Comparison among Texture Compressions on Mobile Device

Android* Texture Compression - a comparison study with code sample

The application of an image, or texture, to a 2D or 3D model to enhance graphical detail is a very common technique in the field of computer graphics. Android* allows the usage of a variety of texture compression file formats, each of which has its own set of advantages and disadvantages. The Android Texture Compression sample allows developers to easily compare textures of five different texture compression file formats: Portable Network Graphics* (PNG), Ericsson Texture Compression* (ETC), Ericsson Texture Compression 2* (ETC2), PowerVR Texture Compression* (PVRTC), and S3 Texture Compression* (S3TC), which is also known as DirectX Texture Compression* (DXTC).  This sample demonstrates how to load and use these different formats with OpenGL ES* on Android. All supported texture formats are shown side-by-side so the relative size and quality can be observed. Choosing the right compression allows the developer to balance app size, visual quality, and performance.

The sample loads each texture format, determines the mapping coordinates of the texture, and displays a portion of each of the textures. The final composition will display the full image/texture, but as four separate, format-specific textures. They are individually labeled at the top of the screen and the file sizes are provided on a small bar at the bottom of the screen.

Background on Principles/Related Terms and Texture Formats

Texture mapping is a method by which an image is applied to the surface of a shape or polygon. A helpful analogy to keep in mind is picturing the texture as wrapping paper, and the 3D model as a gift box to be wrapped.  This is why this process is also called “texture wrapping.”

Mipmaps are an optimized group of images that are generated with the primary texture. They are typically created for the purpose of improving rendering speed and reducing aliasing artifacts. Each mip (bitmap image of the collection of mipmap) is a lower resolution version of the primary texture, utilized when viewing the original texture from a distance or a downsized version of it. The creation and implementation of mipmaps comes from the basic concept that we cannot pick up as much detail in an object when it is located far away from us or when the object is miniscule. Based on this idea, different mips can be used to represent different parts of the texture/image based on the size of the objects. This increases rendering speed because the simplified mips have a much lower texel (overall number of texture pixels) count—less pixels to be processed. Additionally, since mipmaps are essentially anti-aliased, the number of noticeable artifacts is also greatly reduced. The support for mipmaps in PNG, ETC (KTX), ETC2 (KTX), PVRTC, and S3TC are included in the sample.

Portable Network Graphics (PNG)

PNG is a bitmapped image format primarily noted for its lossless data file compression. The image format is equipped with support for palette-based images (24 bit RGB or 32 bit RGBA) or grayscale images (with and without an alpha channel).

Advantages:

• Has a lossless compression scheme and high visual quality
• Handles both 8-bit and 16-bit transparency

Disadvantages:

• Large file size; this will increase app size and memory bandwidth requirements
• Highest GPU cost (i.e. worst performance)

Ericsson Texture Compression (ETC)

Ericsson Texture Compression is a texture compression format that operates on 4x4 blocks of pixels.  Originally Khronos used Ericsson Texture Compression as the standard for OpenGL ES 2.0 (this version is also known as ETC1). Therefore, this texture compression format is available on nearly all Android devices. Recently with the release of OpenGL ES 3.0, a reworked version of ETC1, known as ETC2, was implemented as the new standard.  The main differences between the two schemes is the algorithm which operates on each pixel group.  The improvements in the algorithm result in higher fidelity output when it comes to finer details.  The finer quality of the image comes without the cost of additional space.

ETC1 and ETC2 both support compression of 24-bit RGB data, but they do not support the compression of any images/textures containing alpha components. In addition, there are two different file formats that both fall under the category of ETC texture compression: KTX and PKM. KTX is the standard Khronos Group compression format, and it provides a container for multiple images/textures. When mipmaps are generated with KTX, only one KTX file is created. On the other hand, PKM is a much simpler file format used mainly to contain single compressed images. Generating mipmaps in this case would create multiple PKM files instead of a single file, and so as a consequence, this format is not recommended for that purpose.

Advantages:

• File size is considerably smaller in comparison to the PNG texture compression format
• GPU hardware acceleration supported on nearly all Android devices 

Disadvantages:

• Quality is not as high as PNG texture compression (ETC is a lossy compression format)
• Does not support alpha channels/components

Example of Tools Used For Compression:

• Mali* Texture Compression Tool (Developer Center)
• ETC-Pack Tool

PowerVR Texture Compression (PVRTC)

PowerVR Texture Compression is a lossy, fixed-rate texture compression format utilized primarily in Imagination Technologies’ PowerVR* MBX, SGX, and Rogue technologies. It is currently being employed in all iPhone*, iPod*, and iPad* devices as the standard compression format. Unlike ETC and S3TC, PVRTC is not block-based but rather involves the bilinear upscaling and low-precision blending of two low-resolution images. In addition to the unique process of compression by the PVRTC format, it also supports RGBA (alpha channel supported) for both the 2-bpp (2 bits per pixel) and 4-bpp (4 bits per pixel) options.

Advantages:

• Supports alpha channels/components
• Supports RGBA data for both 2-bpp and 4-bpp modes
• File size is much smaller than one using PNG texture compression
• GPU hardware acceleration on PowerVR GPUs
   

Disadvantages:

• Quality is not as high as PNG texture compression (PVRTC is a lossy compression format)

• PVRTC is only supported on PowerVR hardware
• Only square (power-of-two) dimension textures are determined to work consistently, although in some cases, rectangular support is provided for the compressed texture
• Compressing textures into this format can be slow

Tool Used For Compression:

• PVRTexTool* v3.33 from Imagination Technologies

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Cited URL

2. 压缩纹理的必要性
1）首先要说一下图像文件格式和纹理格式的区别。
常用的图像文件格式有BMP，TGA，JPG，GIF，PNG等；
常用的纹理格式有R5G6B5，A4R4G4B4，A1R5G5B5，R8G8B8, A8R8G8B8等。

    文件格式是图像为了存储信息而使用的对信息的特殊编码方式，它存储在磁盘中，或者内存中，但是并不能被GPU所识别，因为以向量计算见长的GPU对于这些复杂的计算无能为力。这些文件格式当被游戏读入后，还是需要经过CPU解压成R5G6B5，A4R4G4B4，A1R5G5B5，R8G8B8, A8R8G8B8等像素格式，再传送到GPU端进行使用。
    纹理格式是能被GPU所识别的像素格式，能被快速寻址并采样。
举个例子，DDS文件是游戏开发中常用的文件格式，它内部可以包含A4R4G4B4的纹理格式，也可以包含A8R8G8B8的纹理格式，甚至可以包含DXT1的纹理格式。在这里DDS文件有点容器的意味。

    OpenGL ES 2.0支持以上提到的R5G6B5，A4R4G4B4，A1R5G5B5，R8G8B8，A8R8G8B8等纹理格式，其中R5G6B5，A4R4G4B4，A1R5G5B5每个像素占用2个字节(BYTE)，R8G8B8每个像素占用3个字节，A8R8G8B8每个像素占用4个字节。

    

    对于一张512*512的纹理的话，R5G6B5格式的文件需要占用512KB的容量，A8R8G8B8格式的文件需要占用1MB的容量；如果是1024*1024的纹理，则各需要2M和4M的容量，这对于动辄需要几十、几百张甚至更多纹理的游戏，上G容量的游戏在移动平台上是不容易被接受的(当然，还是有1、2G的大作的，里面包含了几千张的纹理)。

    聪明的设计师们在想，有没有其他办法，既能表现丰富的色彩和细节，又能是最小失真的情况下，达到更小的纹理容量呢。压缩纹理格式应运而生(当然，并不是在移动平台后才有的产物)。