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https://issues.apache.org/jira/browse/PDFBOX-4137?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16389356#comment-16389356
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Itai Shaked commented on PDFBOX-4137:
-------------------------------------
Thanks for the tests. I have attached a revision patch (on a technical note -
in such cases is it best to attach a full patch every time, or incremental one?
This time I created an incremental patch to it's easier to see what changed,
but if a full-patch is preferred let me know).
I have fixed both problems, and added isSubsamplingAllowed to PDFRenderer and
PageDrawerParameters. I have also made it so cache may be saved for any
subsampling value, so long as "region" is null. Higher quality (i.e. lower
subsampling value) renders may replace lower-quality (higher subsampling)
renders, but there is no handling of the hypothetical case of a high-quality
cached version being discarded by the GC, so once a request is made with e.g.
subsampling=1, the PDImageXObject instance will never cache versions with
higher subsampling. I assume this won't hurt performance too much (the current
implementation only saves the one version anyway), but it may be worth noting
for the future.
I have modified the problematic code in from1Bit so now it is:
{code:java}
for (int i = 0; i < 8; i++)
{
if (x >= startx + scanWidth)
{
break;
}
int bit = value & mask;
mask >>= 1;
if (x >= startx && x % subsampling == 0)
{
output[idx++] = bit == 0 ? value0 : value1;
}
x++;
}
{code}
I believe it is clearer (only one place where x is incremented, no `continue`),
but looking at it again perhaps the break condition should be in the end of the
loop - algorithmically it makes no difference, but it could be more readable.
> Allow subsampled/downscaled rendering of images, and rendering subimages
> -------------------------------------------------------------------------
>
> Key: PDFBOX-4137
> URL: https://issues.apache.org/jira/browse/PDFBOX-4137
> Project: PDFBox
> Issue Type: Improvement
> Components: Rendering
> Affects Versions: 2.0.8
> Reporter: Itai Shaked
> Priority: Minor
> Attachments: 0001-Image-render-subsample.patch,
> 0001-Image-rendering-subsampling-removed-metadata-options.patch,
> 0001-Image-subsampling-revision-2.patch, 067445.pdf, PDFBOX-1841.pdf,
> PDFJS-4575-noimagedim.pdf, image_rendering_subsampling_hack.patch,
> large-jpeg.pdf
>
>
> Suggested/contributed change to allow subsampling of images and rendering
> sub-regions of images.
> The need arises from having very large images which are highly compressed
> (usually JPEG or JBIG2). The current implementation decodes the entire image
> into memory at full resolution, even if rendering is done at a much lower
> resolution.
> Since the change required augmenting the way Filters work (to allow
> partial/subsampled decoding), it also includes a partial fix for PDFBOX-3340.
>
>
> This change introduces "DecodeOptions" which are currently only applicable
> for images. They include requesting only metadata (for PDImageXObject's
> repair method), subsampling and sub-region (similar to
> javax.imagio.ImageReadParam).
> Since not all filters can or do honor (use) the options, the DecodeOptions
> class contains a flag. Filters that honor the options (subsample / decode
> only requested region) set it to true. If the flag is false, the subsampling
> or cropping should be done after decoding, to ensure consistency.
> PageDrawer was modified so it uses subsampling based on the ratio of the
> desired output to the original image.
>
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