kivy.graphics.transformation.Matrix.view_clip() produces incorrect matrix

[DaytonaJohn]

The code producing the view_clip matrix has an incorrect value for matrix element 10. The kivy code sets element 10 to 2/(far-near), but numerous sources indicate that the correct value is -2/(far-near):

• songho
• Wikipedia
• scratchapixel

Versions

• Python: 3.6.5
• OS: Ubuntu 18.04
• Kivy: v1.11.0.dev0

Description

While trying to use an orthographic projection in the kivy example 3Drendering, I was unable to get satisfactory results. After lengthy investigation, I realized that the matrix produced by Matrix().view_clip() is incorrect when the perspective argument is 0.

Code and Logs

Simply changing the perspective argument in the line:

    proj = Matrix().view_clip(-asp, asp, -1, 1, 1, 100, 1)

from 1 to 0 in update_glsl() in main.py, results in a black screen (no obj displayed). Correcting the element in question with the lines below:

    proj = Matrix().view_clip(-asp, asp, -1, 1, 1, 100, 0)
    proj[10] = -proj[10]

produces the correct result.