchar4, corners[3] + desc.offset));
+    float4 f0 = color_srgb_to_linear_v4(
+        color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, corners[0] + desc.offset)));
+    float4 f1 = color_srgb_to_linear_v4(
+        color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, corners[1] + desc.offset)));
+    float4 f2 = color_srgb_to_linear_v4(
+        color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, corners[2] + desc.offset)));
+    float4 f3 = color_srgb_to_linear_v4(
+        color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, corners[3] + desc.offset)));
 
     if (subd_triangle_patch_num_corners(kg, patch) != 4) {
       f1 = (f1 + f0) * 0.5f;
@@ -614,7 +614,8 @@ ccl_device_noinline float4 subd_triangle_attribute_float4(KernelGlobals *kg,
     if (dy)
       *dy = make_float4(0.0f, 0.0f, 0.0f, 0.0f);
 
-    return color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, desc.offset));
+    return color_srgb_to_linear_v4(
+        color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, desc.offset)));
   }
   else {
     if (dx)
diff --git a/intern/cycles/kernel/geom/geom_triangle.h b/intern/cycles/kernel/geom/geom_triangle.h
index 0278f3ade8e..5ecc2566815 100644
--- a/intern/cycles/kernel/geom/geom_triangle.h
+++ b/intern/cycles/kernel/geom/geom_triangle.h
@@ -317,9 +317,12 @@ ccl_device float4 triangle_attribute_float4(KernelGlobals *kg,
 
     if (desc.element == ATTR_ELEMENT_CORNER_BYTE) {
       int tri = desc.offset + sd->prim * 3;
-      f0 = color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, tri + 0));
-      f1 = color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, tri + 1));
-      f2 = color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, tri + 2));
+      f0 = color_srgb_to_linear_v4(
+          color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, tri + 0)));
+      f1 = color_srgb_to_linear_v4(
+          color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, tri + 1)));
+      f2 = color_srgb_to_linear_v4(
+          color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, tri + 2)));
     }
     else {
       uint4 tri_vindex = kernel_tex_fetch(__tri_vindex, sd->prim);
@@ -343,7 +346,8 @@ ccl_device float4 triangle_attribute_float4(KernelGlobals *kg,
     if (dy)
       *dy = make_float4(0.0f, 0.0f, 0.0f, 0.0f);
 
-    return color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, desc.offset));
+    return color_srgb_to_linear_v4(
+        color_uchar4_to_float4(kernel_tex_fetch(__attributes_uchar4, desc.offset)));
   }
   else {
     if (dx)