[OpenCV4] Cannot use vcpkg_check_features() for "dnn", "gtk", ipp", "openmp", "ovis", "python", "qt", "tbb"

[gblikas]

In the latest opencv4 portfile there is the line

vcpkg/ports/opencv4/portfile.cmake

Lines 98 to 99 in 333ba63

	# Cannot use vcpkg_check_features() for "dnn", "gtk", ipp", "openmp", "ovis", "python", "qt", "tbb"
	set(BUILD_opencv_dnn OFF)

A quick search for \"dnn\" on github, via / command shows that default-features is using the flag, yet it's not in the vcpkg_check_features call:

vcpkg/ports/opencv4/portfile.cmake

Lines 63 to 96 in 333ba63

	vcpkg_check_features(OUT_FEATURE_OPTIONS FEATURE_OPTIONS
	FEATURES
	"ade" WITH_ADE
	"contrib" WITH_CONTRIB
	"cuda" WITH_CUBLAS
	"cuda" WITH_CUDA
	"cudnn" WITH_CUDNN
	"dnn-cuda" OPENCV_DNN_CUDA
	"eigen" WITH_EIGEN
	"ffmpeg" WITH_FFMPEG
	"freetype" WITH_FREETYPE
	"gdcm" WITH_GDCM
	"gstreamer" WITH_GSTREAMER
	"gtk" WITH_GTK
	"halide" WITH_HALIDE
	"jasper" WITH_JASPER
	"openjpeg" WITH_OPENJPEG
	"jpeg" WITH_JPEG
	"lapack" WITH_LAPACK
	"nonfree" OPENCV_ENABLE_NONFREE
	"openvino" WITH_OPENVINO
	"openexr" WITH_OPENEXR
	"opengl" WITH_OPENGL
	"ovis" CMAKE_REQUIRE_FIND_PACKAGE_OGRE
	"png" WITH_PNG
	"quirc" WITH_QUIRC
	"sfm" BUILD_opencv_sfm
	"tiff" WITH_TIFF
	"vtk" WITH_VTK
	"webp" WITH_WEBP
	"world" BUILD_opencv_world
	"dc1394" WITH_1394
	"vulkan" WITH_VULKAN
	)

Can some describe why this line is added, and what it means in context? It appears as if the feature is always "OFF" in opencv4. I'm looking for confirmation that this is the case... But also, why.

@cenit

[cenit]

it's because once there was no "platform" nor "supports" clauses in the manifest. Since the "dnn" feature cannot work in all possible os/arch combinations, it was handled manually like all the others on that line.
In the latest PR in which i am updating opencv i am using all the new features of the manifest to handle all features with vcpkg_check_features

[gblikas]

Is the PR up?

[cenit]

#38928

[dg0yt]

Can some describe why this line is added, and what it means in context? It appears as if the feature is always "OFF" in opencv4. I'm looking for confirmation that this is the case...

It is not always OFF. Just look for BUILD_opencv_dnn in portfile.cmake. To get started, just look at the lines after initialization:

vcpkg/ports/opencv4/portfile.cmake

Lines 98 to 105 in 333ba63

	# Cannot use vcpkg_check_features() for "dnn", "gtk", ipp", "openmp", "ovis", "python", "qt", "tbb"
	set(BUILD_opencv_dnn OFF)
	if("dnn" IN_LIST FEATURES)
	if(NOT VCPKG_TARGET_IS_ANDROID)
	set(BUILD_opencv_dnn ON)
	else()
	message(WARNING "The dnn module cannot be enabled on Android")
	endif()

[gblikas]

Thank you so much! If only I could read:

Check if one or more features are a part of a package installation.
https://learn.microsoft.com/en-us/vcpkg/maintainers/functions/vcpkg_check_features

lol.

[dg0yt]

Well, the FEATURES parameter to that function is not the same as the top-level FEATURES variable.
(Edited my comments to make this clear.)

[gblikas]

uses the top-level FEATURES variable as an input.

@dg0yt Ahhh... I was under the impression that FEATURES was the APPEND'd list, having consumed the <feature-name> <feature-var> tuples below it, and was "unique"; really, it just a keyword, correct?

This would be a great thing to elaborate in the documentation, I think. Totally makes sense, but wasn't clear to me as a new user.

[dg0yt]

As a user, you shouldn't need to look at the portfile cmake code at all. As a port author, opencv4 might not be a suitable starting point for learning. Note opencv4's history, list of features, and network of dependencies.

vcpkg_check_features covers a basic mapping of features to boolean cmake variables. The documentation describes the observable behavior: which variables it sets, subject to <feature-name>. (Fine print: It doesn't only set <out-var> but also each <feature-var>.) Implementation details or alternative processing of ${FEATURES} are somewhat out of scope for this documentation.

[gblikas]

Note opencv4's history, list of features, and network of dependencies.

yeah... the biggest issue is that multiple platforms default OpenCV's ocv_option to different things. So, you cannot use a unified vcpkg_check_features... I think that's exactly what @cenit was saying: #40455 (comment)