title: How to calculate how much video memory or memory cost by a texture
body: <p>I want to know how to calculate how much video memory or memory cost by a texture. for example, how much video memory cost by TGA or DDS or PNG or JPEG. Are the same?</p>
  
  <p>If there are the same, why PVRTC cost less video memory than PNG in IOS.</p>
  
  <p>I know this is such a big topic that can fill a book. So does anyone can give some references for this topic after brief introduce. Thanks in advance.</p>