Most usual methods have the following drawbacks:
- The glass seems milky or smoky
- Forgotten totally that glass affects to the background like a bizarre lens
Check the following and go on reading, if interested:
This method covers milkiness, smokiness and effect to the background.
The starting point is same: A photo of the bottle taken against homogenous greyish background and adjusted for good and realistic contrast & colour.
First steps also are same: Remove the background outside the bottle by using available tools. Separate the transparent parts (=glass) and opague parts (=the cap) to two different layers, the cap to the foreground. Actually there is no need to delete the cap from the glass layer, because it will be hidden.
Background removal can be a challenge, because there is little contrast on many places at the border of the glass. If you feel comfort with clipping paths or lasso tools, then there's no problem. Making a high contrast copy of the glass layer to the foreground helps.
After removing the background outside the bottle and separating the layers, desaturate the glass layer. If the glass is coloured, desaturate it anyway. You can recolorize it later.
Make a copy of the glass layer. Rename it as "Patterns".
Make to the background a new, flat mid-grey layer. This will be the reference. PhotoShop's chessboard is no good for reliable adjustments. Rename it as "Reference"
Now you should have the following sandwich of layers:
- Patterns (=a copy of Glass)
- Glass (= glass parts, desaturated)
Select layer Patterns and take the Curves tool. Deform the luminosity curve until you have created a high, exaggerated contrast. The darkest parts should be black or nearly black and the gloss should be full white. Between then there are several shades of grey.
Select all glossy white areas by using magic wand or "select by colour". Copy the glossy areas into the clipboard and paste them as a new layer just under the layer Cap. Use paste special/paste in place to avoid any shift.
Now your layers are:
Make layers "Patterns" and "Glass" transparent by decreasing their opacity. Proper opacity values to start are about 10 %. Adjust the precense of the glass by adjusting the opacity of Patterns. Adjust the general transparency of the glass by adjusting the opacity of Glass.
If the glass should be milky or smokey, then increase the opacity . Or, should it be clear, then decrease the opacity even to 0%. Adjust the brightness of layer Glass to reach the wanted shade of milkiness or smokiness. If the glass material is colored, then you must colorize layer Glass.
The opacity of layer Gloss must be decreased, if the bottle is not wanted to look out as staying in bright light.
If you have the final new background, then insert it as a new layer under Glass. Proper opacity values can be can be found more accurately.
Some dark areas in Patterns may look out too offensive, but the opacity cannot be smaller because the general precense suffers. In that case use Eraser in 30 % mode to thin the offending areas.
Remove layer Reference when it's not needed . The next step is to generate the lens effect of the glass. It can be done only to the final new background.
Inset the new background, go to layer Glass and select all glass parts of the bottle. With that selection you can modify the background.
Go to the background layer. If you are a genius, you can create the lens effect by tastefully using tools Smudge and Warp. Less talented geezers, like me, must stick with heavy faking. Believe, some preset distortion effect in PhotoShop, for example "Spherize" is often enough. Apply it to the selected area of the background and you're done! The full mathematical falsity does not prevent the right illusion.
Need a shadow? Try a slightly blurred drop shadow, if needed. Erase a great part of it to prevent the glass seem smoky.