Why a transparent window needs a drawing order
A pixel on a transparent surface is usually made by blending its color with the color already in the image. A tinted window, for example, contributes some of its own color while letting the scene behind it show through. That means the engine must already have drawn what lies behind the window when it blends the window into the picture.
Opaque objects are simpler: their surfaces cover whatever is behind them, and a depth buffer—a record of the nearest surface drawn at each pixel—helps the engine reject hidden geometry. Engines typically draw opaque geometry first, building the background that transparent surfaces will blend with afterward.
Why transparent models are sorted back to front
For the blend to look right, a farther transparent surface generally needs to be drawn before a nearer one. The nearer surface then blends with the result, as light passing through one pane and then another would suggest. Drawing them in the opposite order can blend each surface against the wrong colors, producing a sudden visual change rather than a convincing layered view.
This is why engines commonly sort transparent models by distance from the camera and draw them from farthest to nearest. The sorting is a practical shortcut: it works well when objects are separate and their front-to-back relationship is clear.
Where the shortcut can break
A model is often sorted as one unit, even though different parts of it may sit at different depths. Two intersecting transparent objects can therefore have no single correct whole-object order. The engine may draw one model first, then the other, even where their surfaces cross. Some transparent surfaces also need special handling because writing their depth into the buffer can prevent later transparent layers from appearing.
The result is a familiar rendering tradeoff: sorting keeps common scenes efficient, but overlapping glass, smoke, or other layered effects can still look wrong. A window that seems to erase the view behind it is not necessarily hiding the scene in the game world; the engine may simply have blended the surfaces in the wrong sequence.
