Previewing an image in jpg is rather slow

[grignards]

When all its dependencies are installed preview generator uses the ImagePreviewBuilderIMConvert builder. This builder then creates a temporary PNG image before giving it to the ImagePreviewBuilderPillow builder that converts the temporary png in jpeg. For a moderately sized image (around 8MB - 4000x2000 pixels) this takes 4-5 seconds. Converting the same directly with convert -resize does only take 0.6 seconds on the same computer.

Computing a preview should take around the same time as directly using convert.

[PhilippeAccorsi]

• Do we have an idea why we create png and after jpeg file and not directly the jpeg file ?
• When you says Converting the same directly, its about creating PNG first and JPEG at end with convert-resize ?

[grignards]

* Do we have an idea why we create png and after jpeg file and not directly the jpeg file ?

I think it all comes down to successive changes + a specific background management in ImagePreviewBuildPillow that could be easily replicated with ImagePreviewBuildWand.

* When you says `Converting the same directly`, its about creating PNG first and JPEG at end with convert-resize ?

No, without creating the PNG image (so with a command like convert original.jpg -resize 100x100 preview.jpg). Its the PNG image creation which is slow, encoding a PNG is not designed for speed.

[lebouquetin]

The right way to fix this would be to implement another builder which avoid this intermediate file and/or change priority in builders.

[grignards]

In fact there are 3 builders for images, and after a first look at it I think we could keep only one, ImagePreviewBuildWand as it seems to be able to support all the functionalities we need (background + currently supported file formats).