图像文件 ¶

简介 ¶

digiKam 依赖于一些库和支持包来加载和保存图像格式。哪些图像格式能够支持，取决于这些库在你的系统上能不能用，在某些情况下，则取决于这些库的编译方式。大多数的发行版本中，digiKam 的图片查看都支持海量格式。

对第三方库的依赖意味着，无法给出一个明确清单，列好在你的系统上支持的所有格式。至少JPEG、PNG 和 TIFF 应该是可用的。

digiKam only displays files that are in formats that it understands. It does this by looking at the file extension on the files and checking this against a predefined list. If the file extension is in the list and the appropriate library is installed, digiKam will show the file in the Image View. You can change the list of file extensions that digiKam will accept; see this configuration section for more details.

Almost all digital cameras store photographs in one of two formats: JPEG or RAW. Many cameras enable you to select which of these formats to use. digiKam supports both of these formats. A full description of these formats can be found at the Wikipedia.

静态图像压缩 ¶

Image compression is the application of data compression algorithms to digital images. Image compression reduces redundancy in the image data in order to be able to store or transmit data in an efficient form.

Image compression can be lossy or lossless. Lossless compression methods, especially applied before transformations like cropping, resizing, and color corrections, are always preferred for archival purposes because they preserve the original data. In contrast, lossy compression methods introduce compression artifacts, especially when used at low bit rates. Lossy methods are suitable for natural images such as photos in applications where minor (sometimes imperceptible) loss of fidelity is acceptable to achieve a substantial reduction in file size. For example, lossy compression is good for publishing images on the Internet.

JPEG ¶

Joint Photographic Experts Group is a compressed format that trades some image quality for smaller file sizes. In fact, most cameras save their images in this format unless you specify otherwise. A JPEG image is stored using lossy compression, and you can vary the amount of compression. This allows you to choose between lower compression and higher image quality or greater compression and poorer quality. The only reason to choose higher compression is because it creates smaller files so you can store more images, making it easier to send them by e-mail or post them on the Web. Most cameras give you two or three choices equivalent to good, better, best, although the names vary.

JPEG 2000 ¶

JPEG 2000 is an improvement intended to replace the older JPEG standard that was introduced in 2000. It provides for the same compression ratio with better (smoother) results compared to JPEG. The 2000 version also has the option of being lossless, if specified in the settings. It supports metadata and 12-bit color depth.

TIFF ¶

Tagged Image File Format has been widely accepted and widely supported as an image format. Commonly, TIFF may be stored by the camera in uncompressed form or using a lossless compression algorithm (Deflate). It maintains higher image quality but at the expense of much larger file sizes. Some cameras let you save your images in this format, and it is a popular format because of it is a lossless compression algorithm. The problem is that the format has been altered by so many people that there are now 50 or more flavors and not all are recognizable by most programs.

PNG ¶

Portable Network Graphics is an image format that was developed in the 1990’s as an patent-free replacement for the older GIF image file format. It is a lossless format like TIFF but is much more compact, saving disk space. Although your camera is unlikely to support PNG, some people like to convert their photographs to PNG as soon as they get them on their computer. Unlike JPEG, PNG images do not lose quality every time you re-encode them after modification. digiKam fully supports PNG images and the Batch Queue Manager can convert a batch of images from any supported format to PNG (and other formats) in one step.

PNG is designed to work well in on-line viewing applications, such as the World Wide Web, so it is fully streamable with a progressive display option. Also, PNG can store gamma and chromaticity data for improved color matching on heterogeneous platforms. PNG supports bit depths of 8 and 16 bits / color / pixel. It’s the perfect file format to archive your photographs.

PGF ¶

Progressive Graphics File is another not so well known, but open file image format. Wavelet-based, it allows lossless and lossy data compression. PGF compares well with JPEG 2000 but it was developed for speed (compression/decompression) rather than to be the best at compression ratio. At the same file size a PGF file looks significantly better than a JPEG file, while also remaining very good at progressive display. Thus it should be well-suited for photo archiving. digiKam uses this format to store thumbnails in its internal database.

WEBP ¶

Web Photo is an image file format developed by Google intended as a replacement for JPEG, PNG, and GIF file formats for use on the Internet. It supports both lossy and lossless compression as well as animation and alpha transparency. It supports metadata, but color depth is limited to 8 bits. WebP was first announced by Google in 2010 and produces files that are smaller than JPEG files for comparable image quality.

HEIF ¶

High Efficiency Image File Format is a container format introduced in 2015 for storing individual digital images and image sequences. The standard covers multimedia files that can also include other media streams, such as timed text, audio and video. HEIF can store images encoded with multiple coding formats, for example both SDR and HDR images. HEVC is an image and video encoding format and the default image codec used with HEIF. HEIF files containing HEVC-encoded images are also known as HEIC files. Such files require less storage space than the equivalent quality JPEG. HEIF support 12-bit color depth and metadata. This format was adopted in 2017 to store images in Apple devices.

AVIF ¶

AV1 Image File Format is an open, royalty-free image file format specification introduced in 2019 for storing images or image sequences compressed with AV1 in the HEIF container format. It competes with HEIC, which uses the same container format built upon the ISO base media file format (ISOBMFF) but uses HEVC for compression. AVIF showed better compression efficiency than JPEG as well as better detail preservation, fewer blocking artifacts and less color bleeding around hard edges in composites of natural images, text, and graphics. AVIF supports 12-bit color depth and metadata.

JXL ¶

JPEG XL is a royalty-free raster-graphics file format that supports both lossy and lossless compression. It is designed to outperform existing raster formats and thus become their universal replacement.

JPEG XL is particularly optimised for responsive Web environments, so that content renders well on a wide range of devices. Moreover, it includes several features that help transition from the legacy JPEG format. JPEG XL was introduced at the end of 2020. JPEG XL support 16-bit color depth and metadata.

备注

Digital still image format support is provided by digiKam Loader Plugins. You can see a list of available plugins in the Plugins/Image Loaders section of the Setup or Preferences dialog.

The Image Loaders list from digiKam Setup or Preferences dialog¶

RAW ¶

Some, typically more expensive, cameras allow you to store images in RAW format. RAW format is not really an image standard at all. It is different for every make of camera. RAW format images contain all the data that is taken directly from the camera’s image sensor before the software in the camera applies things like white balance, noise reduction, sharpening etc. Storing photographs in a camera’s RAW format allows you to alter settings, such as white balance, after the photograph has been taken. Most professional photographers use RAW format, because it offers them maximum flexibility. The downside is that RAW image files can be very large indeed.

如果您想了解有关 RAW 图像格式的更多信息，请访问这几个非常有用的指南：维基百科和剑桥色彩摄影学习社区。你可以在 digiKam 中使用图像编辑器或批处理管理器将 RAW 格式的图像转换成 JPEG、PNG 或 TIFF 格式。

digiKam 仅支持 RAW 图像的加载读取，它依赖于 Libraw 库，这个库属于 digiKam 核心文件之一，支持超过 1000 种 RAW 文件格式。下表简要介绍了 digiKam 支持的相机 RAW 文件：

RAW 文件格式	描述
CRW, CR2, CR3	佳能（Canon）相机的 RAW 文件格式
NEF, NRW	尼康（Nikon）相机的 RAW 文件格式
ORF, ORI	奥林巴斯/奥之心（Olympus）相机的 RAW 文件格式
RAF	富士（Fuji）相机的 RAW 文件格式
RWL	徕卡（Leica）相机的 RAW 文件格式
PEF, PTX	宾得（Pentax）相机的 RAW 文件格式
X3F	适马（Sigma）相机的 RAW 文件格式
DCR, KDC, DC2, K25	柯达（Kodak）相机的 RAW 文件格式
SRF, SR2, ARW, MRW, MDC	索尼（Sony）/美能达（Minolta）相机的 RAW 文件格式
RAW	松下（Panasonic），卡西欧（Casio），徕卡（Leica）相机的 RAW 文件格式
DNG (CS1, HDR)	Adobe 的 RAW 文件格式（数字负片），Apple ProRAW 文件
BAY	卡西欧（Casio）的 RAW 文件（拜耳阵列）
ERF	爱普生（Epson）相机的 RAW 文件格式
FFF, 3FR	易迈康（Imacon）/ 哈苏（Hasselblad）的 RAW 文件格式
RW2	松下（Panasonic）的 RAW 文件
IIQ, CAP	飞思（Phase One）相机的 RAW 文件格式
RDC	理光（Ricoh）的 RAW 文件

如果想知道 digiKam 是否支持您相机的 RAW 文件，请到 digiKam 程序中的帮助 ‣ 支持的 RAW 相机格式菜单对话框了解详情。

其他格式 ¶

digiKam relies on ImageMagick to support exotic still images when others modules fail to load a file. This is the case for FITS astro-photo, JBIG, PhotoCD, FLIF, BPG, etc.

备注

We recommend you also read the chapter about the file formats used for long time archiving, from the Digital Asset Management section in this manual.