OpenCV中void CannyThreshold(int, void*)函数声明的疑问

Great question—this is a super common point of confusion when starting with OpenCV's interactive features, so let's unpack it step by step.

It's All About OpenCV's Callback Function Requirements #

The void CannyThreshold(int, void*) signature isn't arbitrary—it matches the mandatory callback function format that OpenCV expects for controls like trackbars (created with createTrackbar()).

Here's the breakdown:

• The first int parameter is meant to receive the current position of a trackbar slider (if you use one to adjust Canny thresholds, for example).
• The void* parameter lets you pass custom user data to the callback (like a pointer to your image, a threshold struct, etc.).

Even if your current implementation doesn't use these parameters, the function must have this exact signature to work with OpenCV's callback system. If you declared it as void CannyThreshold(), you'd get a compile error if you tried to hook it up to a trackbar later, because the function signature wouldn't match what OpenCV expects to call.

Why Call It With CannyThreshold(0, 0) in main()? #

That initial call is just a way to trigger the edge detection logic once at program startup, using default values. Since the function expects two arguments (even if it ignores them), you pass 0 for the trackbar position (a reasonable default) and 0 (or NULL) for the user data. It's just a way to satisfy the function's signature while running your initialization logic.

Could You Use a No-Parameter Function Instead? #

Technically, yes—if you never plan to use interactive controls like trackbars. But the tutorial uses this signature for two key reasons:

1. Forward compatibility: If you later decide to add a trackbar to adjust Canny's high/low thresholds, you can directly hook this function up to createTrackbar() without changing its signature.
2. Following OpenCV conventions: Most OpenCV example code uses this callback pattern, so it's good to get familiar with it early—it'll show up again in other interactive features like mouse callbacks.

For context, here's how you'd use this function with a trackbar if you wanted to add interactivity later:

int threshold_val = 100;
const int max_threshold = 255;
createTrackbar("Canny Threshold", "Output Window", &threshold_val, max_threshold, CannyThreshold);

Now, whenever you drag the trackbar, OpenCV automatically passes the current slider position to the int parameter of CannyThreshold().

内容的提问来源于stack exchange，提问作者eirikaso