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to the only part of the function where it is used.
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Currently, the resolution of a resized image that was sent in a chat, depends on the aspect-ratio. Assuming the `balanced`-quality-setting is used, a square image, that is larger than the limits for resolution and file-size, will be resized to 1280x1280 (1,638,400 pixels), an image with an aspect-ratio of 16:9, will be resized to 1280x720 (921,600 pixels), and if the aspect-ratio is 32:9, to 1280x360 (460,800 pixels). This change makes it so, that the number of pixels, in images with different aspect-ratios, will be similar.
The resolution-limits for avatar-images are currently 512x512 or 128x128. Reducing the resolution further, to 2/3 each step, can reduce the quality much more than is necessary to fit within the file-size-limits, which are currently 60 kB or 20 kB. An image made entirely of noise (which results in unusually large file-sizes), encoded with jpeg-quality 75, and 4:2:2-colour-subsampling (the format currently used for encoding images), can be below 60 kB at 227x227. For the lower file-size-limit of 20 kB, such images with a resolution of 128x128 already fit. More normal images will have a lower file-size at the same resolution. Before this change, the target-resolutions for resizing were: 512x512 -> 341x341 -> 227x227. And for the lower file-size-limit: 128x128 (does already fit). After this change, the target-resolutions for resizing will be: 512x512 -> 448x448 -> 392x392 -> 343x343 -> 300x300 -> 263x263 -> 230x230. And for the lower file-size-limit, those will be: 256x256 -> 224x224 -> 196x196 -> 172x172 -> 150x150 -> 131x131. The resolution-limit has been increased to 256x256, because the file-size of many images is still smaller than 20 kB when encoded at 256x256, and it can be a large improvement in quality.
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Here a comparison for the quality of images sent in chats. Tap here to view the original images
*: The 2 columns on the right are supposed to be identical; i mostly included them, to make it visible that square-ish images already get encoded with up to ~1.6 megapixels, on the "main"-branch. |
src/blob.rs
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| let mut target_wh = if !is_avatar && exceeds_wh { | ||
| // Limit resolution to the number of pixels that fit within max_wh * max_wh, | ||
| // so that the image-quality does not depend on the aspect-ratio. | ||
| (f64::from(resolution_of_longest_side_of_image) |
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So the image still can exceed "wh" if it's not square. Then exceeds_wh should be tweaked for images, it should compare not the longest image size, but the number of pixels, with max_wh * max_wh.
Secondly, not the number of pixels matters, but the number of JPEG blocks (which are 8x8), because we recode to JPEG.
| }); | ||
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| if do_scale { | ||
| let resolution_of_longest_side_of_image = max(img.width(), img.height()); |
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Maybe use shorter names for variables: longest_size_len, n_px_sqrt? We use n_* in the many places for "number", it's a standard abbreviation in Core. sqrt is also a standard abbreviation, even the function is called so. "in image" can be omitted because we don't have anything else here, do we?
to encode fewer partial JPEG-blocks.
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