Missing alpha channel in DXT5 texture on android device

Question

I'm porting mobile game to Android and want to use compressed textures in OpenGL the same way I did on iOS with PVR textures.

I've managed to convert my textures from PNG to DXT and run the game on Galaxy Tab 10.1 with Nvidia Tegra 2 chipset. However there were no smooth alpha in my DXT5 formatted textures. They looked like DXT1-textures with 1-bit alpha.

I've read and run the examples from here: http://software.intel.com/en-us/articles/android-texture-compression

I've tried this very good library: https://libregamewiki.org/Simple_OpenGL_Image_Library

But got same results. No alpha channel. Please, help me with this problem. I'm really stuck.

Thanks.

Details:

1. I've used nvcompress tool version 2.1.0 with flags "-nomips -bc3 -alpha" (and of cause a lot of variations but with no success).
2. I'm using OpenGL ES1 library.

3. My openGL code:

   int width = //...
   int height = //...
   const unsigned char* textureData = //...
   int numMipMaps = //...
   int format = //...
   
   GLuint texture = 0;
   glGenTextures(1, &texture);
   
   glBindTexture(GL_TEXTURE_2D, texture);
   glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
   glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
   
   int blockSize;
   GLuint format;
   switch (format) {
       case S3TC_DXT1:
           format = GL_COMPRESSED_RGBA_S3TC_DXT1_EXT;
           blockSize = 8;
           break;
       case S3TC_DXT3:
           format = GL_COMPRESSED_RGBA_S3TC_DXT3_EXT;
           blockSize = 16;
           break;
       case S3TC_DXT5:
           format = GL_COMPRESSED_RGBA_S3TC_DXT5_EXT;
           blockSize = 16;
           break;
       case ATC:
           format = GL_ATC_RGBA_EXPLICIT_ALPHA_AMD;
           blockSize = 16;
           break;
       default:
           //Error...
           break;
   }
   
   int offset = 0;
   for(int i = 0; i < numMipMaps; i++)
   {
       int size = ((width + 3) / 4) * ((height + 3) / 4) * blockSize;
       glCompressedTexImage2D(GL_TEXTURE_2D, i, format, width, height, 0, size, textureData + offset);
       offset += size;
   
       //Scale next level.
       width  /= 2;
       height /= 2;
   }
   

Answer 1

Finally found the problem. OpenGL state of my game was configured to work with premultiplied alpha-channel. I've added special 'premultiply' step to my build system and got proper result.

Blend-function settings:

glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);