Converts image files for Overview.

Methodology

This program always outputs 0.json, 0.txt, 0.blob, and a thumbnail: 0-thumbnail.png or 0-thumbnail.jpg.

If JSON specifies wantOcr, runs tesseract with pdf and txt configurations to generate 0.blob and 0.txt. The output 0.json's metadata will include "isFromOcr":true.

Otherwise, img2pdf wraps the input image into a PDF for 0.blob, and 0.txt is empty. (img2pdf requires an entire Python environment; but that's peanuts next the tesseract-ocr datafiles, and the other viable alternative is pdflatex, which is ~250MB larger.)

The output 0.json has wantOcr:false and wantSplitByPage:false. That makes the output is ready for viewing: no further conversions are needed.

Testing

Write to test/test-*. docker build . will run the tests.

Each test has input.blob (which means the same as in production) and input.json (whose contents are $1 in do-convert-single-file). The files stdout, 0.json, 0.blob, 0.txt, and 0-thumbnail.(png|jpg) in the test directory are expected values. If actual values differ from expected values, the test fails.

PDF, PNG and JPEG are tricky formats to get exactly right. You may need to use the Docker image itself to generate expected output files. For instance, this is how we built test/test-jpg-ocr/0-thumbnail.jpg:

1. Wrote test/test-jpg-ocr/{input.json,input.blob,0.txt,0.blob,stdout}
2. Ran docker build .. The end of the output looked like this: Step 12/13 : RUN [ "/app/test-convert-single-file" ] ---> Running in f65521f3a30c 1..3 Tesseract Open Source OCR Engine v3.04.01 with Leptonica not ok 1 - test-jpg-ocr do-convert-single-file wrote /tmp/test-do-convert-single-file912093989/0-thumbnail.jpg, but we expected it not to exist ...
3. docker cp f65521f3a30c:/tmp/test-do-convert-single-file912093989/0-thumbnail.jpg test/test-jpg-ocr/
4. docker rm -f f65521f3a30c