Pillow rendering multiline text for typewriter overlay

by

Summary

The issue at hand is rendering multiline text for a typewriter overlay using Pillow, a Python imaging library, to input into ffmpeg. The current implementation fails to correctly render multiline text, resulting in an error when trying to measure the length of the text.

Root Cause

The root cause of the issue is that the textlength method in Pillow’s ImageDraw module does not support measuring the length of multiline text. This is because the method is designed to work with single-line text only.

Why This Happens in Real Systems

This issue occurs in real systems due to the following reasons:

  • Insufficient handling of multiline text: Many libraries and frameworks, including Pillow, do not provide built-in support for handling multiline text.
  • Variable font sizes and page widths: Different font sizes and page widths can affect the rendering of text, making it challenging to determine the position of the latest word.
  • Lack of cursor placement logic: The current implementation does not have a robust logic for placing the cursor in the correct position, especially when dealing with multiline text.

Real-World Impact

The impact of this issue is significant, as it affects the overall user experience and the quality of the output. Some of the consequences include:

  • Incorrect rendering of text: Multiline text is not rendered correctly, leading to a poor user experience.
  • Inaccurate cursor placement: The cursor is not placed in the correct position, which can be frustrating for users.
  • Limited flexibility: The current implementation does not provide the flexibility to handle different font sizes, page widths, and other variables that can affect text rendering.

Example or Code

from PIL import Image, ImageDraw, ImageFont

def render_typewriter_frames(text: str, out_dir: str, width: int, height: int, fps: int = 25, duration_s: float = 10.0, chars_per_s: float = 18.0, font_path: str = "/usr/share/fonts/truetype/dejavu/DejaVuSans.ttf", font_size: int = 44, margin: int = 80):
    font = ImageFont.truetype(font_path, font_size)
    lines = text.split('\n')
    total_frames = int(duration_s * fps)
    for i in range(total_frames):
        t = i / fps
        n_chars = min(len(text), int(t * chars_per_s))
        reveal = text[:n_chars]
        img = Image.new("RGBA", (width, height), (0, 0, 0, 0))
        d = ImageDraw.Draw(img)
        x, y = margin, margin
        for line in lines:
            if reveal.startswith(line):
                d.text((x, y), line, font=font, fill=(255, 255, 255, 255))
                y += font_size
            else:
                break
        # blinking cursor if not finished
        if n_chars < len(text) and (int(t * 2) % 2 == 0):
            tw = d.textlength(reveal, font=font)
            d.text((x + tw + 2, y - font_size), "|", font=font, fill=(255, 255, 255, 255))
        img.save(out_dir + f"/tw_{i:05d}.png")

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Implementing a robust text rendering logic: They develop a logic that can handle multiline text, variable font sizes, and page widths.
  • Using a suitable library or framework: They choose a library or framework that provides built-in support for handling multiline text, such as Pygame or Pyglet.
  • Developing a custom cursor placement algorithm: They create an algorithm that can accurately place the cursor in the correct position, taking into account the latest word and font size.

Why Juniors Miss It

Juniors may miss this issue due to:

  • Lack of experience with text rendering: They may not have worked with text rendering before and are unaware of the challenges associated with multiline text.
  • Insufficient understanding of library limitations: They may not be familiar with the limitations of the library or framework they are using, such as Pillow’s lack of support for multiline text.
  • Inadequate testing: They may not have thoroughly tested their implementation, which can lead to incorrect rendering of text and inaccurate cursor placement.

Leave a Comment