How to convert color Image to PPM image to use in TensorRT

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Hi, im working on a program that uses TensorRT API to run an inference with ssd_mobilnet model. My problem is that TensorRT takes ppm format image and must of the forums I have search uses "imwrite" to save the image to ppm but i need the image in memory like a cv::Mat matrix so i dont have to keep writing into disk every converted frame.

Thanks for any response.