A Brief Summary of Image File Formats (WIP)

This page will explain some of the Image file types you can use in your Ogre projects. This list was gathered directly from DevIL, the image library historically used in Ogre. Ogre now uses the FreeImage image library which supports a superset of the image formats listed below.

Now, I'm going to omit some of the lesser known or used formats, and just give a brief description of what they are, how they are typically used, how to create them, and any special requirements for Ogre applications. (Could use help on that part).

The .BMP

Bitmap File
File Type: Raster Image
File Extension: .bmp

Bitmap files have some confusion associated with them. Some refer to any pixel-based image as a bitmap file. However, a true bitmap image file refers to the standard Windows image format. This type of file is mostly used on DOS- and Windows-based machines.
A bitmap file is a raster- (or pixel-) based format that only supports the RGB colour space and bit depths of 1, 4, 8, or 24 bits per channel. The BMP (bitmap) file type is used by many Windows applications. A Bitmap image is a representation of a graphic image, displayed by setting individual pixels to white, black, or a colour.

The bitmap is very accurate in recreating and holding your image (appearance-wise). But the drawback is, this can be the largest file type you can use. It stores all the colour information pixel by pixel, and some would say it's outdated. But this is a good format if you're working on a game that you wish to run on a broad range of computers.

The .JPG

JPGs do not limit colours, making this format the preferred for photographs. Always save your original image in a format other than JPG so you can go back and make changes later. If you save a JPG as a JPG after altering it, you are compressing the image again, causing lots of icky (that's a technical term :-) ) things to happen. Your JPGs can be highly compressed, resulting in really small files, or just a little compressed for larger files. You should experiment and save the image with the most compression, while still maintaining an acceptable image. If you save a JPG in Photoshop, you will not see the effects of the compression until you close the image and reopen it.

The JPG is a savable option in most real (As in NOT MS Paint) image editing packages. Photoshop even has more options that you can use to compress the filetype to your heart's content. So this format is created after you create or manipulate the image. Note: the compression effects of the JPG format will 'stack'; if you save your original image as a JPG, then reopen it and save it again, you have just compresed it twice, and it will look the worse for it.

The .GIF

GIF was created by Compuserve as a machine independent file format. GIF files can contain one or more images with 256 colours or less. The images are compressed using LZW compression, reducing the file size by 30-50% or more. GIFs are flexible, and have capabilities that JPGs don't, such as animation (GIF89a) and transparency. There are interlaced GIFs that load gradually, allowing the user to see parts of the image while it's loading. Interlaced files also take longer to decompress, and may actually take more time to download, although it may seem faster because the user has something to look at. It isn't worth the decompression time for small files. GIFs can also have transparent backgrounds to show the underlying background color or image. GIFs are generally smaller than JPGs, but larger than PNGs. Unless you are doing a large image, there is little to no difference in picture quality, but your palette is limited to 256 colours.

The GIF Is a savable option in most real image editing packages. Photoshop even has more options That you can use to compress the filetype to your heart's content. So this format is created after you create or manuplate the image, but it's recommended that you keep an eye on the colors you use while creating the image. When you convert the image to the GIF format, if you have more than 256 colours the program you use will try to "Match Closest Colour". This may create unintended effects when you save it as a GIF.

There is no reason to use GIF as an image format for games. For paletted textures, use PNG; for true colour textures use JPG or PNG.

The .RAW

The RAW file format is digital photography's equivalent of a negative in film photography: it contains untouched, "raw" pixel information straight from the digital camera's sensor. The RAW file format has yet to undergo demosaicking, and so it contains just one red, green, or blue value at each pixel location. RAW also has the advantage of being smaller (generally) than an uncompressed TIF file. If you want to avoid the lossy compression you get with JPGs, but don't want to put up with the huge file sizes of TIFs, this is a super way to go. Plus your RAW image is kind of like a photographic negative - if you mess up your processed image somehow, you can always use the RAW file to make a new one.

Note that different brands of digital cameras have different sensor mosaics (both in size and topology) and hence different RAW formats. To add to the confusion, various imaging and 3D packages employ their own RAW format. This makes RAW unsuitable as general image exchange or texture format.

In Ogre, this fileformat is used in Greyscale 8, 24 and 32 bit formats for representing -Height Maps. The values must be little endian encoded.

The .PNG

PNG is of principal value in two applications

  1. Large palette. If you have an image with large areas of exactly uniform colour, but containing more than 256 colours, PNG is your choice. PNG's strategy is similar to that of GIF, but it supports 16 million colours, not just 256.
  2. Lossless. If you want to display a photograph without loss on the web, PNG is your choice.


PNG is superior to GIF. It produces smaller files and allows more colours. PNG also supports partial transparency. Partial transparency can be used for many useful purposes, such as fades and antialiasing of text, or grass that fades. The alpha channel can be regarded either as a mask that temporarily hides transparent parts of the image, or as a means for constructing a non-rectangular image. In the first case, the colour values of fully transparent pixels should be preserved for future use. There is no lossy compression in PNG. Existing formats such as JFIF (JPEG File Interchange Format) already handle lossy compression well.

The .TGA

The Targa TGA format was developed by Truevision for their Targa and Vista products.
The TGA format file is widely used by high-end paint programs and ray tracing packages. It is a lossless and uncompressed format, thus producing huge files. For production purposes, the PNG format is better suited as a lossless format.

The .DDS

The DirectDraw Surface graphics file format was established by Microsoft for use with the DirectX SDK. The format is specifically designed for use in real-time rendering applications, such as 3D games. It can be used to store textures, cube maps and mipmap levels, and allows for compression that can be carried onto the graphics hardware (S3TC / DXT) unlike other compression algorithms like JPG and PNG. Although hardware texture compression is not mandatory in DDS files, it remains the most common reason to use to format.

S3 Texture Compression (S3TC) (sometimes also called DXTn or DXTC) is a group of related image compression algorithms originally developed by Iourcha et al of S3 Graphics, Ltd. Its subsequent inclusion in Microsoft's DirectX 6.0 led to widespread adoption of the technology among hardware and software makers.

Wikipedia:
http://en.wikipedia.org/wiki/S3_Texture_Compression

benefits:

  • When using DXT/S3TC, saves video RAM, because the compressed data is directly transfered to vram
  • In Direct3D, there is no decoding since DDS is a raw surface dump. In GL (and also if you manually decode a DDS using the Image class), Ogre decodes the DDS header manually and then deals with the pixel data as per usual in a block
  • different kinds of Alpha Channels (partial alpha if you use DXT2-5)
  • precalculated mipmaps, cube map faces and volume slices can all be stored in the same file




drawbacks:

  • None for uncompressed data except that files are larger than some other formats
  • When using DXT/S3TC, compression is lossy (you lose data when encoding).
  • To use DXT/S3TC you need a GeForce 2 or later. If your video card don't support texture compression, Ogre3D will decompress the texture at runtime and use it like other fileformats (so its no real drawback).
  • When using compression, data is a little harder to manipulate since it is not stored directly per-texel, but as blocks of texels.




additions: Using S3 (dxt) Texture Compression also in OpenGL - NVIDIA document

There are several types of DXT compression offered by DirectX:

DXT1

* 0 or 1 bit alpha




DXT2

* Explicit 4-bit alpha, color data is premultiplied by alpha




DXT3

* Explicit 4-bit alpha, not premultiplied




DXT4

* Interpolated alpha, color data is premultiplied by alpha




DXT5

* Interpolated alpha, not premultiplied




Which DXT Compression to Use?
Obviously, there are some trade-offs between the different formats which make them better or worse for different types of images. Some general rules of thumb for good use of DXT textures are as follows:

1. If your image has no alpha, use DXT1 compression. Using DXT3/5 will double your image size over DXT1 and not gain anything.
2. If your image has 1-bit (on or off) alpha information, use DXT1 with one-bit alpha. If the DXT1 image quality is too low and you don't mind doubling image size, use DXT3 or DXT5 (which one doesn't matter, they'll give the same results).
3. If your image has smooth gradations of alpha (fading in/out slowly), DXT5 is almost certainly your best bet, as it will give you the most accurate transparency representation.
4. If your image has sharp transitions between multiple alpha levels (one pixel is 100%, the next one is 50%, and another neighbor is 12%), DXT3 is probably your best bet. You may want to compare the alpha results in DXT1, DXT3 and DXT5 compression, however, to make sure.


Specification:
http://oss.sgi.com/projects/ogl-sample/registry/EXT/texture_compression_s3tc.txt

Infos:
http://www.sjbrown.co.uk/?code=squish

NVidia has some Tools on their Homepage to produce DDS files from Commandshell or Photoshop.

Tools
To create DDS files in Photoshop you must download the Nvidia DDS plugin. DDS files may also be created using the DirectX utility DxTex, found in the Utilities/bin folder of the DirectX SDK.

http://developer.nvidia.com/object/nv_texture_tools.html

The GIMP also has a plugin which can handle DXT-compressed DDS images.

<HR>
Creative Commons Copyright -- Some rights reserved.


THE WORK (AS DEFINED BELOW) IS PROVIDED UNDER THE TERMS OF THIS CREATIVE COMMONS PUBLIC LICENSE ("CCPL" OR "LICENSE"). THE WORK IS PROTECTED BY COPYRIGHT AND/OR OTHER APPLICABLE LAW. ANY USE OF THE WORK OTHER THAN AS AUTHORIZED UNDER THIS LICENSE OR COPYRIGHT LAW IS PROHIBITED.

BY EXERCISING ANY RIGHTS TO THE WORK PROVIDED HERE, YOU ACCEPT AND AGREE TO BE BOUND BY THE TERMS OF THIS LICENSE. THE LICENSOR GRANTS YOU THE RIGHTS CONTAINED HERE IN CONSIDERATION OF YOUR ACCEPTANCE OF SUCH TERMS AND CONDITIONS.

1. Definitions

  • "Collective Work" means a work, such as a periodical issue, anthology or encyclopedia, in which the Work in its entirety in unmodified form, along with a number of other contributions, constituting separate and independent works in themselves, are assembled into a collective whole. A work that constitutes a Collective Work will not be considered a Derivative Work (as defined below) for the purposes of this License.
  • "Derivative Work" means a work based upon the Work or upon the Work and other pre-existing works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which the Work may be recast, transformed, or adapted, except that a work that constitutes a Collective Work will not be considered a Derivative Work for the purpose of this License. For the avoidance of doubt, where the Work is a musical composition or sound recording, the synchronization of the Work in timed-relation with a moving image ("synching") will be considered a Derivative Work for the purpose of this License.
  • "Licensor" means the individual or entity that offers the Work under the terms of this License.
  • "Original Author" means the individual or entity who created the Work.
  • "Work" means the copyrightable work of authorship offered under the terms of this License.
  • "You" means an individual or entity exercising rights under this License who has not previously violated the terms of this License with respect to the Work, or who has received express permission from the Licensor to exercise rights under this License despite a previous violation.
  • "License Elements" means the following high-level license attributes as selected by Licensor and indicated in the title of this License: Attribution, ShareAlike.

2. Fair Use Rights

Nothing in this license is intended to reduce, limit, or restrict any rights arising from fair use, first sale or other limitations on the exclusive rights of the copyright owner under copyright law or other applicable laws.

3. License Grant

Subject to the terms and conditions of this License, Licensor hereby grants You a worldwide, royalty-free, non-exclusive, perpetual (for the duration of the applicable copyright) license to exercise the rights in the Work as stated below:

  • to reproduce the Work, to incorporate the Work into one or more Collective Works, and to reproduce the Work as incorporated in the Collective Works;
  • to create and reproduce Derivative Works;
  • to distribute copies or phonorecords of, display publicly, perform publicly, and perform publicly by means of a digital audio transmission the Work including as incorporated in Collective Works;
  • to distribute copies or phonorecords of, display publicly, perform publicly, and perform publicly by means of a digital audio transmission Derivative Works.
  • For the avoidance of doubt, where the work is a musical composition:
    • Performance Royalties Under Blanket Licenses. Licensor waives the exclusive right to collect, whether individually or via a performance rights society (e.g. ASCAP, BMI, SESAC), royalties for the public performance or public digital performance (e.g. webcast) of the Work.
    • Mechanical Rights and Statutory Royalties. Licensor waives the exclusive right to collect, whether individually or via a music rights society or designated agent (e.g. Harry Fox Agency), royalties for any phonorecord You create from the Work ("cover version") and distribute, subject to the compulsory license created by 17 USC Section 115 of the US Copyright Act (or the equivalent in other jurisdictions).
    • Webcasting Rights and Statutory Royalties. For the avoidance of doubt, where the Work is a sound recording, Licensor waives the exclusive right to collect, whether individually or via a performance-rights society (e.g. SoundExchange), royalties for the public digital performance (e.g. webcast) of the Work, subject to the compulsory license created by 17 USC Section 114 of the US Copyright Act (or the equivalent in other jurisdictions).


The above rights may be exercised in all media and formats whether now known or hereafter devised. The above rights include the right to make such modifications as are technically necessary to exercise the rights in other media and formats. All rights not expressly granted by Licensor are hereby reserved.

4. Restrictions

The license granted in Section 3 above is expressly made subject to and limited by the following restrictions:

  • You may distribute, publicly display, publicly perform, or publicly digitally perform the Work only under the terms of this License, and You must include a copy of, or the Uniform Resource Identifier for, this License with every copy or phonorecord of the Work You distribute, publicly display, publicly perform, or publicly digitally perform. You may not offer or impose any terms on the Work that alter or restrict the terms of this License or the recipients' exercise of the rights granted hereunder. You may not sublicense the Work. You must keep intact all notices that refer to this License and to the disclaimer of warranties. You may not distribute, publicly display, publicly perform, or publicly digitally perform the Work with any technological measures that control access or use of the Work in a manner inconsistent with the terms of this License Agreement. The above applies to the Work as incorporated in a Collective Work, but this does not require the Collective Work apart from the Work itself to be made subject to the terms of this License. If You create a Collective Work, upon notice from any Licensor You must, to the extent practicable, remove from the Collective Work any credit as required by clause 4(c), as requested. If You create a Derivative Work, upon notice from any Licensor You must, to the extent practicable, remove from the Derivative Work any credit as required by clause 4(c), as requested.
  • You may distribute, publicly display, publicly perform, or publicly digitally perform a Derivative Work only under the terms of this License, a later version of this License with the same License Elements as this License, or a Creative Commons iCommons license that contains the same License Elements as this License (e.g. Attribution-ShareAlike 2.5 Japan). You must include a copy of, or the Uniform Resource Identifier for, this License or other license specified in the previous sentence with every copy or phonorecord of each Derivative Work You distribute, publicly display, publicly perform, or publicly digitally perform. You may not offer or impose any terms on the Derivative Works that alter or restrict the terms of this License or the recipients' exercise of the rights granted hereunder, and You must keep intact all notices that refer to this License and to the disclaimer of warranties. You may not distribute, publicly display, publicly perform, or publicly digitally perform the Derivative Work with any technological measures that control access or use of the Work in a manner inconsistent with the terms of this License Agreement. The above applies to the Derivative Work as incorporated in a Collective Work, but this does not require the Collective Work apart from the Derivative Work itself to be made subject to the terms of this License.
  • If you distribute, publicly display, publicly perform, or publicly digitally perform the Work or any Derivative Works or Collective Works, You must keep intact all copyright notices for the Work and provide, reasonable to the medium or means You are utilizing: (i) the name of the Original Author (or pseudonym, if applicable) if supplied, and/or (ii) if the Original Author and/or Licensor designate another party or parties (e.g. a sponsor institute, publishing entity, journal) for attribution in Licensor's copyright notice, terms of service or by other reasonable means, the name of such party or parties; the title of the Work if supplied; to the extent reasonably practicable, the Uniform Resource Identifier, if any, that Licensor specifies to be associated with the Work, unless such URI does not refer to the copyright notice or licensing information for the Work; and in the case of a Derivative Work, a credit identifying the use of the Work in the Derivative Work (e.g., "French translation of the Work by Original Author," or "Screenplay based on original Work by Original Author"). Such credit may be implemented in any reasonable manner; provided, however, that in the case of a Derivative Work or Collective Work, at a minimum such credit will appear where any other comparable authorship credit appears and in a manner at least as prominent as such other comparable authorship credit.

5. Representations, Warranties and Disclaimer

UNLESS OTHERWISE AGREED TO BY THE PARTIES IN WRITING, LICENSOR OFFERS THE WORK AS-IS AND MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND CONCERNING THE MATERIALS, EXPRESS, IMPLIED, STATUTORY OR OTHERWISE, INCLUDING, WITHOUT LIMITATION, WARRANTIES OF TITLE, MERCHANTIBILITY, FITNESS FOR A PARTICULAR PURPOSE, NONINFRINGEMENT, OR THE ABSENCE OF LATENT OR OTHER DEFECTS, ACCURACY, OR THE PRESENCE OF ABSENCE OF ERRORS, WHETHER OR NOT DISCOVERABLE. SOME JURISDICTIONS DO NOT ALLOW THE EXCLUSION OF IMPLIED WARRANTIES, SO SUCH EXCLUSION MAY NOT APPLY TO YOU.

6. Limitation on Liability.

EXCEPT TO THE EXTENT REQUIRED BY APPLICABLE LAW, IN NO EVENT WILL LICENSOR BE LIABLE TO YOU ON ANY LEGAL THEORY FOR ANY SPECIAL, INCIDENTAL, CONSEQUENTIAL, PUNITIVE OR EXEMPLARY DAMAGES ARISING OUT OF THIS LICENSE OR THE USE OF THE WORK, EVEN IF LICENSOR HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

7. Termination

  • This License and the rights granted hereunder will terminate automatically upon any breach by You of the terms of this License. Individuals or entities who have received Derivative Works or Collective Works from You under this License, however, will not have their licenses terminated provided such individuals or entities remain in full compliance with those licenses. Sections 1, 2, 5, 6, 7, and 8 will survive any termination of this License.
  • Subject to the above terms and conditions, the license granted here is perpetual (for the duration of the applicable copyright in the Work). Notwithstanding the above, Licensor reserves the right to release the Work under different license terms or to stop distributing the Work at any time; provided, however that any such election will not serve to withdraw this License (or any other license that has been, or is required to be, granted under the terms of this License), and this License will continue in full force and effect unless terminated as stated above.

8. Miscellaneous

  • Each time You distribute or publicly digitally perform the Work or a Collective Work, the Licensor offers to the recipient a license to the Work on the same terms and conditions as the license granted to You under this License.
  • Each time You distribute or publicly digitally perform a Derivative Work, Licensor offers to the recipient a license to the original Work on the same terms and conditions as the license granted to You under this License.
  • If any provision of this License is invalid or unenforceable under applicable law, it shall not affect the validity or enforceability of the remainder of the terms of this License, and without further action by the parties to this agreement, such provision shall be reformed to the minimum extent necessary to make such provision valid and enforceable.
  • No term or provision of this License shall be deemed waived and no breach consented to unless such waiver or consent shall be in writing and signed by the party to be charged with such waiver or consent.
  • This License constitutes the entire agreement between the parties with respect to the Work licensed here. There are no understandings, agreements or representations with respect to the Work not specified here. Licensor shall not be bound by any additional provisions that may appear in any communication from You. This License may not be modified without the mutual written agreement of the Licensor and You.