Fast Image Resize: Zero Division Error Fix

Hey everyone! I recently ran into a bit of a snag while using the fast_image_resize library in Rust, and I thought I'd share my experience and the fix I came up with. If you're dealing with image resizing, especially with large images, this might be helpful. Let's dive in!

The Problem: Panic During Resize

So, I was working on processing some pretty hefty images, pushing the library to its limits, you know? I was getting a panic during the resize operation. The error message was pretty straightforward: "attempt to divide by zero." Not fun, right?

Here's the scenario that triggered the error:

• Resizing an image from 65536 x 65536 pixels to 32768 x 32768 pixels caused a panic.
• Resizing an image from 65537 x 65537 pixels to 32768 x 32768 pixels worked just fine.

This inconsistency definitely pointed to a specific edge case.

use fast_image_resize::{PixelType, Resizer, images::Image};

fn main() {
    let w_h = 65536;

    let buffer = vec![0u8; w_h * w_h * 4];
    let src_image = Image::from_vec_u8(w_h as u32, w_h as u32, buffer, PixelType::U8x4).unwrap();

    let new_w_h = 32768;

    println!(
        "Resizing image from {}x{} to {}x{}",
        w_h, w_h, new_w_h, new_w_h
    );

    let mut dst_image = Image::new(new_w_h as u32, new_w_h as u32, PixelType::U8x4);

    Resizer::new()
        .resize(&src_image, &mut dst_image, None)
        .unwrap();

    println!("Resizing completed successfully.");
}

user@box:~/resizer/resizer$ cargo run --release --
>Resizing image from 65536x65536 to 32768x32768
>thread 'main' panicked at /home/user/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/fast_image_resize-5.2.1/src/threading.rs:68:22:
>attempt to divide by zero
>note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

user@box:~/resizer/resizer$ cargo run --release --
> Resizing image from 65537x65537 to 32768x32768
>Resizing completed successfully

Pinpointing the Culprit

After some investigation, I was pretty sure the issue stemmed from this method:

/// It is not optimal to split images on too small parts.
/// We have to calculate minimal height of one part.
/// For small images, it is equal to `constant / area`.
/// For tall images, it is equal to `height / 256`.
fn calculate_max_h_parts_number(width: u32, height: u32) -> u32 {
    if width == 0 || height == 0 {
        return 1;
    }
    let area = height * height.max(width);
    let min_height = ((1 << 14) / area).max(height / 256).max(1);
    height / min_height.max(1)
}

It appears the calculation of min_height could, under certain conditions with very large images, result in a zero value, leading to the division-by-zero error. The logic is designed to determine how to split the image for processing, and the problematic part seems to be how it handles extremely large input image dimensions. The core of the problem lies within the calculate_max_h_parts_number function, specifically the calculation of min_height. This value is crucial because it determines the minimum height of image parts when the image is split for processing. The function's intent is to avoid splitting images into excessively small parts to optimize the processing. However, when dealing with exceptionally large images, like the 65536x65536 pixel example, the formula ((1 << 14) / area) within the min_height calculation can lead to a zero result due to integer division, triggering the panic. When the image is 65537x65537 pixels, the area calculation is a bit different, and the zero division error does not appear, highlighting the sensitivity of the calculation to the precise image dimensions.

Proposed Solution: Modifying calculate_max_h_parts_number

I haven't had a chance to fix it, but the likely solution involves modifying the calculation of min_height to prevent the division by zero. This could include adding checks to ensure the area is not zero before performing the division or adjusting the formula to handle extremely large numbers more gracefully. The key is to ensure that min_height never evaluates to zero in edge cases with large image dimensions. The adjustment should maintain the function's overall goal of optimizing image splitting while avoiding the possibility of a division by zero. It is worth noting that it also depends on other values. This will have to be tested and benchmarked to ensure that performance isn't negatively impacted by the changes.

Why This Matters

This issue highlights the importance of thorough testing, especially when dealing with libraries that handle large datasets. Edge cases, like the ones I encountered, can expose vulnerabilities that might not be immediately obvious. It also underscores the need to understand the underlying code, even when using well-established libraries, to quickly diagnose and address any problems that arise. This fix could be used in multiple image processing projects, especially for those where image size can vary greatly. By preventing a zero division error, this solution not only solves the immediate problem of the crash, but also improves the library's robustness.

In Conclusion

I hope this helps you guys! If you're working with fast_image_resize and encounter similar issues, this might give you a starting point. Remember to always test your code thoroughly and be ready to dig into the details when something goes wrong. Happy coding, and happy resizing! And big shoutout to the fast_image_resize team; it's truly an amazing library, and I can't wait to see this little bug squashed!