[kvii]

Fix #26

This code is inspired from pdfcpu, gopdf and image/png

[kvii]

Test data for TIFF pdfminer/pdfminer.six#1058 (review)
You can use this file to test the logic of predictor 2 manually.

[ledongthuc]

@kvii, Thank you for this PR. It adds TIFF predictor 2 and PNG predictors 10 to 15. This change is useful.

Issue 1 (must fix): The Average filter gives wrong values.
In pngPredictorReader.Read, case 3 adds two byte values before it divides by 2. The sum can go over 255. The result is then wrong.
Example: previous row = [200, 200]. Current row raw = [0, 0].
Got: [100, 22]. Expected: [100, 150].
Change the sum to int. Add a test with values above 127.

Issue 2 (should fix): The new panics can stop valid PDFs.
decodeParams panics when Colors is not 1 or 3. It also panics when BitsPerComponent is not 8.
Before this change, these streams decoded without a panic. Now a CMYK image (Colors 4) can crash the caller.
Return an error, or pass the stream through unchanged.

Minor:

• Add a comment in tiffPredictorReader. Explain what happens to a truncated last row.
• Add a PR description. State which PDFs failed and what behavior changes.
• The PR comment links TIFF test data. That file is not in this PR. Add a test PDF if you can.

Thanks.

[kvii]

@ledongthuc Issue 1 is fixed. But I don't think this PR changes any behavior. YMCK(Color 4) is valid if and only if Predictor is not 1 or null. The previous code also caused a panic when Predictor was not 12 or null. Perhaps I can stop parsing other params when predictor is 1 or null, like this:

func decodeParams(param Value) (predictor, rawLen, bpp int64) {
	if v := param.Key("Predictor"); v.Kind() == Null {
		predictor = 1
	} else {
		predictor = v.Int64()
	}
	switch predictor {
	case 1:
		return // <---
	case 2, 10, 11, 12, 13, 14, 15:
	default:
		panic(fmt.Errorf("Predictor %d is currently unsupported", predictor))
	}

	...
	switch colors {
	case 1, 3:
	default:
		panic(fmt.Errorf("Colors %d is currently unsupported", colors))
	}
	...
}

[kvii]

predictor 15
predictor15.pdf
tiff
tiff.pdf