Painting glass & liquid
Painting transparent materials like glass and liquid is a challenge that can be both scary and exciting. When you look at a glass that is filled with your favorite drink you notice many reflections and effects that occur on its surface. Sometimes they seem to be even more whimsical in reality than what you would come up with from your imagination. As you turn the glass in your hand and see how the reflections shift across the surface you probably ask yourself, where would I start if I wanted to paint this?
In this tutorial I am going answer this question for you. I will show you an approach that will create a reliable result that will impress your client or art director. You can either use the accompanying PSD and the layers I prepared, or you can come up with your own creation.
Our final result
Creating a drawing
A great painting begins well planned. In this approach we start with a neat and clean drawing. There are a few things to keep in mind when planning out our piece: the liquid inside the glass will visually extend to the border of the glass at both sides. Only where the glass is really thick (in this case at the bottom), will you be able see the thickness of the glass itself. This is due to the way light is bent when entering a transparent medium other than air. There are kinks and bends in the straw where it enters the liquid and where it is behind the glass. This is due to the same effect.
The effect that causes light to bend when entering a different medium is called refraction. Each transparent material has its own refraction index.
Layer masks and color composition
Create layers for each element or feel free to use the layers from the accompanying PSD. Lock their transparency. Then, create a new one on top of each mask layer and turn each of them into clipping masks. Use the new layers to assign your material colors. My advice is to stay within a color range in order to create a compelling piece. In my case I went for a range from orange to red and then to purple. Only the lemon slice stands out with its yellow-greenish color. Don‘t worry about the glass layer, it will stay invisible for now and we will mostly use it to quick-select the area in order to add shadows and reflection.
Having assigned the colors to our layers will give us a foreshadowing of how our final glass will look.
Add a new clipping mask to each layer and set it to multiply. Because the glass itself is transparent, ctrl-click its layer, turn it to invisible, and use the selection for painting the shadow on a new and separate layer that is placed at the very top. As I don‘t want to introduce any new colors at this point, I use a vibrant purple similar to the drink‘s color. 50% opacity for each layer worked well in my case but feel free to experiment.
Use some reference images as inspiration or put a glass of water on your desk. This will help you to create convincing shadows.
Adding light and reflection
Repeat step 3 and create new layers as clipping masks. But this time turn them to screen, 50% again, and as a color we use white to paint your primary reflections. Keep in mind that the glass itself reflects both on the inside as well as on the outside. While the lemon and the straw can have a soft transition, the reflection on the glass should have a hard edge in order to give a shiny impression.
The desaturated white reflection indicates that it is light that did not have to pass through the liquid before reaching the viewer‘s eye.
Adding a back light
In order to create a deeper sense of volume, I add a second light. For this repeat the last step, create new layers as clipping masks and set them to screen again. But this time use a vibrant light blue. I made a soft transition for all materials but you can also give the glass and the liquid a hard edge if you want to emphasize their reflectivity.
Adding a second light makes things more interesting.
At this point the reflection looks nice but our glass still does not really pop out from the canvas. In order to change that, add a layer on top and paint the highlights in white. At this point I am also worried a bit about the liquid being too dark, so I add an overlay layer and paint in some red in order to create a sense of translucency.
Adding highlights will make the glass jump out into the viewer‘s eye.
Now that we have prepared everything carefully, we reach my favorite step. Turn off the layer with the lines, create a new one at the very top, take your favorite brush and use the color-picker to start shifting around the colors. Most colors you need should be on your canvas by now. Since the glass itself is still transparent I have an easy time to add some additional background.
Not every area needs the same amount of detail. I like to keep my focus on the drink itself, add some bubbles and some swirly effects in order to give some personality.
Effects and final touch ups
Once you are happy with your painting you can add some final touch ups. I now use the hidden glass layer, turn it back on and while setting its fill to 0%, I add an inner glow to create the so-called Fresnel effect. This will pull the last straw and really make the drink stand out from the background. If you group this layer by ctrl+G and add a layer mask to the group, you can mask out the effect around the straw and the lemon. It‘s a small trick that will save you from having to paint on top and fix it manually. I also add some sparkling highlights, and a reflection of the liquid on the ground. A final touch up is the bokeh effect in the background.
Play around and add some final effects. Maybe you can even come up with some of your own.
Make sure to check out Gregor's other tutorial "Paint a fantasy character concept with simple renderpasses"