This is an example how to compress textures via OpenGL. No software compression, everything is done on the hardware by the GPU. No special OpenGL version, all you need is OpenGL 3.3 core profile (or later) and GL_EXT_texture_compression_rgtc + GL_EXT_texture_compression_s3tc extensions.

The compression is done in src/Compressor.cpp file. This works by rendering the source image into a framebuffer, then copying the pixels into a destination texture while specifying the target format (for example DXT5). You can also use the glGetCompressedTexImage function to get the compressed pixels (maybe save them to a file?). See the compress function for more details.

It works like this:

• Load the source image file into pixel array.
• Create a "source" texture from the source image pixels, in any format.
• Create an empty "fboColor" texture.
• Create a framebuffer object.
• Optionally attach a depth buffer to the framebuffer object (might be needed on some hardware).
• For each mimap level:
  • Set the "fboColor" texture storage via glTexImage2D with no source data (last parameter is NULL).
  • Attach the "fboColor" texture to the framebuffer with the mipmap level.
  • Bind and render the "source" texture.
• Create an empty "destination" texture.
• For each mimap level:
  • Attach the "fboColor" texture to the framebuffer with the mipmap level.
  • Bind the "destination" texture.
  • Call glCopyTexImage2D with DXT or RGTC format (3rd param).
  • Optionally call glGetTexLevelParameteriv with GL_TEXTURE_COMPRESSED_IMAGE_SIZE to get the size of the current mimap level (in bytes).
  • Optionally call glGetCompressedTexImage to get the raw compressed pixels.
• Delete framebuffer, renderbuffer, "source" texture, "fboColor" texture.
• Update the "destination" texture max and min mipmap levels via glTexParameteri (the GL_TEXTURE_BASE_LEVEL and GL_TEXTURE_MAX_LEVEL).
• Keep the "destination" texture.

Sample image lena.png was converted from lena.tiff downloaded from http://eeweb.poly.edu/~yao/EL5123/SampleData.html

Compile the application via CMake and vcpkg (steps below). Run the TextureCompression.exe executable. Press spacebar on your keyboard to switch between compression types.

• Make sure you have vcpkg installed and integrated.
• Install glfw3 glm glad vcpkg packages.
• Build the project with vcpkg toolchain.

# Vcpkg dependencies
vcpkg install --triplet x64-windows glfw3 glm glad

# Clone
git clone https://github.com/matusnovak/texture-compression.git
cd texture-compression

# Create build dir and configure
mkdir build
cd build
cmake \
  -G "Visual Studio 16 2019" \
  -DCMAKE_TOOLCHAIN_FILE=C:/tools/vcpkg/scripts/buildsystems/vcpkg.cmake \
  ..

# Build it
cmake --build .

# Run it
./Debug/TextureCompression.exe