LovyanGFX: Fix For Pixel Copying Large Images

by Alex Johnson 46 views

When working with LovyanGFX and attempting to display images larger than 64k pixels, users have encountered a blending issue where the image copy restarts from the beginning after reaching the 64k position. This article delves into the problem, its causes, and potential solutions.

Environment

  • MCU or Board name: ESP32-S3
  • Panel Driver IC: Any
  • Bus type: Any
  • LovyanGFX version: Latest develop
  • FrameWork version: ESP-IDF v5.5.1
  • Build Environment: ESP-IDF
  • Operating System: Any

Problem Description

When copying large images from a buffer to the display using LovyanGFX, the process fails, leading to image corruption. Specifically, the image starts blending back from the beginning once it reaches the 64k pixel mark. The issue arises during the pixel copy operation, where the data transfer appears to loop prematurely.

param->fp_copy(dmabuf, 0, step, param);

Expected Behavior

The expected behavior is that the entire image data should be copied correctly from the buffer to the display without any data loss or repetition. Every pixel from the source buffer should be accurately represented on the display, ensuring a complete and untruncated image.

Actual Behavior

In reality, only the first 64k pixels of the image are copied correctly. After this point, the copy process restarts from the beginning of the image data, causing a blending effect where the initial part of the image overwrites the subsequent sections. This results in a distorted and incomplete display of the intended image.

Steps to Reproduce

To reproduce this issue, you need to call param->fp_copy() in a loop until the end of the image data. Crucially, you should avoid incrementing pos_y after each step. This mimics the scenario where a large horizontal strip of the image is being copied, triggering the described behavior.

Root Cause

The root cause of this problem lies in the insufficient precision used in some of the pixelcopy_t::copy_* functions. The functions utilize src_x and src_y as 16-bit integers instead of the more precise src_x32 and src_y32. This limitation leads to an overflow when dealing with large images, causing the copy operation to wrap around after 64k pixels.

uint32_t i = (param->src_x + param->src_y * param->src_bitwidth) * param->src_bits;

Understanding the Code Snippet:

The problematic line of code calculates the index i based on param->src_x, param->src_y, param->src_bitwidth, and param->src_bits. The issue arises because src_x and src_y are 16-bit integers. When the result of param->src_x + param->src_y * param->src_bitwidth exceeds the maximum value that a 16-bit integer can hold (65535), it wraps around to zero, effectively resetting the starting position of the copy operation. This is why the image starts blending from the beginning after reaching the 64k pixel mark.

Proposed Solution

To resolve this issue, the precision of src_x and src_y within the pixelcopy_t::copy_* functions must be increased. Replacing the 16-bit integers with 32-bit integers (src_x32 and src_y32) will provide the necessary range to handle larger image dimensions without overflow. Here’s how to implement the solution:

  1. Modify Data Types:

    • Locate the declarations of src_x and src_y in the pixelcopy_t structure.
    • Change their data types from uint16_t to uint32_t.
    typedef struct {
    uint32_t src_x32;
    uint32_t src_y32;
    // Other members
} pixelcopy_t;

  2. Update Calculation:

    • Update the calculation of the index i to use the new 32-bit variables.

uint32_t i = (param->src_x32 + param->src_y32 * param->src_bitwidth) * param->src_bits; ```

  1. Review and Adjust Related Code:
    • Check all instances where src_x and src_y are used within the pixelcopy_t::copy_* functions and ensure they are correctly updated to src_x32 and src_y32. This step is crucial to avoid introducing new issues.

Detailed Explanation of the Solution

To fully understand the fix, let's break down why using 32-bit integers resolves the problem. The original 16-bit integers (uint16_t) have a maximum value of 65535 (2^16 - 1). When the calculated pixel index exceeds this value, it wraps around to zero, causing the pixel copy operation to restart from the beginning of the image buffer. By switching to 32-bit integers (uint32_t), the maximum value increases dramatically to 4294967295 (2^32 - 1). This significantly larger range ensures that the pixel index will not overflow, even for very large images.

Impact of the Change:

  • Accuracy: The pixel copy operation will now accurately copy pixels from the correct positions in the source buffer, eliminating the blending issue.
  • Support for Larger Images: The fix allows LovyanGFX to handle images with dimensions exceeding 64k pixels without any data loss or corruption.
  • Stability: By preventing the overflow, the fix ensures the stability and reliability of the pixel copy process, especially when dealing with large image datasets.

Testing the Solution

After implementing the proposed solution, thorough testing is necessary to ensure its effectiveness. Here are some testing strategies:

  1. Create Large Test Images:

    • Generate test images with dimensions significantly larger than 64k pixels. These images should contain distinct patterns or colors to easily identify any blending or repetition issues.

  2. Implement Test Cases:

    • Write test cases that specifically target the param->fp_copy() function. These test cases should simulate the conditions under which the original issue was observed.

  3. Monitor Pixel Copy Output:

    • Carefully monitor the output of the pixel copy operation. Verify that all pixels are copied correctly and that there is no blending or repetition of image data.

  4. Performance Evaluation:

    • Evaluate the performance impact of the fix. While the use of 32-bit integers may introduce a slight overhead, it should be minimal and not significantly affect the overall performance of LovyanGFX.

Additional Considerations

While implementing the fix, consider the following aspects:

  • Memory Usage:

    • The use of 32-bit integers will increase the memory footprint of the pixelcopy_t structure. Ensure that the target platform has sufficient memory to accommodate this increase. This is particularly important for memory-constrained devices.

  • Compatibility:

    • Verify that the fix is compatible with all target platforms and architectures. Test LovyanGFX on a variety of devices to ensure consistent behavior.

  • Code Review:

    • Conduct a thorough code review to identify any potential issues or areas for improvement. Involve multiple developers in the review process to ensure a comprehensive assessment.

Conclusion

The pixel copy issue encountered when displaying large images with LovyanGFX stems from the limited precision of 16-bit integers used for pixel indexing. By upgrading to 32-bit integers, the overflow problem is eliminated, enabling accurate and reliable rendering of images exceeding 64k pixels. Implementing this fix ensures that LovyanGFX can handle a broader range of image sizes, providing a more robust and versatile graphics library for embedded systems.

By addressing this issue, developers can confidently use LovyanGFX for applications requiring high-resolution displays and large image assets, enhancing the overall user experience. This fix is essential for projects involving detailed graphics and complex visual elements.

For further reading on graphics programming and embedded systems, consider exploring resources like the Embedded Graphics Library Documentation