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Re^7: About GD Image Data Output Methods

by BrowserUk (Patriarch)
on Jun 07, 2015 at 21:27 UTC ( #1129383=note: print w/replies, xml ) Need Help??

in reply to Re^6: About GD Image Data Output Methods
in thread About GD Image Data Output Methods [SOLVED]

some explanations about how your code works.

Both formats are very simple, uncompressed files, with almost the same data format; which makes conversion simpler.

I detailed the gd format header in Re: About GD Image Data Output Methods.

I used this for the bmp format details.

Hopefully these comments will explain the rest.

## Get the gd format data as a string my $gd = $image->gd; ## Unpack the header (we're only interested in the width & height) and + strip that header from the image data my( undef, $width, $height, undef, undef ) = unpack 'nnnCV', substr( $ +gd, 0, 11, '' ); ## GD image data is 4 bytes (rgba); BMP image data is 3 bytes; so calu +late the length the bmp image data with occupy my $len = length( $gd ) / 4 * 3; ## Pack the required information to create a .bmp header (in two parts +). (see link above) my $bmp = pack 'a2 V V V l< l< l< v v V V l< l< V V', ## The .bmp file header has 5 fields (but 2 U16s do nothing so pack 1x +U32 value 0 for them) ## sig file-length (data+header length), 2 unused shorts as U32, an +d the offset to the rgb data. 'BM', $len + 54, 0, 54, ## BITMAPINFOHEADER 40 bytes, image width & height (negative to turn i +t the right way up!), ## color planes (1), bits/pel(24), compression type(none:0), length of + rgb data, 3 unused fields. 40, $width, -$height, 1, 24, 0, $len, 0, 0, 0; ## Convert the rgba gd data to rgb bmp data (x skips the alpha bytes) ## And tack it onto the end of the header. $bmp .= join'', unpack '(xaaa)*', $gd;

Of course, the code is crude as .bmps have a multitude of other possibilities, and (as you've seen) I don't even check for truecolor .v. palette in the gd format; but the intention was to give you something that did the job, not duplicate the functionality of image conversion software.

It wouldn't be too hard to handle palette images; but I don't think it is necessary for your purpose.

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Re^8: About GD Image Data Output Methods
by karlgoethebier (Abbot) on Jun 08, 2015 at 09:36 UTC

    Thank you very much for your explanation.

    Best regards, Karl

    «The Crux of the Biscuit is the Apostrophe»

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