Working texture compression. See https://discourse.libcinder.org/t/opengl-extensions-glew-glee-gloader/1195 . Also read: https://www.oldunreal.com/editing/s3tc/ARB_texture_compression.pdf

CompressedTextureProblemApp.cpp

#include "cinder/Log.h"
#include "cinder/app/App.h"
#include "cinder/app/RendererGl.h"
#include "cinder/gl/gl.h"

using namespace ci;
using namespace ci::app;
using namespace std;

 void APIENTRY logDebug( GLenum source​, GLenum type​, GLuint id​, GLenum severity​, GLsizei length​, const GLchar* message​, const void* userParam​ )
 {
   CI_LOG_E(std::string( message​ ));
 }

 class CompressedTextureProblemApp : public App {
   public:
     void setup() override;
     void draw() override;

     gl::Texture2dRef mTexture;
 };

 void CompressedTextureProblemApp::setup()
 {
     glEnable( GL_DEBUG_OUTPUT );
     glDebugMessageCallback( logDebug, this );

     // Create the data for a simple gradient.
     const int width = 256;
     const int height = 256;
     const int bytesPerPixel = 3;

     std::vector<uint8_t> pixels;
     pixels.resize( width * height * bytesPerPixel );

     for( int y = 0; y < height; ++y ) {
         memset( pixels.data() + y * width * bytesPerPixel, uint8_t( y ), width * bytesPerPixel );
     }

     // Create texture. Use default format.
     mTexture = gl::Texture2d::create( width, height );

     // Bind texture.
     gl::ScopedTextureBind stb( mTexture );

     // Upload uncompressed data. OpenGL will compress the data for us.
     glTexImage2D( mTexture->getTarget(), 0, GL_COMPRESSED_RGB_ARB, width, height, 0, GL_BGR_EXT, GL_UNSIGNED_BYTE, pixels.data() );

     // Find out if our texture was compressed.
     GLint isCompressed = GL_FALSE;
     glGetTexLevelParameteriv( mTexture->getTarget(), 0, GL_TEXTURE_COMPRESSED_ARB, &isCompressed );

     if( isCompressed == GL_TRUE ) {
         // Find out the compressed format used.
         GLint internalFormat = 0;
         glGetTexLevelParameteriv( mTexture->getTarget(), 0, GL_TEXTURE_INTERNAL_FORMAT, &internalFormat );

         // Retrieve the compressed image size in bytes.
         GLint imageSize = 0;
         glGetTexLevelParameteriv( mTexture->getTarget(), 0, GL_TEXTURE_COMPRESSED_IMAGE_SIZE, &imageSize );

         // Obtain the image data.
         std::vector<uint8_t> image;
         image.resize( imageSize, 0 );

         glGetCompressedTexImage( mTexture->getTarget(), 0, image.data() );

         // TODO: write file to disk.
     }
 }

 void CompressedTextureProblemApp::draw()
 {
     gl::clear( Color( 0.4f, 0.6f, 0.8f ) );

     if( mTexture ) {
         gl::draw( mTexture );
     }
 }

 CINDER_APP( CompressedTextureProblemApp, RendererGl )