imgui-cmake

Simple project with CMake files to build ImGui's example_sdl_opengl3 on multiple platforms. The main focus is RaspberryPi support, but this will work just fine on Windows, Mac and Linux.

To test the example:

git clone https://github.com/Pesc0/imgui-cmake --recurse-submodules
cd imgui-cmake
#git submodule foreach git pull origin HEAD
mkdir build && cd build
cmake -D BUILD_EXAMPLES=ON ..
cmake --build . -j4
./example_sdl_opengl3

To add the library to your project simply add to your CMakeLists.txt:

add_subdirectory("path/to/imgui-cmake")
target_link_libraries(YourTarget IMGUI)

Raspberry support

Building

You may need these dependencies:

sudo apt install libxext-dev libgles-dev libgles2-mesa-dev libgdm-dev libgbm-dev libdrm-dev libudev-dev libasound2-dev libdbus-1-dev

GPU acceleration

Regarding graphics, to render on Raspberry we have different options:

• No hardware acceleration, using the Mesa software renderer (slow)
• With GPU acceleration, which can be achieved in two ways (text adapted from here):

  1. Use the old firmware-side Broadcom driver usually located in /opt/vc/lib. This is an older driver that kind of works, but doesn’t integrate well with X11: you just get one fullscreen window, with or without X11. SDL has the rpi video backend for this driver. You may have to export SDL_VIDEODRIVER=rpi before running the executable to force SDL to use this backend, which doens't always get automatically selected. imgui-cmake builds SDL from source since the build in apt repos doesn't have the rpi backend compiled in (VIDEO_RPI option), although this may have been fixed in RaspberrypiOS. Note that Broadcom drivers are not available in the 64-bit version of RaspberrypiOS, only the 32-bit one. Broadcom drivers are no longer shipped in RaspberrypiOS:

     git clone https://github.com/raspberrypi/userland
     cd userland
     # Build and install
     ./buildme
     
     # You may need to add the following to your shell env
     # export LD_LIBRARY_PATH=/opt/vc/lib:$LD_LIBRARY_PATH
     

  2. Use the new vc4 driver by Eric Anholt. Broadcom hired him to write an open source driver that uses open APIs (specifically: It’s a gallium driver). If you activate this driver (which deactivates the older driver), X11 itself and the x11 backend of SDL will get hardware acceleration. This works from X11, Wayland, or starting with SDL 2.0.6 there's a new KMSDRM driver which will make it possible to run SDL applications directly from the console with no display server. To activate this driver use the raspi-config program of the Raspbian distribution. Under “Advanced Options” -> “GL Driver” you can select the “GL (Full KMS)” option.

It is also reccommended in the old SDL Raspberry documentation to assign 128MB of ram (or more) to the GPU. The document has been removed long ago.

Additional video info here and here. It seems it is recommended on lower hardware (like the RPi 1 or Zero) to use the broadcom drivers for performance reasons, even if they are old/unsupported.

If not using raspi-config you can manually edit /boot/config.txt:

• Set dtoverlay=vc4-kms-v3d to select the VC4 KMS driver
• Comment #dtoverlay=vc4-kms-v3d to select the broadcom diver
• Do not use the Fake KMS (FKMS) driver.

Compiler notes

In old raspbian there's a bug with the latest versions of the gcc compiler: the <limits.h> file has been modified, and the PATH_MAX macro is missing. This causes an error when compiling some SDL files. Possible solutions are:

• Use gcc-4.9 or an earlier version: when running cmake specify -D CMAKE_C_COMPILER=gcc-4.9 -D CMAKE_CXX_COMPILER=g++-4.9. This is not optimal since newer compilers tend to produce better code in general.
• Add #define PATH_MAX 4096 at the beginning of the broken SDL files, or add the same define in the compiler options, but this is not an elegant solution.
• Use the new RaspberrypiOS, where this issue is not present.

Expected performance

On a Raspberry Pi 0-3 (VideoCore IV) the performance is around 100-130 fps at 1080p for the imgui demo. It can drop down to as low as 30 fps or even less if certain demanding features are being displayed.

An example of such features is the colored buttons present in the demo under "layout & scrolling -> scrolling"