OpenCV environment variables reference

Table of Contents

• Introduction
  • Setting environment variable in Windows
  • Setting environment variable in Linux
  • Setting environment variable in Python
• Types
• General, core
• Logging
  • Levels
• core/parallel_for
• backends
• plugins
• OpenCL
  • SVM (Shared Virtual Memory) - disabled by default
  • Links:
• Tracing/Profiling
  • Links:
• Cache
• dnn
• Tests
  • Links:
• videoio
  • videoio tests
• gapi
  • gapi tests/samples
  • Links:
• highgui
• imgproc
• imgcodecs

Prev Tutorial: OpenCV configuration options reference
Next Tutorial: Installation in Linux

Introduction

OpenCV can change its behavior depending on the runtime environment:

• enable extra debugging output or performance tracing
• modify default locations and search paths
• tune some algorithms or general behavior
• enable or disable workarounds, safety features and optimizations

Notes:

• ⭐ marks most popular variables
• variables with names like this VAR_${NAME} describes family of variables, where ${NAME} should be changed to one of predefined values, e.g. VAR_TBB, VAR_OPENMP, ...

Setting environment variable in Windows

In terminal or cmd-file (bat-file):

set MY_ENV_VARIABLE=true
C:\my_app.exe

In GUI:

• Go to "Settings -> System -> About"
• Click on "Advanced system settings" in the right part
• In new window click on the "Environment variables" button
• Add an entry to the "User variables" list

Setting environment variable in Linux

In terminal or shell script:

export MY_ENV_VARIABLE=true
./my_app

or as a single command:

MY_ENV_VARIABLE=true ./my_app

Setting environment variable in Python

import os
os.environ["MY_ENV_VARIABLE"] = "True" # value must be a string
import cv2 # variables set after this may not have effect

Note

This method may not work on all operating systems and/or Python distributions. For example, it works on Ubuntu Linux with system Python interpreter, but doesn't work on Windows 10 with the official Python package. It depends on the ability of a process to change its own environment (OpenCV uses getenv from C++ runtime to read variables).

See also:

• https://docs.python.org/3.12/library/os.html#os.environ
• https://stackoverflow.com/questions/69199708/setenvironmentvariable-does-not-seem-to-set-values-that-can-be-retrieved-by-ge

Types

• bool - 1, True, true, TRUE / 0, False, false, FALSE
• number/_size_ - unsigned number, suffixes MB, Mb, mb, KB, Kb, kb
• string - plain string or can have a structure
• path - to file, to directory
• paths - ;-separated on Windows, :-separated on others

General, core

name	type	default	description
OPENCV_SKIP_CPU_BASELINE_CHECK	bool	false	do not check that current CPU supports all features used by the build (baseline)
OPENCV_CPU_DISABLE	, or ;-separated		disable code branches which use CPU features (dispatched code)
OPENCV_SETUP_TERMINATE_HANDLER	bool	true (Windows)	use std::set_terminate to install own termination handler
OPENCV_LIBVA_RUNTIME	file path		libva for VA interoperability utils
OPENCV_ENABLE_MEMALIGN	bool	true (except static analysis, memory sanitizer, fuzzying, _WIN32?)	enable aligned memory allocations
OPENCV_BUFFER_AREA_ALWAYS_SAFE	bool	false	enable safe mode for multi-buffer allocations (each buffer separately)
OPENCV_KMEANS_PARALLEL_GRANULARITY	num	1000	tune algorithm parallel work distribution parameter parallel_for_(..., ..., ..., granularity)
OPENCV_DUMP_ERRORS	bool	true (Debug or Android), false (others)	print extra information on exception (log to Android)
OPENCV_DUMP_CONFIG	bool	false	print build configuration to stderr (getBuildInformation)
OPENCV_PYTHON_DEBUG	bool	false	enable extra warnings in Python bindings
OPENCV_TEMP_PATH	path	/tmp/ (Linux), /data/local/tmp/ (Android), GetTempPathA (Windows)	directory for temporary files
OPENCV_DATA_PATH_HINT	paths		paths for findDataFile
OPENCV_DATA_PATH	paths		paths for findDataFile
OPENCV_SAMPLES_DATA_PATH_HINT	paths		paths for findDataFile
OPENCV_SAMPLES_DATA_PATH	paths		paths for findDataFile

Links:

• https://github.com/opencv/opencv/wiki/CPU-optimizations-build-options

Logging

name	type	default	description
⭐ OPENCV_LOG_LEVEL	string		logging level (see accepted values below)
OPENCV_LOG_TIMESTAMP	bool	true	logging with timestamps
OPENCV_LOG_TIMESTAMP_NS	bool	false	add nsec to logging timestamps

Levels

• 0, O, OFF, S, SILENT, DISABLE, DISABLED
• F, FATAL
• E, ERROR
• W, WARNING, WARN, WARNINGS
• I, INFO
• D, DEBUG
• V, VERBOSE

core/parallel_for

name	type	default	description
⭐ OPENCV_FOR_THREADS_NUM	num	0	set number of threads
OPENCV_THREAD_POOL_ACTIVE_WAIT_PAUSE_LIMIT	num	16	tune pthreads parallel_for backend
OPENCV_THREAD_POOL_ACTIVE_WAIT_WORKER	num	2000	tune pthreads parallel_for backend
OPENCV_THREAD_POOL_ACTIVE_WAIT_MAIN	num	10000	tune pthreads parallel_for backend
OPENCV_THREAD_POOL_ACTIVE_WAIT_THREADS_LIMIT	num	0	tune pthreads parallel_for backend
OPENCV_FOR_OPENMP_DYNAMIC_DISABLE	bool	false	use single OpenMP thread

backends

OPENCV_LEGACY_WAITKEY Some modules have multiple available backends, following variables allow choosing specific backend or changing default priorities in which backends will be probed (e.g. when opening a video file).

name	type	default	description
OPENCV_PARALLEL_BACKEND	string		choose specific paralel_for backend (one of TBB, ONETBB, OPENMP)
OPENCV_PARALLEL_PRIORITY_${NAME}	num		set backend priority, default is 1000
OPENCV_PARALLEL_PRIORITY_LIST	string, ,-separated		list of backends in priority order
OPENCV_UI_BACKEND	string		choose highgui backend for window rendering (one of GTK, GTK3, GTK2, QT, WIN32)
OPENCV_UI_PRIORITY_${NAME}	num		set highgui backend priority, default is 1000
OPENCV_UI_PRIORITY_LIST	string, ,-separated		list of highgui backends in priority order
OPENCV_VIDEOIO_PRIORITY_${NAME}	num		set videoio backend priority, default is 1000
OPENCV_VIDEOIO_PRIORITY_LIST	string, ,-separated		list of videoio backends in priority order

plugins

Some external dependencies can be detached into a dynamic library, which will be loaded at runtime (plugin). Following variables allow changing default search locations and naming pattern for these plugins.

name	type	default	description
OPENCV_CORE_PLUGIN_PATH	paths		directories to search for core plugins
OPENCV_CORE_PARALLEL_PLUGIN_${NAME}	string, glob		parallel_for plugin library name (glob), e.g. default for TBB is "opencv_core_parallel_tbb*.so"
OPENCV_DNN_PLUGIN_PATH	paths		directories to search for dnn plugins
OPENCV_DNN_PLUGIN_${NAME}	string, glob		parallel_for plugin library name (glob), e.g. default for TBB is "opencv_core_parallel_tbb*.so"
OPENCV_CORE_PLUGIN_PATH	paths		directories to search for highgui plugins (YES it is CORE)
OPENCV_UI_PLUGIN_${NAME}	string, glob		highgui plugin library name (glob)
OPENCV_VIDEOIO_PLUGIN_PATH	paths		directories to search for videoio plugins
OPENCV_VIDEOIO_PLUGIN_${NAME}	string, glob		videoio plugin library name (glob)

OpenCL

Note: OpenCL device specification format is <Platform>:<CPU|GPU|ACCELERATOR|nothing=GPU/CPU>:<deviceName>, e.g. AMD:GPU:

name	type	default	description
OPENCV_OPENCL_RUNTIME	filepath or disabled		path to OpenCL runtime library (e.g. OpenCL.dll, libOpenCL.so)
⭐ OPENCV_OPENCL_DEVICE	string or disabled		choose specific OpenCL device. See specification format in the note above. See more details in the Links section.
OPENCV_OPENCL_RAISE_ERROR	bool	false	raise exception if something fails during OpenCL kernel preparation and execution (Release builds only)
OPENCV_OPENCL_ABORT_ON_BUILD_ERROR	bool	false	abort if OpenCL kernel compilation failed
OPENCV_OPENCL_CACHE_ENABLE	bool	true	enable OpenCL kernel cache
OPENCV_OPENCL_CACHE_WRITE	bool	true	allow writing to the cache, otherwise cache will be read-only
OPENCV_OPENCL_CACHE_LOCK_ENABLE	bool	true	use .lock files to synchronize between multiple applications using the same OpenCL cache (may not work on network drives)
OPENCV_OPENCL_CACHE_CLEANUP	bool	true	automatically remove old entries from cache (leftovers from older OpenCL runtimes)
OPENCV_OPENCL_VALIDATE_BINARY_PROGRAMS	bool	false	validate loaded binary OpenCL kernels
OPENCV_OPENCL_DISABLE_BUFFER_RECT_OPERATIONS	bool	true (Apple), false (others)	enable workaround for non-continuos data downloads
OPENCV_OPENCL_BUILD_EXTRA_OPTIONS	string		pass extra options to OpenCL kernel compilation
OPENCV_OPENCL_ENABLE_MEM_USE_HOST_PTR	bool	true	workaround/optimization for buffer allocation
OPENCV_OPENCL_ALIGNMENT_MEM_USE_HOST_PTR	num	4	parameter for OPENCV_OPENCL_ENABLE_MEM_USE_HOST_PTR
OPENCV_OPENCL_DEVICE_MAX_WORK_GROUP_SIZE	num	0	allow to decrease maxWorkGroupSize
OPENCV_OPENCL_PROGRAM_CACHE	num	0	limit number of programs in OpenCL kernel cache
OPENCV_OPENCL_RAISE_ERROR_REUSE_ASYNC_KERNEL	bool	false	raise exception if async kernel failed
OPENCV_OPENCL_BUFFERPOOL_LIMIT	num	1 << 27 (Intel device), 0 (others)	limit memory used by buffer bool
OPENCV_OPENCL_HOST_PTR_BUFFERPOOL_LIMIT	num		same as OPENCV_OPENCL_BUFFERPOOL_LIMIT, but for HOST_PTR buffers
OPENCV_OPENCL_BUFFER_FORCE_MAPPING	bool	false	force clEnqueueMapBuffer
OPENCV_OPENCL_BUFFER_FORCE_COPYING	bool	false	force clEnqueueReadBuffer/clEnqueueWriteBuffer
OPENCV_OPENCL_FORCE	bool	false	force running OpenCL kernel even if usual conditions are not met (e.g. dst.isUMat)
OPENCV_OPENCL_PERF_CHECK_BYPASS	bool	false	force running OpenCL kernel even if usual performance-related conditions are not met (e.g. image is very small)

SVM (Shared Virtual Memory) - disabled by default

name	type	default	description
OPENCV_OPENCL_SVM_DISABLE	bool	false	disable SVM
OPENCV_OPENCL_SVM_FORCE_UMAT_USAGE	bool	false
OPENCV_OPENCL_SVM_DISABLE_UMAT_USAGE	bool	false
OPENCV_OPENCL_SVM_CAPABILITIES_MASK	num
OPENCV_OPENCL_SVM_BUFFERPOOL_LIMIT	num		same as OPENCV_OPENCL_BUFFERPOOL_LIMIT, but for SVM buffers

Links:

• https://github.com/opencv/opencv/wiki/OpenCL-optimizations

Tracing/Profiling

name	type	default	description
⭐ OPENCV_TRACE	bool	false	enable trace
OPENCV_TRACE_LOCATION	string	OpenCVTrace	trace file name ("${name}-$03d.txt")
OPENCV_TRACE_DEPTH_OPENCV	num	1
OPENCV_TRACE_MAX_CHILDREN_OPENCV	num	1000
OPENCV_TRACE_MAX_CHILDREN	num	1000
OPENCV_TRACE_SYNC_OPENCL	bool	false	wait for OpenCL kernels to finish
OPENCV_TRACE_ITT_ENABLE	bool	true
OPENCV_TRACE_ITT_PARENT	bool	false	set parentID for ITT task
OPENCV_TRACE_ITT_SET_THREAD_NAME	bool	false	set name for OpenCV's threads "OpenCVThread-%03d"

Links:

• https://github.com/opencv/opencv/wiki/Profiling-OpenCV-Applications

Cache

Note: Default tmp location is TMPDIR% (Windows); $XDG_CACHE_HOME, $HOME/.cache, /var/tmp, /tmp (others)

name	type	default	description
OPENCV_CACHE_SHOW_CLEANUP_MESSAGE	bool	true	show cache cleanup message
OPENCV_DOWNLOAD_CACHE_DIR	path	default tmp location	cache directory for downloaded files (subdirectory downloads)
OPENCV_DNN_IE_GPU_CACHE_DIR	path	default tmp location	cache directory for OpenVINO OpenCL kernels (subdirectory dnn_ie_cache_${device})
OPENCV_OPENCL_CACHE_DIR	path	default tmp location	cache directory for OpenCL kernels cache (subdirectory opencl_cache)

dnn

Note: In the table below dump_base_name equals to ocv_dnn_net_%05d_%02d where first argument is internal network ID and the second - dump level.

name	type	default	description
OPENCV_DNN_BACKEND_DEFAULT	num	3 (OpenCV)	set default DNN backend, see dnn.hpp for backends enumeration
OPENCV_DNN_NETWORK_DUMP	num	0	level of information dumps, 0 - no dumps (default file name ${dump_base_name}.dot)
OPENCV_DNN_DISABLE_MEMORY_OPTIMIZATIONS	bool	false
OPENCV_DNN_CHECK_NAN_INF	bool	false	check for NaNs in layer outputs
OPENCV_DNN_CHECK_NAN_INF_DUMP	bool	false	print layer data when NaN check has failed
OPENCV_DNN_CHECK_NAN_INF_RAISE_ERROR	bool	false	also raise exception when NaN check has failed
OPENCV_DNN_ONNX_USE_LEGACY_NAMES	bool	false	use ONNX node names as-is instead of "onnx_node!${node_name}"
OPENCV_DNN_CUSTOM_ONNX_TYPE_INCLUDE_DOMAIN_NAME	bool	true	prepend layer domain to layer types ("domain.type")
OPENCV_VULKAN_RUNTIME	file path		set location of Vulkan runtime library for DNN Vulkan backend
OPENCV_DNN_IE_SERIALIZE	bool	false	dump intermediate OpenVINO graph (default file names ${dump_base_name}_ngraph.xml, ${dump_base_name}_ngraph.bin)
OPENCV_DNN_IE_EXTRA_PLUGIN_PATH	path		path to extra OpenVINO plugins
OPENCV_DNN_IE_VPU_TYPE	string		Force using specific OpenVINO VPU device type ("Myriad2" or "MyriadX")
OPENCV_TEST_DNN_IE_VPU_TYPE	string		same as OPENCV_DNN_IE_VPU_TYPE, but for tests
OPENCV_DNN_INFERENCE_ENGINE_HOLD_PLUGINS	bool	true	always hold one existing OpenVINO instance to avoid crashes on unloading
OPENCV_DNN_INFERENCE_ENGINE_CORE_LIFETIME_WORKAROUND	bool	true (Windows), false (other)	another OpenVINO lifetime workaround
OPENCV_DNN_OPENCL_ALLOW_ALL_DEVICES	bool	false	allow running on CPU devices, allow FP16 on non-Intel device
OPENCV_OCL4DNN_CONVOLUTION_IGNORE_INPUT_DIMS_4_CHECK	bool	false	workaround for OpenCL backend, see https://github.com/opencv/opencv/issues/20833
OPENCV_OCL4DNN_WORKAROUND_IDLF	bool	true	another workaround for OpenCL backend
OPENCV_OCL4DNN_CONFIG_PATH	path		path to kernel configuration cache for auto-tuning (must be existing directory), set this variable to enable auto-tuning
OPENCV_OCL4DNN_DISABLE_AUTO_TUNING	bool	false	disable auto-tuning
OPENCV_OCL4DNN_FORCE_AUTO_TUNING	bool	false	force auto-tuning
OPENCV_OCL4DNN_TEST_ALL_KERNELS	num	0	test convolution kernels, number of iterations (auto-tuning)
OPENCV_OCL4DNN_DUMP_FAILED_RESULT	bool	false	dump extra information on errors (auto-tuning)
OPENCV_OCL4DNN_TUNING_RAISE_CHECK_ERROR	bool	false	raise exception on errors (auto-tuning)

Tests

name	type	default	description
⭐ OPENCV_TEST_DATA_PATH	dir path		set test data search location (e.g. /home/user/opencv_extra/testdata)
⭐ OPENCV_DNN_TEST_DATA_PATH	dir path	$OPENCV_TEST_DATA_PATH/dnn	set DNN model search location for tests (used by dnn, gapi, objdetect, video modules)
OPENCV_OPEN_MODEL_ZOO_DATA_PATH	dir path	$OPENCV_DNN_TEST_DATA_PATH/omz_intel_models	set OpenVINO models search location for tests (used by dnn, gapi modules)
INTEL_CVSDK_DIR			some dnn tests can search OpenVINO models here too
OPENCV_TEST_DEBUG	num	0	debug level for tests, same as --test_debug (0 - no debug (default), 1 - basic test debug information, >1 - extra debug information)
OPENCV_TEST_REQUIRE_DATA	bool	false	same as --test_require_data option (fail on missing non-required test data instead of skip)
OPENCV_TEST_CHECK_OPTIONAL_DATA	bool	false	assert when optional data is not found
OPENCV_IPP_CHECK	bool	false	default value for --test_ipp_check and --perf_ipp_check
OPENCV_PERF_VALIDATION_DIR	dir path		location of files read/written by --perf_read_validation_results/--perf_write_validation_results
⭐ OPENCV_PYTEST_FILTER	string (glob)		test filter for Python tests

Links:

• https://github.com/opencv/opencv/wiki/QA_in_OpenCV

videoio

Note: extra FFmpeg options should be pased in form key;value|key;value|key;value, for example hwaccel;cuvid|video_codec;h264_cuvid|vsync;0 or vcodec;x264|vprofile;high|vlevel;4.0

name	type	default	description
⭐ OPENCV_FFMPEG_CAPTURE_OPTIONS	string (see note)		extra options for VideoCapture FFmpeg backend
⭐ OPENCV_FFMPEG_WRITER_OPTIONS	string (see note)		extra options for VideoWriter FFmpeg backend
OPENCV_FFMPEG_THREADS	num		set FFmpeg thread count
OPENCV_FFMPEG_DEBUG	bool	false	enable logging messages from FFmpeg
OPENCV_FFMPEG_LOGLEVEL	num		set FFmpeg logging level
OPENCV_FFMPEG_DLL_DIR	dir path		directory with FFmpeg plugin (legacy)
OPENCV_FFMPEG_IS_THREAD_SAFE	bool	false	enabling this option will turn off thread safety locks in the FFmpeg backend (use only if you are sure FFmpeg is built with threading support, tested on Linux)
OPENCV_FFMPEG_READ_ATTEMPTS	num	4096	number of failed av_read_frame attempts before failing read procedure
OPENCV_FFMPEG_DECODE_ATTEMPTS	num	64	number of failed avcodec_receive_frame attempts before failing decoding procedure
OPENCV_VIDEOIO_GSTREAMER_CALL_DEINIT	bool	false	close GStreamer instance on end
OPENCV_VIDEOIO_GSTREAMER_START_MAINLOOP	bool	false	start GStreamer loop in separate thread
OPENCV_VIDEOIO_MFX_IMPL	num		set specific MFX implementation (see MFX docs for enumeration)
OPENCV_VIDEOIO_MFX_EXTRA_SURFACE_NUM	num	1	add extra surfaces to the surface pool
OPENCV_VIDEOIO_MFX_POOL_TIMEOUT	num	1	timeout for waiting for free surface from the pool (in seconds)
OPENCV_VIDEOIO_MFX_BITRATE_DIVISOR	num	300	this option allows to tune encoding bitrate (video quality/size)
OPENCV_VIDEOIO_MFX_WRITER_TIMEOUT	num	1	timeout for encoding operation (in seconds)
OPENCV_VIDEOIO_MSMF_ENABLE_HW_TRANSFORMS	bool	true	allow HW-accelerated transformations (DXVA) in MediaFoundation processing graph (may slow down camera probing process)
OPENCV_DSHOW_DEBUG	bool	false	enable verbose logging in the DShow backend
OPENCV_DSHOW_SAVEGRAPH_FILENAME	file path		enable processing graph tump in the DShow backend
OPENCV_VIDEOIO_V4L_RANGE_NORMALIZED	bool	false	use (0, 1) range for properties (V4L)
OPENCV_VIDEOIO_V4L_SELECT_TIMEOUT	num	10	timeout for select call (in seconds) (V4L)
OPENCV_VIDEOCAPTURE_DEBUG	bool	false	enable debug messages for VideoCapture
OPENCV_VIDEOWRITER_DEBUG	bool	false	enable debug messages for VideoWriter
⭐ OPENCV_VIDEOIO_DEBUG	bool	false	debug messages for both VideoCapture and VideoWriter

videoio tests

name	type	default	description
OPENCV_TEST_VIDEOIO_BACKEND_REQUIRE_FFMPEG	bool	false	test app will exit if no FFmpeg backend is available
OPENCV_TEST_V4L2_VIVID_DEVICE	file path		path to VIVID virtual camera device for V4L2 test (e.g. /dev/video5)
OPENCV_TEST_PERF_CAMERA_LIST	paths		cameras to use in performance test (waitAny_V4L test)
OPENCV_TEST_CAMERA_d_FPS	num		fps to set for N-th camera (0-based index) (waitAny_V4L test)

gapi

name	type	default	description
⭐ GRAPH_DUMP_PATH	file path		dump graph (dot format)
PIPELINE_MODELS_PATH	dir path		pipeline_modeling_tool sample application uses this var
OPENCV_GAPI_INFERENCE_ENGINE_CORE_LIFETIME_WORKAROUND	bool	true (Windows, Apple), false (others)	similar to OPENCV_DNN_INFERENCE_ENGINE_CORE_LIFETIME_WORKAROUND

gapi tests/samples

name	type	default	description
PLAIDML_DEVICE	string		specific to PlaidML backend test
PLAIDML_TARGET	string		specific to PlaidML backend test
OPENCV_GAPI_ONNX_MODEL_PATH	dir path		search location for ONNX models test
OPENCV_TEST_FREETYPE_FONT_PATH	file path		location of TrueType font for one of tests

Links:

• https://github.com/opencv/opencv/wiki/Using-G-API-with-OpenVINO-Toolkit
• https://github.com/opencv/opencv/wiki/Using-G-API-with-MS-ONNX-Runtime

highgui

name	type	default	description
OPENCV_LEGACY_WAITKEY	bool	false	switch waitKey return result (default behavior: return code & 0xff (or -1), legacy behavior: return code)
$XDG_RUNTIME_DIR			Wayland backend specific - create shared memory-mapped file for interprocess communication (named opencv-shared-??????)
OPENCV_HIGHGUI_FB_MODE	string	FB	Selects output mode for the framebuffer backend (FB - regular frambuffer, EMU - emulation, perform internal checks but does nothing, XVFB - compatible with xvfb virtual frambuffer)
OPENCV_HIGHGUI_FB_DEVICE	file path		Path to frambuffer device to use (will be checked first)
FRAMEBUFFER	file path	/dev/fb0	Same as OPENCV_HIGHGUI_FB_DEVICE, commonly used variable for the same purpose (will be checked second)

imgproc

name	type	default	description
OPENCV_OPENCL_IMGPROC_MORPH_SPECIAL_KERNEL	bool	true (Apple), false (others)	use special OpenCL kernel for small morph kernel (Intel devices)
OPENCV_GAUSSIANBLUR_CHECK_BITEXACT_KERNELS	bool	false	validate Gaussian kernels before running (src is CV_16U, bit-exact version)

imgcodecs

name	type	default	description
OPENCV_IMGCODECS_AVIF_MAX_FILE_SIZE	num	64MB	limit input AVIF size
OPENCV_IMGCODECS_WEBP_MAX_FILE_SIZE	num	64MB	limit input WEBM size
OPENCV_IO_MAX_IMAGE_PARAMS	num	50	limit maximum allowed number of parameters in imwrite and imencode
OPENCV_IO_MAX_IMAGE_WIDTH	num	1 << 20, limit input image size to avoid large memory allocations
OPENCV_IO_MAX_IMAGE_HEIGHT	num	1 << 20
OPENCV_IO_MAX_IMAGE_PIXELS	num	1 << 30
OPENCV_IO_ENABLE_OPENEXR	bool	true (set build option OPENCV_IO_FORCE_OPENEXR or use external OpenEXR), false (otherwise)	enable OpenEXR backend
OPENCV_IO_ENABLE_JASPER	bool	true (set build option OPENCV_IO_FORCE_JASPER), false (otherwise)	enable Jasper backend