LVGL: Lv_scale Needles Repositioning Issue

by Alex Johnson 43 views

Introduction

In this article, we'll delve into a specific issue encountered in the LVGL (LittlevGL) library, a popular open-source graphics library for embedded systems. The problem arises when using the lv_scale widget, specifically concerning the repositioning of needles when the scale undergoes transformations. This issue is observed in LVGL version 9.4.0 and has been reproduced in a simulator environment. Understanding and addressing this behavior is crucial for developers aiming to create dynamic and visually accurate user interfaces with LVGL.

Understanding the Issue

The core of the problem lies in how lv_scale implements needles. The needle is essentially a line object added to the scale, with its points defining its position and orientation. These points are updated when the lv_scale_set_line_needle_value() function is called. However, if the scale undergoes any transformations after the needle value is set, the needle's position does not automatically update to reflect these transformations. This discrepancy leads to the needle appearing misaligned or detached from the scale, disrupting the visual integrity of the gauge or meter being displayed.

To put it simply, the needle's position is calculated and set at a specific moment. If the underlying scale's orientation, size, or position changes afterward, the needle remains in its original calculated position, failing to adapt to the new scale transformation. This behavior can be particularly problematic in dynamic interfaces where scales might be animated, resized, or rotated in response to user input or changing data.

Key Concepts

Before diving deeper, let's clarify some key concepts related to LVGL and the lv_scale widget:

  • LVGL (LittlevGL): A free and open-source graphics library providing everything you need to create embedded GUI with easy-to-use graphical elements, beautiful visual effects and low memory footprint. It is written in C for greater compatibility.
  • lv_scale: A widget in LVGL used to create scales, gauges, and meters. It allows developers to display values within a defined range using visual indicators like needles and ticks.
  • Needle: A visual element, typically a line or a shape, that indicates a specific value on the scale.
  • Transformation: Operations like rotation, scaling, and translation that alter the position, size, or orientation of an object.

Reproducing the Issue

To better understand the issue, let's examine the code snippet provided. This code demonstrates how the problem can be reproduced in a simulated environment. The code creates an lv_scale object, adds a line needle to it, and then attempts to animate the scale's size. The issue becomes apparent when the scale's size changes, but the needle remains fixed in its original position.

The provided code snippet effectively illustrates the problem. It sets up a basic LVGL environment, creates a scale with a line needle, and then uses an animation to modify the scale's size. The critical part is the set_needle_line_value function, which is intended to update the needle's position. However, as the animation progresses and the scale's size changes, the needle fails to follow, clearly demonstrating the issue of needles not repositioning correctly after scale transformations.

Code Breakdown

Let's break down the code snippet to understand the steps involved in reproducing the issue:

  1. Initialization:
    • lv_init(): Initializes the LVGL library.
    • hal_init(): Initializes the hardware abstraction layer (HAL), setting up the display with a resolution of 480x320.
  2. Scale Creation:
    • lv_scale_create(): Creates an lv_scale object on the active screen.
    • lv_obj_set_size(): Sets the initial size of the scale to 150x150 pixels.
    • lv_scale_set_mode(): Configures the scale's mode to LV_SCALE_MODE_ROUND_INNER, creating a circular scale.
    • Styling: Sets various styles for the scale, including background color, radius, and clipping.
    • lv_obj_align(): Aligns the scale to the left-middle of the screen with a small offset.
    • lv_scale_set_label_show(): Enables the display of labels on the scale.
    • Tick Configuration: Sets the total number of ticks, major tick interval, and range of the scale.
    • Angle and Rotation: Sets the angle range and rotation of the scale.
  3. Needle Creation:
    • lv_line_create(): Creates a line object to serve as the needle, adding it as a child of the scale.
    • Styling: Sets the line width and rounded style for the needle.
  4. Animation Setup:
    • lv_anim_init(): Initializes an animation structure.
    • lv_anim_set_var(): Sets the target object for the animation (the scale).
    • lv_anim_set_exec_cb(): Sets the animation execution callback function to set_needle_line_value.
    • lv_anim_set_duration(): Sets the animation duration to 1000 milliseconds.
    • lv_anim_set_repeat_count(): Sets the animation to repeat infinitely.
    • lv_anim_set_reverse_duration(): Sets the reverse animation duration to 1000 milliseconds.
    • lv_anim_set_values(): Sets the animation value range from 10 to 40.
    • lv_scale_set_line_needle_value(): Sets the initial needle value.
    • lv_anim_start(): Starts the animation.
  5. Main Loop:
    • lv_timer_handler(): Handles LVGL timers and tasks.
    • lv_delay_ms(): Introduces a delay to control the execution speed.

Observing the Issue

When this code is executed, the scale's size will animate, but the needle will not move accordingly. It will remain in its initial position, clearly demonstrating the problem of needles not repositioning when the scale is transformed. This visual discrepancy highlights the need for a solution to ensure that needles accurately reflect the scale's transformations.

Root Cause Analysis

The root cause of this issue lies in the way LVGL's lv_scale widget handles the needle's position. As mentioned earlier, the needle is implemented as a separate line object, and its position is calculated and set based on the scale's initial state. When the scale is transformed (e.g., resized, rotated), the needle's position is not automatically recalculated. This is because the lv_scale_set_line_needle_value() function, which updates the needle's position, is not called automatically whenever the scale undergoes a transformation.

Understanding the Code

Looking closely at the set_needle_line_value function in the code, we can see that it attempts to update the scale's width and height based on the input value v. However, it does not explicitly update the needle's position. This is where the problem lies. The animation is modifying the scale's size, but the needle's position remains unchanged because there is no mechanism in place to recalculate and update it based on the new scale dimensions.

Potential Solutions

To address this issue, we need to ensure that the needle's position is updated whenever the scale undergoes a transformation. There are several potential solutions, each with its own trade-offs:

  1. Recalculate Needle Position in Animation Callback:
    • Modify the set_needle_line_value function to recalculate the needle's position based on the current scale size and the needle's value.
    • This approach ensures that the needle's position is updated with every animation frame.
  2. Event-Driven Update:
    • Use LVGL's event system to detect scale transformation events (e.g., LV_EVENT_SIZE_CHANGED, LV_EVENT_POS_CHANGED).
    • In the event handler, recalculate and update the needle's position.
    • This approach is more efficient as it only updates the needle when a transformation actually occurs.
  3. Overriding Transformation Functions:
    • Subclass the lv_scale widget and override the transformation functions (e.g., lv_obj_set_size, lv_obj_set_pos).
    • In the overridden functions, recalculate and update the needle's position after the transformation is applied.
    • This approach provides a more encapsulated solution but requires more code modification.

Proposed Solution: Recalculate Needle Position in Animation Callback

For this article, we will focus on the first solution: recalculating the needle's position in the animation callback. This approach is relatively straightforward to implement and provides a good balance between performance and code complexity.

The idea is to modify the set_needle_line_value function to not only update the scale's size but also recalculate the needle's position based on the new scale dimensions and the needle's current value. This ensures that the needle stays aligned with the scale during the animation.

Implementing the Solution

To implement this solution, we need to modify the set_needle_line_value function to include the logic for recalculating the needle's position. This will involve the following steps:

  1. Get the Scale's Current Size:
    • Retrieve the current width and height of the scale object.
  2. Calculate Needle Endpoints:
    • Calculate the x and y coordinates of the needle's endpoints based on the scale's size, the needle's value, and the scale's range and angle.
  3. Update Needle Points:
    • Use lv_line_set_points() to update the needle's points with the newly calculated coordinates.

Modified Code

Here's how the modified set_needle_line_value function would look:

static void set_needle_line_value(void * obj, int32_t v)
{
    lv_obj_t * scale = (lv_obj_t *)obj;
    lv_obj_set_style_width(scale, v * 5, 0);
    lv_obj_set_style_height(scale, v * 5, 0);

    // 1. Get the Scale's Current Size
    lv_coord_t scale_width = lv_obj_get_width(scale);
    lv_coord_t scale_height = lv_obj_get_height(scale);

    // 2. Calculate Needle Endpoints
    int32_t needle_value = 30; // Assuming needle_value is fixed for this example
    int32_t range_min = 10;
    int32_t range_max = 40;
    int32_t angle_range = 270;
    int32_t rotation = 135;

    // Calculate the angle corresponding to the needle value
    float angle = (float)(needle_value - range_min) / (range_max - range_min) * angle_range + rotation;

    // Convert angle to radians
    float angle_rad = angle * LV_DEG_TO_RAD;

    // Calculate the endpoint coordinates
    lv_coord_t center_x = scale_width / 2;
    lv_coord_t center_y = scale_height / 2;
    lv_coord_t needle_length = MIN(scale_width, scale_height) / 2; // Adjust needle length as needed
    lv_point_t needle_points[2];
    needle_points[0].x = center_x;
    needle_points[0].y = center_y;
    needle_points[1].x = center_x + needle_length * lv_trigo_sin(angle_rad) / LV_TRIGO_SIN_MAX;
    needle_points[1].y = center_y - needle_length * lv_trigo_cos(angle_rad) / LV_TRIGO_SIN_MAX; // Note the minus sign for Y

    // 3. Update Needle Points
    lv_line_set_points(needle_line, needle_points, 2);
}

Explanation of the Modified Code

  1. Get the Scale's Current Size:
    • lv_obj_get_width(scale) and lv_obj_get_height(scale) are used to retrieve the current width and height of the scale object.
    • These values are crucial for calculating the needle's new position based on the transformed scale dimensions.
  2. Calculate Needle Endpoints:
    • The code calculates the angle corresponding to the needle value within the scale's range.
    • It then converts the angle to radians for trigonometric calculations.
    • The lv_trigo_sin and lv_trigo_cos functions are used to calculate the x and y offsets of the needle's endpoint relative to the center of the scale.
    • The needle's length is adjusted based on the minimum of the scale's width and height to ensure it fits within the scale.
    • The endpoint coordinates are calculated and stored in the needle_points array.
  3. Update Needle Points:
    • lv_line_set_points(needle_line, needle_points, 2) is used to update the needle's points with the newly calculated coordinates.
    • This function sets the line's starting and ending points, effectively repositioning the needle on the scale.

How the Solution Works

With this modification, the set_needle_line_value function now recalculates the needle's position whenever the scale's size changes due to the animation. This ensures that the needle stays aligned with the scale, providing a visually accurate representation of the value being indicated.

By recalculating the needle's position in the animation callback, we are effectively making the needle responsive to scale transformations. This approach ensures that the needle remains visually consistent with the scale, even as the scale's size changes dynamically.

Testing the Solution

To test the solution, replace the original set_needle_line_value function in the code snippet with the modified version. When you run the code, you should observe that the needle now correctly repositions itself as the scale's size changes during the animation. This confirms that the solution effectively addresses the issue of needles not repositioning when lv_scale is transformed.

Expected Outcome

After applying the solution, the expected outcome is that the needle will move in sync with the scale's animation. As the scale grows and shrinks, the needle will adjust its position and orientation to accurately reflect its value on the transformed scale. This visual consistency is crucial for creating a polished and intuitive user interface.

Further Testing

To ensure the robustness of the solution, it is recommended to perform further testing under various conditions. This includes:

  • Different Scale Sizes: Test the solution with different initial scale sizes and animation ranges.
  • Different Needle Values: Test with different needle values to ensure accurate positioning across the entire scale range.
  • Different Transformations: Test with other transformations, such as rotation and position changes, to verify that the needle remains aligned in all scenarios.
  • Real Hardware: Test the solution on the target hardware platform to ensure compatibility and performance.

Conclusion

In this article, we have explored an issue in LVGL where needles do not get repositioned when the lv_scale widget is transformed. We analyzed the root cause of the problem, presented a solution involving recalculating the needle's position in the animation callback, and provided a modified code snippet to implement the solution.

By understanding the intricacies of LVGL's lv_scale widget and implementing appropriate solutions, developers can create dynamic and visually appealing user interfaces for embedded systems. The solution presented in this article ensures that needles accurately reflect the scale's transformations, enhancing the overall user experience.

Key Takeaways

  • The lv_scale widget in LVGL implements needles as separate line objects.
  • Needle positions are not automatically updated when the scale is transformed.
  • Recalculating the needle's position in the animation callback is an effective solution.
  • Thorough testing is crucial to ensure the robustness of the solution.

This issue highlights the importance of understanding the underlying mechanisms of UI libraries and frameworks. By delving into the implementation details and identifying potential pitfalls, developers can create more robust and visually appealing applications.

For further information on LVGL and its features, visit the official LVGL website: LVGL Official Website