Tips for aligning the camera to ensure the object maintains a consistent pixel width and height on the screen

I am grappling with a challenge that I'm unsure how to approach, hoping someone can offer me a clue on the solution.

My goal is to position the camera at a specific z index so that a cube appears on the screen with consistent pixel dimensions regardless of window size or aspect ratio. The cube itself is located at z position 0, requiring the camera to be positioned facing it from a certain distance.

The desired outcome is for users to always see the cube displayed with the exact same pixel width and height on their screens. To achieve this, I believe the camera's z position should be a calculated function based on window width, height, aspect ratio, and a constant value.

How do I determine A, B, C, and D for this scenario? This seems like a geometric problem, but I'm uncertain about how to tackle it. Should I include a constraint specifying that the object must match exactly 100 pixels wide and 100 pixels high?

var aspectRatio = window.innerWidth / window.innerHeight;
var camera = new PerspectiveCamera( 60.0, aspectRatio, 1.0, 10000.0 );

var A = 1.0;
var B = 1.0;
var C = 1.0;
var D = 1.0;
camera.position.z = A * window.innerWidth + B * window.innerHeight +
                    (C * aspectRatio) + D;
var geometry = new CubeGeometry( 100.0, 100.0, 0.0001 );

Update: I successfully resolved this issue through trial and error.

I lack comprehension of the geometry and mathematics involved in this problem, yet I discovered that the object's size was linked to the window's height rather than its width. Although I cannot explain why, adjusting the height changed the object's size while adjusting the width had no effect on it.

This observation led me to assume that the height plays a crucial role in determining the function. Through trial and error, I adjusted values until achieving the correct size of 100 by 100 pixels. Subsequently, altering the height confirmed the object remained consistently sized. I am thrilled to have achieved this result.

num A = 0.0;
num B = -0.867;
num C = 0.0;
num D = 0.0;

Answer №1

It seems in your scenario, the dependency is more likely related to the axis with the smaller window size! This is because aspect ratio equations typically vary based on whether:

  1. width > height
  2. width < height

Most rendering engines follow this behavior inspired by OpenGL, so perhaps adding an if condition to your code would make it complete. For confirmation, try resizing your window to have greater height than width and observe the outcome.

By the way, the mathematical concept behind this is quite straightforward, similar to triangle math like below:

One angle is 90 degrees and the second one is

atan (h1/z1) = atan (h0/z0)
h1/z1 = h0/z0   <- Triangle similarity
z1 = z0*h1/h0   <- The desired calculation

Where:
h0 represents half the size in the controlled axis (x or y)
h1 signifies half of the cube's size
z0 indicates the near plane of your frustum
z1 denotes the cube position (remember to factor in the offset to the center of the cube)

Therefore, the cube's central position can be determined as follows:

z1' = (z0*h1/h0)+h1

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