Make it possible to get predictable compressed cubemap image sizes.

I expect the drivers to return size of *one* face when I'm querying that
particular face using the pre-DSA or EXT_DSA API and size of *all* six
faces when I'm querying whole texture using DSA API.

One can dream, eh?

It appears that, at least on my NVidia, the returned value does not
depend on whether I'm querying all faces or a single one, but RATHER
is based on whether the texture is immutable or not. How's that
predictable at all?! Workaround in the next commit.

src/Magnum/CubeMapTexture.cpp

@@ -109,8 +109,7 @@ void CubeMapTexture::compressedImage(const Int level, CompressedImage3D& image)
     createIfNotAlready();
 
     const Vector3i size{imageSize(level), 6};
-    GLint textureDataSize;
-    (this->*Context::current()->state().texture->getCubeLevelParameterivImplementation)(level, GL_TEXTURE_COMPRESSED_IMAGE_SIZE, &textureDataSize);
+    const GLint textureDataSize = (this->*Context::current()->state().texture->getCubeLevelCompressedImageSizeImplementation)(level);
     const std::size_t dataSize = Implementation::compressedImageDataSizeFor(image, size, textureDataSize);
     GLint format;
     (this->*Context::current()->state().texture->getCubeLevelParameterivImplementation)(level, GL_TEXTURE_INTERNAL_FORMAT, &format);
@@ -135,8 +134,7 @@ void CubeMapTexture::compressedImage(const Int level, CompressedBufferImage3D& i
     createIfNotAlready();
 
     const Vector3i size{imageSize(level), 6};
-    GLint textureDataSize;
-    (this->*Context::current()->state().texture->getCubeLevelParameterivImplementation)(level, GL_TEXTURE_COMPRESSED_IMAGE_SIZE, &textureDataSize);
+    const GLint textureDataSize = (this->*Context::current()->state().texture->getCubeLevelCompressedImageSizeImplementation)(level);
     const std::size_t dataSize = Implementation::compressedImageDataSizeFor(image, size, textureDataSize);
     GLint format;
     (this->*Context::current()->state().texture->getCubeLevelParameterivImplementation)(level, GL_TEXTURE_INTERNAL_FORMAT, &format);
@@ -199,8 +197,9 @@ BufferImage2D CubeMapTexture::image(const Coordinate coordinate, const Int level
 
 void CubeMapTexture::compressedImage(const Coordinate coordinate, const Int level, CompressedImage2D& image) {
     const Vector2i size = imageSize(level);
-    GLint textureDataSize;
-    (this->*Context::current()->state().texture->getCubeLevelParameterivImplementation)(level, GL_TEXTURE_COMPRESSED_IMAGE_SIZE, &textureDataSize);
+    /* The function returns size of all six faces, divide the result to get size
+       of one face */
+    const GLint textureDataSize = (this->*Context::current()->state().texture->getCubeLevelCompressedImageSizeImplementation)(level)/6;
     const std::size_t dataSize = Implementation::compressedImageDataSizeFor(image, size, textureDataSize);
     GLint format;
     (this->*Context::current()->state().texture->getCubeLevelParameterivImplementation)(level, GL_TEXTURE_INTERNAL_FORMAT, &format);
@@ -223,8 +222,9 @@ CompressedImage2D CubeMapTexture::compressedImage(const Coordinate coordinate, c
 
 void CubeMapTexture::compressedImage(const Coordinate coordinate, const Int level, CompressedBufferImage2D& image, const BufferUsage usage) {
     const Vector2i size = imageSize(level);
-    GLint textureDataSize;
-    (this->*Context::current()->state().texture->getCubeLevelParameterivImplementation)(level, GL_TEXTURE_COMPRESSED_IMAGE_SIZE, &textureDataSize);
+    /* The function returns size of all six faces, divide the result to get size
+       of one face */
+    const GLint textureDataSize = (this->*Context::current()->state().texture->getCubeLevelCompressedImageSizeImplementation)(level)/6;
     const std::size_t dataSize = Implementation::compressedImageDataSizeFor(image, size, textureDataSize);
     GLint format;
     (this->*Context::current()->state().texture->getCubeLevelParameterivImplementation)(level, GL_TEXTURE_INTERNAL_FORMAT, &format);
@@ -362,6 +362,38 @@ void CubeMapTexture::getLevelParameterImplementationDSAEXT(const GLint level, co
 #endif
 #endif
 
+#if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
+GLint CubeMapTexture::getLevelCompressedImageSizeImplementationDefault(const GLint level) {
+    bindInternal();
+    /* Using only parameters of +X in pre-DSA code path and assuming that all
+       other faces are the same */
+    GLint value;
+    glGetTexLevelParameteriv(GL_TEXTURE_CUBE_MAP_POSITIVE_X, level, GL_TEXTURE_COMPRESSED_IMAGE_SIZE, &value);
+
+    /* Size of all six faces */
+    return value*6;
+}
+
+#ifndef MAGNUM_TARGET_GLES
+GLint CubeMapTexture::getLevelCompressedImageSizeImplementationDSA(const GLint level) {
+    GLint value;
+    glGetTextureLevelParameteriv(_id, level, GL_TEXTURE_COMPRESSED_IMAGE_SIZE, &value);
+    return value;
+}
+
+GLint CubeMapTexture::getLevelCompressedImageSizeImplementationDSAEXT(const GLint level) {
+    _flags |= ObjectFlag::Created;
+    /* Using only parameters of +X in pre-DSA code path and assuming that all
+       other faces are the same */
+    GLint value;
+    glGetTextureLevelParameterivEXT(_id, GL_TEXTURE_CUBE_MAP_POSITIVE_X, level, GL_TEXTURE_COMPRESSED_IMAGE_SIZE, &value);
+
+    /* Size of all six faces */
+    return value*6;
+}
+#endif
+#endif
+
 #ifndef MAGNUM_TARGET_GLES
 void CubeMapTexture::getImageImplementationDefault(const Coordinate coordinate, const GLint level, const Vector2i&, const PixelFormat format, const PixelType type, std::size_t, GLvoid* const data) {
     bindInternal();

src/Magnum/CubeMapTexture.h

@@ -952,6 +952,14 @@ class MAGNUM_EXPORT CubeMapTexture: public AbstractTexture {
         #endif
         #endif
 
+        #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
+        GLint MAGNUM_LOCAL getLevelCompressedImageSizeImplementationDefault(GLint level);
+        #ifndef MAGNUM_TARGET_GLES
+        GLint MAGNUM_LOCAL getLevelCompressedImageSizeImplementationDSA(GLint level);
+        GLint MAGNUM_LOCAL getLevelCompressedImageSizeImplementationDSAEXT(GLint level);
+        #endif
+        #endif
+
         #ifndef MAGNUM_TARGET_GLES
         void MAGNUM_LOCAL getImageImplementationDefault(Coordinate coordinate, GLint level, const Vector2i& size, PixelFormat format, PixelType type, std::size_t dataSize, GLvoid* data);
         void MAGNUM_LOCAL getImageImplementationDSA(Coordinate coordinate, GLint level, const Vector2i& size, PixelFormat format, PixelType type, std::size_t dataSize, GLvoid* data);

src/Magnum/Implementation/TextureState.cpp

@@ -136,6 +136,7 @@ TextureState::TextureState(Context& context, std::vector<std::string>& extension
         setBufferRangeImplementation = &BufferTexture::setBufferRangeImplementationDSA;
 
         getCubeLevelParameterivImplementation = &CubeMapTexture::getLevelParameterImplementationDSA;
+        getCubeLevelCompressedImageSizeImplementation = &CubeMapTexture::getLevelCompressedImageSizeImplementationDSA;
         cubeSubImageImplementation = &CubeMapTexture::subImageImplementationDSA;
         cubeCompressedSubImageImplementation = &CubeMapTexture::compressedSubImageImplementationDSA;
 
@@ -161,6 +162,7 @@ TextureState::TextureState(Context& context, std::vector<std::string>& extension
         setBufferRangeImplementation = &BufferTexture::setBufferRangeImplementationDSAEXT;
 
         getCubeLevelParameterivImplementation = &CubeMapTexture::getLevelParameterImplementationDSAEXT;
+        getCubeLevelCompressedImageSizeImplementation = &CubeMapTexture::getLevelCompressedImageSizeImplementationDSAEXT;
         cubeSubImageImplementation = &CubeMapTexture::subImageImplementationDSAEXT;
         cubeCompressedSubImageImplementation = &CubeMapTexture::compressedSubImageImplementationDSAEXT;
 
@@ -199,6 +201,7 @@ TextureState::TextureState(Context& context, std::vector<std::string>& extension
 
         #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
         getCubeLevelParameterivImplementation = &CubeMapTexture::getLevelParameterImplementationDefault;
+        getCubeLevelCompressedImageSizeImplementation = &CubeMapTexture::getLevelCompressedImageSizeImplementationDefault;
         #endif
         cubeSubImageImplementation = &CubeMapTexture::subImageImplementationDefault;
         cubeCompressedSubImageImplementation = &CubeMapTexture::compressedSubImageImplementationDefault;

src/Magnum/Implementation/TextureState.h

@@ -99,6 +99,7 @@ struct TextureState {
 
     #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
     void(CubeMapTexture::*getCubeLevelParameterivImplementation)(GLint, GLenum, GLint*);
+    GLint(CubeMapTexture::*getCubeLevelCompressedImageSizeImplementation)(GLint);
     #endif
     #ifndef MAGNUM_TARGET_GLES
     void(CubeMapTexture::*getCubeImageImplementation)(CubeMapTexture::Coordinate, GLint, const Vector2i&, PixelFormat, PixelType, std::size_t, GLvoid*);