Basic pre-vetting of a PNG file using StringTheory

[jslarve]

This doesn't guarantee that a PNG file is 100% valid, but it gives a few sanity checks before attempting to save and process the file as PNG.

Makes sure that there's a valid PNG header and checks the dimensions to make sure they're in range of your specifications.

This way, the image never needs to hit the disk if it doesn't pass the vetting.

Usage:

MyImageST    StringTheory
MyMessageST StringTheory

   CODE

  !Assuming MyImageST already contains the PNG data

    IF NOT ValidatePNGFile(MyImageST,MyMessageST)
        !Don't bother saving/processing
        MESSAGE('Invalid Image: ' & MyMessageST.GetValue())
    END

ValidatePNGFile      PROCEDURE  (StringTheory pPngContentsST,StringTheory pResultMsgST,|
                            LONG pMinWidth=10,LONG pMaxWidth=1200,LONG pMinHeight=10,LONG pMaxHeight=1200)!,LONG
S                           STRING(4) !4 bytes to be examined as LONG using L.
L                           LONG,OVER(S)
IHDRPos                     LONG !Position of the IHDR chunk
ST                          StringTheory !Utility StringTheory object.

  CODE

    LOOP 1 TIMES     
        pResultMsgST.SetValue('')
        IF pPngContentsST.Slice(1,8) <> '<089h,050h,04Eh,047h,00Dh,00Ah,01Ah,00Ah>'
            pResultMsgST.SetValue('Invalid PNG file')
            BREAK
        END
        IHDRPos = pPngContentsST.Instring('IHDR')
        IF NOT IHDRPos
            pResultMsgST.SetValue('Cannot Determine PNG Dimensions')
            BREAK
        END
       
        S = pPngContentsST.Slice(IHDRPos + 4,IHDRPos + 8 ) !Getting The Width (but it's BigEndian)
        L = ST.SwitchEndian(L)                            !Switch to LittleEndian so we can read it.
        IF L < pMinWidth OR L > pMaxWidth
            pResultMsgST.SetValue('Width is out of Bounds: Value=' & L & ' Minimum=' & pMinWidth & ' Maximum=' & pMaxWidth)
            BREAK
        END
       
        S = pPngContentsST.Slice(IHDRPos + 8,IHDRPos + 12) !Getting The Height (but it's BigEndian)
        L = ST.SwitchEndian(L)                             !Switch to LittleEndian so we can read it.
        IF L < pMinHeight OR L > pMaxHeight
            pResultMsgST.SetValue('Height is out of Bounds: Value=' & L & ' Minimum=' & pMinHeight & ' Maximum=' & pMaxHeight)
            BREAK
        END
        RETURN 1
    END
    RETURN 0