Problem in OGL Texture...

I see no problem in your screenshots. As the AP said, the banding in the first picture comes from the 16bit screen colour resolution. Older graphics cards dealt with that using dithering. But I don't think modern GPUs do that anymore, because 16bit desktop colour modes are effectively dead nowadays. They're only supported for legacy purposes, and should never be used in practice (32bit is both faster and a lot better looking).

I think this is a problem in software, in my software, any error in values in glTexImage2D or a OGL limitation...

Is this on NVIDIA hardware? If you create a texture and pass an unsized internal format into the glTexImage call, the default texture size depends on the desktop color depth. In other words, if you request GL_RGB, you will get RGB565 or RGB888 depending on the desktop. If you want to control this, you can give a hint by requesting a sized internal format, i.e. GL_RGB8 means you want 8 bits per component regardless of the desktop color depth.

The rendered image will still be dithered on a 16-bit desktop but the banding will be less apparent. The banding is obvious if you get a 16-bit internal format because the data is thrown away during the TexImage call, where the LSBs of each texel are truncated. Dither during rendering cannot recover this lost information.

The control panel includes a setting for the default texel size but its better to do this programmatically as I described.

Thanks for the info gold!