Astronomical CCD images are often affected by bleeding from bright sources, producing column-aligned streaks that degrade source detection and photometry. We present a pixel-level masking algorithm for bleeding in Korea Microlensing Telescope Network (KMTNet) images, in which these streaks extend along detector columns away from the readout register. The algorithm identifies bleed-generating pixels using a conservative bleeding threshold and a bleeding index that quantifies excess signal along the expected bleeding direction. The extent of each bleed trail is then determined by termination criteria based on a backgroundrelative detection threshold and a continuity condition. To optimize these termination parameters, we generate multiple candidate masks and evaluate them with an Fβ-like score that favors high purity while preserving recovery of sources whose photometry is biased by bleeding. Using the final mask, we apply interpolation-based cleaning and assess the performance in terms of completeness and photometric accuracy in a crowded KMTNet field, using Gaia DR3 as a reference. The source completeness improves from 94.08% to 98.61%, primarily by enabling proper aperture placement that was previously hindered by bleeding. For selected bleeding-affected sources, the root-mean-square photometric offset decreases from 0.623 mag to 0.068 mag. These results demonstrate that the algorithm provides a practical framework for mitigating bleeding artifacts in KMTNet images. We further examine the stability of the algorithm across different observing conditions and discuss limitations of the current masking and cleaning procedure.

Introduction
Data
Bleeding Masking Algorithm
    Parameter Selection
        Bleeding Threshold
        Bleeding Index
        Detection Threshold & Continuity Length
Application
    Completeness
    Photometry
Discussion
    Stability of the Bleeding Index Criterion
    Limitations
        Artifacts Introduced by Interpolation
        Residual Bleeding Caused by Incomplete Masking
Conclusion
Acknowledgments
References

• Jiseop Shin(SNU Astronomy Research Center, Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea)
• Mankeun Jeong(SNU Astronomy Research Center, Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea)
• Myungshin Im(SNU Astronomy Research Center, Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea) Corresponding author
• Seo-Won Chang(SNU Astronomy Research Center, Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea)
• Seong-Kook Lee(SNU Astronomy Research Center, Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea)
• Sang-Mok Cha(Korea Astronomy and Space Science Institute, Daejeon 34055, Republic of Korea)