I was recently working on a demonstration of a Framos IMX 253 mono-chrome camera with a 12-bit sensor supported on a Jetson Xavier. For the demo, I needed to save the images as jpeg format. I thought it may be useful for others to see the inner-workings of a jpeg compression implementation Jetson, using the hardware assisted jpeg compressor.<\/p>\n\n\n\n
The image came from the camera driver as 12 bit format in 16 bit integer array format. The sensor is mono-chrome, but the Jetson jpeg compressor only takes a single YUV format as input.<\/p>\n\n\n\n
For this demo, I skip the step of re-mapping the luminance values from a 12 bit range to an 8 bit range and simply take the lower 8-bits. A production implementation needs a scheme for this re-mapping of luminance range. The implementation on a Jetson should take advantage of the hardware assist.<\/p>\n\n\n\n
<\/p>\n\n\n\n
Step 1: get things setup<\/p>\n\n\n\n
\n\/\/ Info is a structure provided from the caller that contains info about the frame \n\n\/\/ prepare to time execution of the jpeg compression \nauto start = std::chrono::steady_clock::now();\n\n\/\/ prepare the output file\nstd::string outFile=\"\/path\/to\/file.jpg\";\nstd::ofstream* outFileStr = new std::ofstream(outFile);\nif(!outFileStr->is_open())\n return false;\n\n\/\/ create an instance of the nvidia jetson jpeg encoder\n\nNvJPEGEncoder* jpegenc = NvJPEGEncoder::createJPEGEncoder(\"jpenenc\");\n\n\/\/ the jpeg output buffer size is 1.5 times the width*height \nunsigned long out_buf_size = Info.Width * Info.Height * 3 \/ 2;\nunsigned char *out_buf = new unsigned char[out_buf_size];\n<\/code><\/pre>\n\n\n\n<\/p>\n\n\n\n
Step 2: create an nvidia native buffer<\/p>\n\n\n\n
\/\/ V4L2_PIX_FMT_YUV420M = is the only format which appears to be supported by the Jetson jpeg encoder\n\n\/\/ allocate the buffer \nNvBuffer buffer(V4L2_PIX_FMT_YUV420M, Info.Width, Info.Height , 0);\n\nbuffer.allocateMemory();\n\nNvBuffer::NvBufferPlane* plane = &buffer.planes[0];\n\n\/\/convert the image luminance from uint16 to 8 bits and copy into the nvidia buffer\nfor(int y=0; y < Info.Height;y++)\n{\n for(int x=0; x < Info.Width;x++)\n {\n plane->data[x+(y*plane->fmt.stride)] = (unsigned char) (m_img[x+(y*Info.Width)]);\n }\n}\n\nplane->bytesused = 1 * plane->fmt.stride * plane->fmt.height;\n<\/code><\/pre>\n\n\n\nStep 3: the Framos camera driver provides a mono-chrome image, so make the image actually mono-chrome by setting the UV vectors to neutral color (127d). <\/p>\n\n\n\n
\/\/ initialize the Cb plan to the 127d value which means 0 color\n\nplane = &buffer.planes[1];\nchar* data = (char *) plane->data;\nplane->bytesused = 0;\nfor (int j = 0; j < plane->fmt.height; j++)\n{\n memset(data,127,plane->fmt.width);\n data += plane->fmt.stride;\n}\nplane->bytesused = plane->fmt.stride * plane->fmt.height;\n\n\/\/ initialize the Cr plan to the 127d value which means 0 color\n\nplane = &buffer.planes[2];\ndata = (char *) plane->data;\nplane->bytesused = 0;\nfor (int j = 0; j < plane->fmt.height; j++)\n{\n memset(data,127,plane->fmt.width);\n data += plane->fmt.stride;\n}\nplane->bytesused = plane->fmt.stride * plane->fmt.height;<\/code><\/pre>\n\n\n\nStep 4: run the actual jpeg encode function, save the file, and measure the results:<\/p>\n\n\n\n
jpegenc->encodeFromBuffer(buffer, JCS_YCbCr, &out_buf, out_buf_size, 95);\n\nauto end = std::chrono::steady_clock::now();\n\noutFileStr->write((char *) out_buf, out_buf_size);\noutFileStr->close();\n\nprintf( \"Jpeg Encode Elapsed time in nanoseconds: %d\\n\",std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count());\n\ndelete[] out_buf;\ndelete outFileStr;\n<\/code><\/pre>\n\n\n\nResult:<\/p>\n\n\n\n
I am seeing roughly 25 milliseconds for the encode and save on an image 3840 x 2160 pixels.<\/p>\n\n\n\n
<\/p>\n","protected":false},"excerpt":{"rendered":"
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