Debugging OpenWebUI’s Hidden Image Processing Limits: A Technical Deep Dive

Get the final OWUI pipe here:

https://openwebui.com/f/rabbithole/replicate_custom_model_owui_pipe

When integrating image generation capabilities into OpenWebUI using Replicate.com’s API, I encountered an interesting challenge that led to the discovery of undocumented image processing limits. This post details the journey of debugging and solving this issue, which might help others working with similar integrations.

The Initial Problem

I was developing a custom image generation pipe for OpenWebUI that needed to handle various user inputs:

man sitting in a chair              # Default: 1440x1440, 1 image
16:9 man sitting in a chair        # Aspect ratio specified
x3 man sitting in a chair          # Multiple images requested
16:9 x3 man sitting in a chair     # Both aspect ratio and multiple images

Everything worked perfectly when generating single images or when specifying an aspect ratio. However, when requesting multiple images without an aspect ratio, I encountered a cryptic error:

Error: Generation incomplete after processing status

The Investigation Begins

The error was particularly puzzling because:

1. It only occurred when generating multiple images without a specified aspect ratio
2. The error message didn’t appear anywhere in OpenWebUI’s source code
3. Replicate.com’s logs showed successful image generation

Breaking Down the Problem

Through systematic testing, I discovered some interesting patterns:

1. 16:9 x3 man sitting in a chair worked perfectly
2. x3 man sitting in a chair failed
3. 1:1 x3 man sitting in a chair also failed

This was the first breakthrough – the issue wasn’t with the number of images per se, but something specific about square (1:1) images when generating multiple copies.

The Megapixel Mystery

I started analyzing the image dimensions:

# For 1440x1440 images:
1 image  = 2.07 MP                 # Works
2 images = 4.14 MP total           # Works
3 images = 6.21 MP total           # Fails
4 images = 8.28 MP total           # Fails

# For 1440x810 (16:9) images:
3 images = 3.49 MP total           # Works
4 images = 4.66 MP total           # Works

Then came the second breakthrough – by reducing square images to 1024×1024:

3 images at 1024x1024 = 3.15 MP total  # Works!
4 images at 1088x1088 = 4.73 MP total  # Fails
4 images at 1024x1024 = 4.19 MP total  # Works!

The Solution

Through careful testing, I found that OpenWebUI has an undocumented limit of approximately 4.7 megapixels for the total output of all images combined. This led to the development of a dynamic scaling solution:

def calculate_dimensions(
    self,
    aspect_ratio,
    num_outputs,
    max_total_mp=4.7,
    base_dimension=1440
):
    """
    Calculate image dimensions based on aspect ratio and number of outputs,
    ensuring total megapixels stays under max_total_mp.
    """
    width_ratio, height_ratio = aspect_ratio

    # Calculate initial dimensions maintaining aspect ratio
    if width_ratio > height_ratio:
        initial_width = base_dimension
        initial_height = int(base_dimension / width_ratio * height_ratio)
    else:
        initial_height = base_dimension
        initial_width = int(base_dimension / height_ratio * width_ratio)

    # Calculate megapixels for these dimensions
    mp_per_image = (initial_width * initial_height) / 1_000_000
    total_mp = mp_per_image * num_outputs

    # If we're under the limit, use these dimensions
    if total_mp <= max_total_mp:
        return initial_width, initial_height

    # If we're over the limit, scale down
    scale_factor = (max_total_mp / num_outputs / mp_per_image) ** 0.5
    new_width = int(initial_width * scale_factor)
    new_height = int(initial_height * scale_factor)

    return new_width, new_height

This solution automatically scales image dimensions based on:

• The requested aspect ratio
• The number of images
• OpenWebUI’s total megapixel limit

The Final Implementation

To make this solution configurable, I added a valve to the pipe configuration:

class Valves(BaseModel):
    OWUI_TOTAL_MEGAPIXEL_LIMIT: float = Field(
        default=4.7,
        description="The maximum megapixels OpenWebUI can handle..."
    )

Now the system dynamically handles all cases:

• Single images at full resolution (up to 1440×1440)
• Multiple images with automatic scaling to stay under the 4.7MP limit
• Any aspect ratio while maintaining proper proportions

Lessons Learned

1. Sometimes system limits aren’t in the obvious places – OpenWebUI’s image processing limit wasn’t documented or visible in the source code
2. When debugging image processing issues, consider total memory/processing requirements, not just individual image sizes
3. The error “Generation incomplete after processing status” was actually a clue – it indicated a processing limitation rather than a generation issue

Conclusion

This investigation revealed an important but undocumented limitation in OpenWebUI’s image processing capabilities. By understanding and working around this limit, we can now reliably generate multiple images while maintaining the highest possible quality within the system’s constraints.

The solution is now implemented as a configurable component that automatically handles all edge cases, making it robust for production use. The key is the dynamic scaling system that maintains aspect ratios while ensuring the total output stays within OpenWebUI’s processing capabilities.