Author Topic: Rounding error on bitmap edges?  (Read 1053 times)

anna105

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Rounding error on bitmap edges?
« on: October 04, 2017, 12:48:13 am »
I am working on translated comics. I have scanned and well positioned images that fill entire pages. Sometimes there are facing pages with one big divided picture on them. They have to fit perfectly together.

When I put these images in the frames filling the pages and exporting them as bitmaps, there are strange effects on some edges. First (or last) pixel rows (or columns) are brighter then needed. Sizes and positions are perfect (I think). It seems to be a rounding effect. It's not a simple screen rendering issue. Exported bitmaps show the same effect.

Its not a big deal when you have big images (let's say 2000×3000 pixel). You can't notice it unless you put together two exported facing pages. There will be an ugly bright line in the middle.

In short: I have input bitmaps that fit perfectly and seamless together in GIMP or PS. I import them into Scribus and do some work (new text layer). I export it as bitmap images. These images have the mentioned wrong lines on edges.

My workaround: I slightly increase the image size by decreasing the original 300 dpi to 299,7. Starting coordinates must be changed to center the image in the frame. I will lost some image information on edges. In case of facing pages there are no bright lines but missing parts and that can be noticeable too.

I made an example. To make it more obvious I made the image and the page size very little. The input image is black to make brightened lines more noticeable. On the exported bitmap you can see on the left side a nearly white pixel column. On the top a light gray pixel row. A rounding/rendering bug?
« Last Edit: October 14, 2017, 01:50:06 am by anna105 »

anna105

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Re: Rounding error on bitmap edges?
« Reply #1 on: December 27, 2017, 02:35:24 am »
Finally I solved my own problem!

First I stripped the ICC profile from my input image with imagemagick:
  convert input.png -strip input_new.png

Then I imported this in Scribus and saved my export file:
  File --> Export --> Save as Image --> No Background must be checked.

I made two export versions. PNG and BMP. The generated PNG has now an alpha channel. The BMP is fine!

I think it is definitely a bug. My original PNG is not transparent but Scribus makes it on two edges (semi)transparent and one can see the background. If you need a background it makes sense that you get transparent output files. But if I don't need the background, I don't need transparency too. Moreover the alpha channel is incorrect on some edges.

See my attachments. Can you reproduce it? I am using Scribus 1.5.2 on Linux.
« Last Edit: December 27, 2017, 03:27:01 am by anna105 »

 

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