PyOpenGL

Quick Guide

Parameter	Type	Description
driver	cui.Windower	Currently only cui.GLFW is supported

Introduction

PyOpenGLBackend is CoshUI's backend for raw OpenGL rendering via PyOpenGL, and serves as an alternative to ModernGLBackend for projects that prefer PyOpenGL's more direct, classic OpenGL-style API over ModernGL's higher-level wrapper.

Setup

import glfw
import coshui as cui

glfw.init()
window = glfw.create_window(800, 600, "My App", None, None)
glfw.make_context_current(window)

# Created once, outside the loop, since shader compilation is expensive
backend = cui.PyOpenGLBackend(cui.GLFW)

while not glfw.window_should_close(window):
    glfw.poll_events()
    with cui.CoshUIRenderer(backend):
        cui.Label(id="hello", text="Hello, PyOpenGL!")
    glfw.swap_buffers(window)

Unlike ModernGLBackend, PyOpenGLBackend doesn't take a context object as a parameter. PyOpenGL operates directly on whichever OpenGL context is currently active on the calling thread, so as long as glfw.make_context_current() (or equivalent) has been called beforehand, PyOpenGL will use it automatically.

As with ModernGLBackend, construct PyOpenGLBackend once outside your render loop — its constructor compiles and links all of CoshUI's shaders, and doing that every frame would be wasteful and slow.

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Windowers

Currently, PyOpenGLBackend only supports cui.GLFW as a windower. The GLFW driver handles reporting window size and polling mouse input, both called automatically by CoshUI each frame — you don't need to call these yourself.

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How Rendering Works

PyOpenGLBackend.flush() walks the render stack and dispatches each RenderContext to _draw_rect, _draw_text, or _draw_image, mirroring ModernGLBackend's structure closely but using PyOpenGL's lower-level, function-based API (glGenBuffers, glBufferData, glDrawArrays, and so on) instead of ModernGL's object-oriented wrappers.

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Rects and Borders

Like ModernGLBackend, rects are drawn using the same SDF-based rect.frag shader, giving anti-aliased rounded corners and asymmetric border radii support entirely on the GPU. The vertex and fragment shader source files are shared between both GL backends.

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Text

Text rendering uses the same glyph atlas system as ModernGLBackend (gl_font.py), generating a 512x512 texture atlas per unique (font_path, font_size) pair using freetype-py. The key difference is purely in how the texture and buffers are created and bound — using raw glGenTextures/glTexImage2D calls instead of ModernGL's context.texture() wrapper.

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Images

Images are loaded with Pillow and uploaded via glTexImage2D, cached by file path so repeated draws of the same image don't re-upload the texture.

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Clipping

Clipping uses the OpenGL scissor test via glScissor(), with the same top-left-to-bottom-left coordinate flip as ModernGLBackend to account for OpenGL's coordinate system.

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Input Polling

PyOpenGLBackend.poll_input() delegates to self.driver._poll_input(), same as ModernGLBackend. Since only GLFW is supported here, input is always read via glfw.get_cursor_pos() and glfw.get_mouse_button().