1. September 2017

SDL2 for Android API level 19

SDL2 example application works without problem with newer Androids API 21+. The problem was with API level 19. It took me some time to figure out reasons for very interesting errors.

Let’s examine error messages and investigate how to fix them.

When I’ve started sdl2-android-example on a device with API 19, it failed with error message: dlopen failed: cannot locate symbol “signal” referenced by libSDL2.so”

The root cause is not that obvious from the error message. It failed because the application has been built for API 21 instead of API 19.

The fixture is easy, just set sdkVersion to 19.

For this purpose, I’ve extracted sdkVersion to settings.gradle which can be found in the root directory of the project.

gradle.ext.sdkVersion = 19

Each module then has reference to this value:

model {
    android {
        compileSdkVersion = gradle.sdkVersion
...

Make complete clean and distributeLib to rebuild for proper API.

gradle clean dL

Hooray! Fixed. Let’s start the application.

Yey. New error: dlopen failed: could not load library “libSDL2_image.so” needed by “libmain.so”; caused by cannot locate symbol “png_set_longjmp_fn” referenced by “libSDL2_image.so”

The root cause of this issue is similar to the problem with JPEG described in the previous article. Paul Asiimwe found out that there is another library the name png and it has a different version so the application crashes.

To fix this issue it was necessary to rename whole png submodule to SDL2_png. Renaming requires clean and new distributeLib:

gradle clean dL

After these small tricks, the sdl2-android-example works even with API level 19.

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9. July 2017

SDL2_image for Android with PNG image format

Update of the article #1: If you’d like to support API level 19, the library must have a different name than png. Therefore I changed names in the article to SDL2_png. Read more about SDL2 API level 19 support.

Update of the article #2: Original article contained a dependency on the custom version of zip lib. It is not necessary because it is sufficient to link libz from Android. The library was removed from the example.

In the previous article, we were discussing how to add support for XPM image format to SDL2 application for Android. Let’s add another more common format. What about Portable Network Graphic (a.k.a. PNG)?

The first step is to enable PNG in SDL2_image. Just add a proper definition to SDL2_image/build.gradle.

ndk {
...
CFlags.addAll([
    "-DLOAD_PNG",
    "-DLOAD_XPM"
])}

Unfortunately, it’s not that simple. PNG support has a dependency on PNG library with a patch and PNG library. To get PNG working you need to add these one C library into the project. The approach is the same like when adding SDL2_image. The library is stored as a separate module in the directory with the same name like module name.

Register modules in settings.gradle:

include ':SDL2_image'
include ':SDL2_png'

Add build.gradle: SDL2_png/build.gradle.

You can verify compilation of the whole suite by following commands:

gradle :SDL2_png:distributeLib
gradle :SDL2_image:distributeLib

or simply:

gradle distributeLib

The library is ready. Now it’s necessary to add PNG file to the project. The location for graphic assets is app/src/main/assets.

Here is a sample code for loading and displaying PNG image:

SDL_Surface *loadedSurface = IMG_Load("smiley.png");
SDL_Texture *smileyTexture = SDL_CreateTextureFromSurface(renderer, loadedSurface);
SDL_RenderCopy(renderer, smileyTexture, NULL, NULL);

Result looks like this:

As you can see the PNG version of smiley has support for background transparency.

You can find the whole source code at https://github.com/georgik/sdl2-android-example.

We will discuss how to use further SDL2 libraries in next articles. You can find more under topic SDL2.

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