Introduction
In this guide, I will explain how to accurately align 3D objects in your scene with a perspective camera. While this may seem challenging when working alongside other objects in the same scene, it is important to consider whether absolute alignment of 3D objects is necessary. For instance, for text or flat images, utilizing HTML with CSS can be a simpler and more manageable approach.
An alternative method, not discussed here, involves rendering items using an orthographic camera. This setup is straightforward due to the simplicity of orthographic math, and you can find numerous examples (1, 2) online for reference. Using an orthographic camera might be the ideal solution for your needs.
My personal project required creating a 3D visor for a game I am developing. I needed the player to have the ability to remove the visor while ensuring dynamic alignment with screen resolution and the flexibility to align objects to any edge of the screen, which I will discuss further.
Key Assumptions
This tutorial assumes that your objects will not move farther away from the camera spontaneously (if they do, they will move towards the center of the canvas). Achieving precise alignment requires complex calculations based on the geometry of a cone, which is beyond the scope of this discussion. Instead, I will provide constants that have proven effective in my workflow.
Step 1: Determine Ratios and Constants
Upon examining the screenshots below, under 1:1 resolution, the spaceship's dashboard appears at the bottom of the screen, while the windshield nearly reaches the top edge. This alignment remains consistent even at a widescreen 16:9 resolution. However, the cockpit's side view becomes less visible at a 1:1 ratio compared to 16:9.
This observation indicates that resolution changes primarily impact the x-axis (left-to-right) while leaving the y-axis (up/down) unaffected by resolution adjustments. In other words, the aspect ratio predominantly affects the x-axis in this scenario.
1:1 resolution:
https://i.sstatic.net/s0O1l.png
Widescreen resolution:
https://i.sstatic.net/1z2Dc.png
Step 2: Define the Center Point
To simplify alignment processes, establish a clear central point at 0, connecting your 3D objects directly to the camera. Assuming your object's origin is centered, this ensures automatic centering. If the object's origin deviates from the center, leverage a bounding box to enforce centering.
Note: If attaching objects to the camera is not ideal, consider attaching only during resizing operations and detaching afterwards. Since attachment does not alter world position, this workaround effectively maintains alignment.
Step 3: Establish Forward Axis
In 3D alignment, positioning objects in front of the viewer at a specified distance is crucial. For example, when designing a visor, place the objects approximately 5 centimeters from the player's face. To streamline the process, ensure that Blender models maintain a precisely measured longest edge of 1 meter, without variations like "at least" or "at most."
Step 4: Implementation Code
The following code assumes a standard unit length where 1 unit equates to 1 meter. This simplifies computations and maintains consistency, especially when referencing dimensions in tools like Blender (focusing on actual dimensions rather than scale).
I will present demonstrations showcasing center alignment, right alignment, and bottom-right alignment. These examples should provide sufficient guidance for extrapolating additional alignment scenarios by adjusting simple parameters such as changing the sign from - to + for top alignment instead of bottom alignment.
[Code snippets and implementation details provided]
Achieved real-world results are showcased below, featuring aligned 3D representations including a curved bar and bottom-right text elements.
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https://i.sstatic.net/oLuTg.gif