Improve the image quality of thumbnails rendered by PDFThumbnailView.draw (issue 8233)
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…riables These properties are always updated/used together, and there's no other methods which depend on just one of them, hence they're changed into local variables instead. Looking through the history of this code, it seems they were converted *from* local variables and to properties all the way back in PR 2914; however as far as I can tell from that diff it doesn't seem to have been necessary even back then!?
…oving the thumbnail A loop is less efficient than just overwriting the content, which is what we've generally been using (for years) in other parts of the code-base (see e.g. `BaseViewer` and `PDFThumbnailViewer`).
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I needed some time to think through all of the code-paths involved here (hence why it's submitted as a "draft"), and hopefully this patch now covers all the edge-cases involved here. As mentioned above this isn't perfect, but I hope it's deemed good enough since we need to consider the memory usage as well. /botio-linux preview |
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….draw` (issue 8233)
The reason for the fairly large discrepancy, in the thumbnail quality, between the `draw`/`setImage`-methods is that in the former case we *directly* render the thumbnails at the final size that they'll appear at in the sidebar. In the latter case, we instead downsize the (generally) much larger "regular" pages.
To address this, I'm thus proposing that we let `PDFThumbnailView.draw` render thumbnails at *twice* their intended size and then downsize them to the final size.
Obviously this will increase *peak* memory usage during thumbnail rendering in `PDFThumbnailView.draw`, since doubling the width/height of a `canvas` will lead to its pixel-count increasing by a factor of `4`. Furthermore, since you need four components per pixel (given that it's RGBA-data), this will thus lead to the *temporary* thumbnail `canvas`-sizes increasing by a factor of `16` during rendering. Hence why rendering thumbnails at their "original" scale, i.e. using something like `PDFPageProxy.getViewport({ scale: 1 });`, would be an absolutely terrible idea!
To reduce the size and scope of these changes, I've tried to re-factor and re-use as much of the existing downsizing-implementation already present in `PDFThumbnailView` as possible.
While this will generally *not* make thumbnails rendered by `PDFThumbnailView.draw` look *identical* to those based on the rendered pages (via `PDFThumbnailView.setImage`), it's a considerable improvement as far as I'm concerned and enough to call the issue fixed.
*Please note:* This patch will not lead to *any* additional overhead, in either memory usage or parsing, for thumbnails which are based on the rendered pages.
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Off-topic for this PR, but: I'm wondering if we should simply remove pdf.js/web/pdf_thumbnail_view.js Lines 35 to 37 in 6429ccc AppOptions) besides manually editing the code?
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Looks better to me too. Thank you for looking into this! Regarding |
The reason for the fairly large discrepancy, in the thumbnail quality, between the
draw/setImage-methods is that in the former case we directly render the thumbnails at the final size that they'll appear at in the sidebar. In the latter case, we instead downsize the (generally) much larger "regular" pages.To address this, I'm thus proposing that we let
PDFThumbnailView.drawrender thumbnails at twice their intended size and then downsize them to the final size.Obviously this will increase peak memory usage during thumbnail rendering in
PDFThumbnailView.draw, since doubling the width/height of acanvaswill lead to its pixel-count increasing by a factor of4. Furthermore, since you need four components per pixel (given that it's RGBA-data), this will thus lead to the temporary thumbnailcanvas-sizes increasing by a factor of16during rendering. Hence why rendering thumbnails at their "original" scale, i.e. using something likePDFPageProxy.getViewport({ scale: 1 });, would be an absolutely terrible idea!To reduce the size and scope of these changes, I've tried to re-factor and re-use as much of the existing downsizing-implementation already present in
PDFThumbnailViewas possible.While this will generally not make thumbnails rendered by
PDFThumbnailView.drawlook identical to those based on the rendered pages (viaPDFThumbnailView.setImage), it's a considerable improvement as far as I'm concerned and enough to call the issue fixed.Please note: This patch will not lead to any additional overhead, in either memory usage or parsing, for thumbnails which are based on the rendered pages.
Fixes #8233