There is no single correct “mobile image size.” Set dimensions from the image’s rendered layout and any higher-density display needs, then resize before encoding. A reliable workflow preserves or deliberately crops the composition, avoids unnecessary pixel processing, handles transparency correctly, and uses explicit encoder settings—while treating a requested file size as a target rather than a promise.
1. Set dimensions from the rendered layout
Start with the largest pixel dimensions the mobile presentation actually needs. Consider the rendered width and height, the image’s role, and whether the design needs a higher-density version. The source material does not establish a universal mobile width, height, or density multiplier: those values must come from the layout and delivery requirements.
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With ImageMagick, -resize accepts width and height geometry. Supplying just one dimension preserves the image’s aspect ratio; supplying both constrains the resize to those dimensions while preserving the ratio by default. Add > to limit the operation to images larger than the requested geometry, so smaller inputs are not enlarged. See the ImageMagick resize documentation.
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For example, a maximum-width constraint can be written as magick input.jpg -resize 640x output.jpg. The value is illustrative, not a recommended mobile standard; choose it from the actual rendered requirement. A bounded box can be expressed as magick input.jpg -resize 640x480> output.jpg.
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2. Choose fit or crop to match the design
Resizing geometry determines composition as well as dimensions. Choose a mode based on whether the whole image must remain visible, the frame must be filled, or empty space is acceptable; do not distort the source merely to force it into a box.
| Mode | Composition | Use when |
|---|---|---|
inside |
Preserves the aspect ratio and keeps the image within both target dimensions. | The whole image must fit inside a maximum-size box. |
cover |
Fills the target frame while preserving the aspect ratio; excess image area is cropped. | The design requires a fully filled frame and the crop is acceptable. |
contain |
Fits the whole image within the frame and may leave unused space. | The whole image must remain visible, even if the frame is not filled. |
fill |
Fills the target dimensions by changing the aspect ratio. | Only when distortion is intentional. |
Sharp documents these resize modes in its resize API. For a face, product, logo, or other composition-sensitive subject, inspect the resulting crop at the final frame size rather than assuming a mathematically valid crop is visually suitable.
3. Reduce pixels efficiently, then resample to the target
When the input is much larger than the delivery size, processing every source pixel can be wasteful. The libvips guide describes shrink-on-load for supported JPEG and WebP inputs, followed by a fast block-shrink stage and a final reduction to the exact dimensions. Its documented workflow uses Lanczos3 for the final reduction; that is a technique, not a universal requirement or a benchmark proving it is best for every image.
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Where the input format and processing library support decode-time shrinking, use it to reduce the data being handled before the final resize. Then evaluate the final resampling choice against both output appearance and processing cost in the actual pipeline. Details are in the libvips image shrinking guide.
4. Preserve transparency through the resize
For images with alpha transparency, the libvips guide’s processing sequence is to premultiply before shrinking and unpremultiply afterward. This matters for transparent logos and interface graphics, where resized edges can reveal halos or other artifacts against a background.
Include transparent assets in visual checks, ideally against the backgrounds on which they will appear. That check is a practical quality-control step; it is not evidence that one particular output will always show an artifact.
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5. Encode after resizing, with explicit settings
Settle the pixel dimensions and composition before final encoding. Compression controls affect the encoded output; they do not replace choosing an appropriate image size. Specify the format and relevant encoder settings, then inspect representative images for byte size and visual quality.
ImageMagick’s WebP encoder documents target-size as a byte target that triggers multiple passes to approach the requested size. It also documents a method option that trades encoding time against compressed size and quality, along with quality and lossless options. A target size is an aim, not a guarantee, and a setting that works for one image may not suit another. Consult the ImageMagick WebP options and evaluate the resulting files.
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How to make delivery predictable
- Map each delivery context. Record the mobile layout’s maximum rendered dimensions and any higher-density variant it needs.
- Choose composition behavior. Use a fit mode that keeps the full image, or an intentional crop where the frame must be filled.
- Resize before encoding. Use decode-time shrinking where supported, then resample to the exact delivery dimensions.
- Check transparency. Premultiply before shrinking and unpremultiply afterward for alpha-bearing images, then inspect the output.
- Encode and verify. Make format and encoder settings explicit; compare representative outputs for bytes, visual quality, and encoding time.
This workflow makes dimensions and trade-offs explicit, but the cited documentation does not establish one mobile size, quality setting, or byte budget that fits every layout and asset.
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