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add webp details

Dane Springmeyer 2013-10-04 12:14:01 -07:00
parent 6133afd296
commit e6ec371a20
1 changed files with 56 additions and 2 deletions
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Image-IO.md

@ -65,7 +65,7 @@ There generally two types of PNG Mapnik can author: 1) reduced color paletted pn
So, the two main types can be requested using the formats names:
| Name | Type |
| ---- | -----: |
| ---- | ----- |
| `png`, `png8` , or `png256` | Creates reduced color/quantized paletted png NOTE: in Mapnik versions older than 2.3.x the `png` keyword used to map to full color png|
| `png24` or `png32` | Creates full color png with millions of possible colors (and much larger file size) |
@ -82,3 +82,57 @@ And the key:value options can be controlled as follows:
| e | string, `miniz` or `libpng` | 'libpng` | Experimental: not recommended to set this option. `e=miniz` triggers using the experimental miniz encoder support instead of `libpng`. In some cases this encoder provides better encoding speeds with minor size differences but it also does not work fully yet (bugs we need to track down) |
### WEBP output options
We support one custom option called `alpha`. If you pass `alpha=false` then an `rgb` webp image will be created instead of an `rgba` image. It is unclear yet whether there is any major benefit to encoding `rgb` webp images and so this option may be removed in the future.
We support every option in the [Advanced Encoding API](https://developers.google.com/speed/webp/docs/api#advanced_encoding_api) except `show_compressed`.
The `image_hint` details (from the webp sources) are:
| value | meaning |
| ----- | ------- |
| 0 | default / `WEBP_HINT_DEFAUL` |
| 1 | picture / `WEBP_HINT_PICTURE` (digital picture, like portrait, inner shot) |
| 2 | photo / `WEBP_HINT_PHOTO` (outdoor photograph, with natural lighting) |
| 2 | graph / `WEBP_HINT_GRAPH` (Discrete tone image (graph, map-tile etc).) |
And all the `WebPConfig` based advanced options are:
```c++
struct WebPConfig {
int lossless; // Lossless encoding (0=lossy(default), 1=lossless).
float quality; // between 0 (smallest file) and 100 (biggest)
int method; // quality/speed trade-off (0=fast, 6=slower-better)
WebPImageHint image_hint; // Hint for image type (lossless only for now).
// Parameters related to lossy compression only:
int target_size; // if non-zero, set the desired target size in bytes.
// Takes precedence over the 'compression' parameter.
float target_PSNR; // if non-zero, specifies the minimal distortion to
// try to achieve. Takes precedence over target_size.
int segments; // maximum number of segments to use, in [1..4]
int sns_strength; // Spatial Noise Shaping. 0=off, 100=maximum.
int filter_strength; // range: [0 = off .. 100 = strongest]
int filter_sharpness; // range: [0 = off .. 7 = least sharp]
int filter_type; // filtering type: 0 = simple, 1 = strong (only used
// if filter_strength > 0 or autofilter > 0)
int autofilter; // Auto adjust filter's strength [0 = off, 1 = on]
int alpha_compression; // Algorithm for encoding the alpha plane (0 = none,
// 1 = compressed with WebP lossless). Default is 1.
int alpha_filtering; // Predictive filtering method for alpha plane.
// 0: none, 1: fast, 2: best. Default if 1.
int alpha_quality; // Between 0 (smallest size) and 100 (lossless).
// Default is 100.
int pass; // number of entropy-analysis passes (in [1..10]).
int show_compressed; // if true, export the compressed picture back.
// In-loop filtering is not applied.
int preprocessing; // preprocessing filter (0=none, 1=segment-smooth)
int partitions; // log2(number of token partitions) in [0..3]
// Default is set to 0 for easier progressive decoding.
int partition_limit; // quality degradation allowed to fit the 512k limit on
// prediction modes coding (0: no degradation,
// 100: maximum possible degradation).
};
```