[Fix, Test] (TextureUtil, GLImpl): accept GL_STENCIL_INDEX as a stencil-only texture internal format

MobileGL/MG_Impl/GLImpl/Texture/GL_Texture.cpp

@@ -4303,8 +4303,8 @@ namespace MobileGL::MG_Impl::GLImpl {
         }
 
         // Shared format/type/internal-format matrix (packed-type pairing, depth-vs-color mismatch,
-        // integer-ness). Also rejects STENCIL_INDEX readback, which needs GL_ARB_texture_stencil8
-        // (not advertised by MobileGL).
+        // integer-ness). Also rejects a STENCIL_INDEX readback of anything but stencil-only
+        // storage, which is the only pairing GL 4.4 / ARB_texture_stencil8 ever made legal.
         if (!TextureImpl::ValidateTextureInternalFormatCompatibleWithInput(
                 textureInputFormat, textureObject->GetFormat(), texturePixelDataType)) {
             return false;

MobileGL/MG_Impl/GLImpl/Texture/Validators.cpp

@@ -313,9 +313,13 @@ namespace MobileGL::MG_Impl::GLImpl::TextureImpl {
             return false;
         }
 
-        // TexImage in core 3.3 has no stencil-only upload path (that arrived with GL 4.4).
-        if (format == TextureInputFormat::StencilIndex) {
-            return recordInvalidOperation("STENCIL_INDEX is not a valid texture upload format");
+        // The stencil-only transfer path arrived with GL 4.4 / ARB_texture_stencil8, and only ever
+        // pairs with stencil-only storage: against a depth, depth-stencil or colour internal format
+        // STENCIL_INDEX keeps the pre-4.4 answer (GL CTS packed_pixels feeds exactly that pairing
+        // and expects INVALID_OPERATION).
+        if (format == TextureInputFormat::StencilIndex &&
+            internalFormat != TextureInternalFormat::StencilIndex8) {
+            return recordInvalidOperation("STENCIL_INDEX requires a stencil-only internal format");
         }
 
         if (IsDepthLikeInputFormat(format) != IsDepthLikeInternalFormat(internalFormat)) {

MobileGL/MG_Test/Texture/TextureTest.cpp

@@ -1057,6 +1057,32 @@ TEST_F(TextureTest, TexImage2DAcceptsSpecCompliantFormatCombinations) {
     EXPECT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
 }
 
+// GL_STENCIL_INDEX is the unsized base format for stencil-only storage, and refusing it as an
+// internal format killed the ARB_clear_texture stencil case in its own setup - before it could
+// reach the calls it actually tests. The stencil-only transfer format stays paired with
+// stencil-only storage in both directions, which is what keeps those clears erroring.
+TEST_F(TextureTest, StencilIndexIsATextureInternalFormatPairedOnlyWithStencilStorage) {
+    GLuint texture = 0;
+    MG_Impl::GLImpl::GenTextures(1, &texture);
+    MG_Impl::GLImpl::BindTexture(GL_TEXTURE_2D, texture);
+    MG_Impl::GLImpl::TexImage2D(GL_TEXTURE_2D, 0, GL_STENCIL_INDEX, 4, 4, 0, GL_STENCIL_INDEX, GL_UNSIGNED_BYTE,
+                                nullptr);
+    EXPECT_EQ(MG_Impl::GLImpl::GetError(), GL_NO_ERROR);
+
+    const auto textureObject = MG_State::pGLContext->GetTextureObject(texture);
+    ASSERT_NE(textureObject, nullptr);
+    EXPECT_EQ(textureObject->GetFormat(), TextureInternalFormat::StencilIndex8);
+
+    // A colour transfer format against stencil storage is still INVALID_OPERATION, so the clear
+    // the conformance case makes next fails the way it is supposed to.
+    MG_Impl::GLImpl::ClearTexImage(texture, 0, GL_RGBA, GL_UNSIGNED_BYTE, nullptr);
+    ExpectSingleGlError(GL_INVALID_OPERATION);
+
+    // ...and the other direction: GL_STENCIL_INDEX against colour storage stays illegal.
+    MG_Impl::GLImpl::TexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 4, 4, 0, GL_STENCIL_INDEX, GL_UNSIGNED_BYTE, nullptr);
+    ExpectSingleGlError(GL_INVALID_OPERATION);
+}
+
 // Desktop GL table 3.3 lists GREEN and BLUE as TexImage client formats (GL CTS packed_pixels
 // rgba8_format_green/blue upload with them and verify the readback): the single input component
 // feeds the named channel, the other color channels default to 0 and alpha to 1.

MobileGL/MG_Util/Converters/GLToMG/TextureEnumConverter.cpp

@@ -253,6 +253,12 @@ namespace MobileGL {
                 return TextureInternalFormat::Depth32FStencil8;
             case GL_STENCIL_INDEX8:
                 return TextureInternalFormat::StencilIndex8;
+            // The unsized stencil base format resolves to the only stencil storage there is, the
+            // same way the unsized colour and depth base formats below resolve to theirs. Returning
+            // Unknown made glTexImage2D(GL_STENCIL_INDEX) an error, which killed the negative
+            // clear-texture cases in their own setup before they could reach the call they test.
+            case GL_STENCIL_INDEX:
+                return TextureInternalFormat::StencilIndex8;
             case GL_DEPTH_COMPONENT:
                 return TextureInternalFormat::DepthComponent;
             case GL_DEPTH_STENCIL:

MobileGL/MG_Util/Converters/MGToMG/TextureEnumConverter.cpp

@@ -124,6 +124,9 @@ namespace MobileGL {
             case TextureInternalFormat::DepthComponent32F:
             case TextureInternalFormat::Depth24Stencil8:
             case TextureInternalFormat::Depth32FStencil8:
+            // Already sized: both GL_STENCIL_INDEX8 and the unsized GL_STENCIL_INDEX resolve here,
+            // and there is only one stencil storage to infer.
+            case TextureInternalFormat::StencilIndex8:
                 return internalformat;
             // probably we should assume unorm here?
             case TextureInternalFormat::RGBA: {

MobileGL/MG_Util/Texture/PixelStoreProcessor.cpp

@@ -138,6 +138,11 @@ namespace MobileGL::MG_Util::PixelStoreProcessor {
             case TextureInternalFormat::DepthComponent32F:
                 out = {1, ShadowComponent::Float32, false};
                 return true;
+            // Stencil is the one single-channel INTEGER shadow that is not a colour format: eight
+            // bits, held as an unsigned index rather than a normalized value.
+            case TextureInternalFormat::StencilIndex8:
+                out = {1, ShadowComponent::UInt8, true};
+                return true;
 
             case TextureInternalFormat::R8:
             case TextureInternalFormat::Red:     out = {1, ShadowComponent::UNorm8, false}; return true;
@@ -336,8 +341,13 @@ namespace MobileGL::MG_Util::PixelStoreProcessor {
             case TextureInputFormat::BGRAInteger:  out = {{2, 1, 0, 3}, 4, true};     return true;
             // A depth value converts like a single normalized/float channel.
             case TextureInputFormat::DepthComponent: out = {{0, -1, -1, -1}, 1, false}; return true;
+            // A stencil index is a single INTEGER channel (GL 4.6 core 8.4.4.3). Without this the
+            // upload fell to the raw-memcpy branch, which copies the client element width into the
+            // one-byte STENCIL_INDEX8 shadow verbatim - right for GL_UNSIGNED_BYTE and wrong for
+            // every wider type. The state layer keeps this paired with stencil-only storage.
+            case TextureInputFormat::StencilIndex: out = {{0, -1, -1, -1}, 1, true}; return true;
             default:
-                return false; // stencil / packed depth-stencil / unknown
+                return false; // packed depth-stencil / unknown
             }
         }
 
@@ -998,6 +1008,11 @@ namespace MobileGL::MG_Util::PixelStoreProcessor {
                                    const void* inputPixel,
                                    Vector<Uint8>& outputPixel) {
         outputPixel.clear();
+        // A stencil index became a transferable format when STENCIL_INDEX8 texture storage did (see
+        // GetUnpackChannelMapping), but this helper serves glClearBufferData, whose internal formats
+        // are all colour (GL 4.6 core table 8.20): a stencil pattern would otherwise pass the size
+        // check and land silently in an equally-sized colour store.
+        if (textureInputFormat == TextureInputFormat::StencilIndex) return false;
         if (inputPixel == nullptr || !IsValidUnpackPixelPair(textureInputFormat, inputDataType)) return false;
 
         PixelStoreParameters params{};