Best way to call c++ methods in visualizers #1291

ahrm · 2025-06-08T12:38:04Z

ahrm
Jun 8, 2025

I am trying to write a python visualizer for QPixmap images. In the visualize I need to call methods on the c++ object passed to the visualizer. Currently I have it working using a combination of GetName and EvaluateExpression. However I am wondering if there is a better way to do it. For example is there a way to directly call a c++ method on a codelldb.value.Value? Here is my current working code:

import io
import lldb
import debugger
import base64
import matplotlib
import time
matplotlib.use('agg')
import matplotlib.pyplot as plt

def show_pixmap(pixmap, column=2):
    import numpy as np

    target = lldb.debugger.GetSelectedTarget()

    pixmap_name = pixmap.unwrap(pixmap).GetName()
    process = target.GetProcess()
    thread = process.GetSelectedThread()
    frame = thread.GetSelectedFrame()
    
    image = frame.EvaluateExpression(f"{pixmap_name}.toImage()")
    image_width = frame.EvaluateExpression(f"{image.GetName()}.width()").GetValueAsSigned()
    image_height = frame.EvaluateExpression(f"{image.GetName()}.height()").GetValueAsSigned()

    image_data = frame.EvaluateExpression(f"{image.GetName()}.constBits()").GetValueAsUnsigned()
    bytes_per_line = frame.EvaluateExpression(f"{image.GetName()}.bytesPerLine()").GetValueAsSigned()
    image_size = image_height * bytes_per_line

    memory = process.ReadMemory(image_data, image_size, lldb.SBError())

    image_array = np.frombuffer(memory, dtype=np.uint8)
    image_array = image_array.reshape((image_height, bytes_per_line // 4, 4))
    image_array = image_array[:, :, :3]

    plt.imshow(image_array)
    image_bytes = io.BytesIO()
    plt.savefig(image_bytes, format='png')
    document = '<html><img src="data:image/png;base64,%s"></html>' % base64.b64encode(image_bytes.getvalue()).decode('utf-8')
    debugger.create_webview(document, view_column=column)


    return str(image_array.shape)

Overall any comments on the things that I am doing wrong/could be doing better is greatly appreciated.

Also I was wondering if it it is possible to reuse a webview? For example when the value of a variable is updated, I don't want to create a new webview, I just want to update the webview corresponding to that variable. I guess it is possible to do so by keeping track of already created webviews corresponding to variables in the python file, but I was wondering if there is a better solution?

Answered by vadimcn

Jun 8, 2025

I don't think you can do much better than this. You could avoid some overheads associated with compiling C++ expressions by reading data directly from private fields of the pixmap object, but I'm not sure if that's worth it.

Regarding webviews: yes, if you want to modify contents in-place, you'll need to keep track of them yourself.

View full answer

vadimcn · 2025-06-08T18:12:43Z

vadimcn
Jun 8, 2025
Maintainer

I don't think you can do much better than this. You could avoid some overheads associated with compiling C++ expressions by reading data directly from private fields of the pixmap object, but I'm not sure if that's worth it.

Regarding webviews: yes, if you want to modify contents in-place, you'll need to keep track of them yourself.

0 replies

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

Best way to call c++ methods in visualizers #1291

Uh oh!

{{title}}

Uh oh!

Uh oh!

{{editor}}'s edit

{{editor}}'s edit

Uh oh!

Replies: 1 comment

Uh oh!

{{title}}

Uh oh!

Select a reply

Uh oh!

Best way to call c++ methods in visualizers #1291

Uh oh!

Uh oh!

ahrm Jun 8, 2025

Replies: 1 comment

Uh oh!

vadimcn Jun 8, 2025 Maintainer

ahrm
Jun 8, 2025

vadimcn
Jun 8, 2025
Maintainer