Improve table rendering speed

[schooltech]

I use unipdf to generate a pdf file, the file has 10 columns, 652 lines, Chinese characters, the generated table is 42 pages long, the entire process consumes 33.76 seconds, the font used is microsoft.ttf, how to optimize to provide performance, improve rendering speed?

[gunnsth]

@schooltech Can you provide a full example that we can run to reproduce this?

[schooltech]

pdf_test.zip

[gunnsth]

Thanks for reporting. That certainly looks like a bug. We will look into it.

[gunnsth]

Profiling reveals: Seems like the font serialization is not cached (PdfFont.ToPdfObject())

I tried adding caching in font.go:

// ToPdfObject converts the PdfFont object to its PDF representation.
func (font *PdfFont) ToPdfObject() core.PdfObject {
    if font.context == nil {
        common.Log.Debug("ERROR: font context is nil")
        return core.MakeNull()
    }
    if font.cached == nil {
        font.cached = font.context.ToPdfObject()
    }
    return font.cached

which reduces the report generation time to 1.5sec.

The file is also pretty big (9MB), which would be improved with font subsetting (#321, #17 ).

@adrg can you take a look at this? Is this the right place to do the caching or would it be better in the creator? It seems like this info should not be changing, so seems like caching would make sense.

[schooltech]

@gunnsth thanks for your reply, has the code been updated?

[adrg]

@gunnsth I think it might be safer to cache the CMap data returned by the CMap.Bytes method as the CMap content is very unlikely to change.

I made some tests on my computer and the difference in execution time is minor:

font ToPdfObject caching: 1.59636693s
CMap data caching: 1.612317699s

[gunnsth]

@adrg OK. Can you make a PR for this?

[adrg]

Sure.

[gunnsth]

@schooltech This is completed and included in v3.6.2

[schooltech]

@gunnsth render slowly again after upgrade to the new version 3.8.0, you can run the same sample project above