使用 PVRTC 压缩格式创建纹理（Creating textures in the PVRTC compression format）

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本文遵循“署名-非商业用途-保持一致”创作公用协议

转载请保留此句：太阳火神的美丽人生 -  本博客专注于 敏捷开发及移动和物联设备研究：iOS、Android、Html5、Arduino、pcDuino，否则，出自本博客的文章拒绝转载或再转载，谢谢合作。

      有关该篇官方文档的相关信息，可以看到很久一直保持应用着，下面具体来看一下 PVRTC 是否能解决纹理加载过程中的延迟大的问题，以便能使其它方法处理结果速度有所提升。

针对于跨平台的 OpenGLES 应用，尚不知这种压缩后的纹理是否能正常工作，有待进一步证实。

使用 PVRTC 压缩格式创建纹理
Creating textures in the PVRTC compression format

Q:  我如何使用 PVRTC 压缩格式创建纹理？
Q:  How do I create textures in the PVRTC compression format?

A: 我如何使用 PVRTC 压缩格式创建纹理？
A: How do I create textures in the PVRTC compression format?

The iPhone SDK includes a tool that allows you to create textures in the PVRTC compression format, aptly named texturetool. If you have Xcode installed with the iPhone OS 2.2 SDK in the default location (/Developer/Platforms/), then texturetool is located at: /Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/texturetool.

texturetool allows you to create four variants of PVRTC data, the primary difference being tradeoffs between quality and size. You will have to experiment with these variants to determine which setting is the best compromise for each individual texture image.

The parameters that may be passed to texturetool and their function is presented in the rest of this document.

Listing 1  Encoding Options.

user$ texturetool -l  Encoders:  PVRTC  --channel-weighting-linear  --channel-weighting-perceptual  --bits-per-pixel-2  --bits-per-pixel-4  Formats:  Raw PVR

The texturetool will default to --bits-per-pixel-4, --channel-weighting-linear, and -f Raw if no other options are provided.

The --bits-per-pixel-2 and --bits-per-pixel-4 options create PVRTC data that encodes source pixels into 2 or 4 bits per pixel. These options represent a fixed 16:1 and 8:1 compression ratio over the uncompressed 32-bit RGBA image data. There is a minimum data size of 32 bytes; the compressor will never produce files smaller than this, and at least that many bytes are expected when uploading compressed texture data.

When compressing specifying --channel-weighting-linear will spread compression error equally across all channels. By contrast specifying --channel-weighting-perceptual attempts to reduce error in the green channel compared to the linear option. In general, PVRTC compression does better with photographic images than with line art.

The -m option allows you to automatically generate mipmap levels for the source image. This levels are provided as additional image data in the archive created. If you use the Raw image format, then each level of image data will be appended one after another to the archive. If you use the PVR archive format, then each mipmap image will be provided as a separate image in the archive.

With iPhone OS 2.2, the texturetool now additional supports a format (-f) parameter that allows you to control the format of its output file. While this parameter is not available with iPhone OS 2.1 or earlier, the data files produced are compatible with those versions if iPhone OS.

The default format is Raw, which is equivalent to the format that texturetool produced under iPhone SDK 2.0 and 2.1. This format is raw compressed texture data, either for a single texture level (without the -m option) or for each texture level concatenated together (with the -m option). Each texture level stored in the file will be at least 32 bytes in size, and must be uploaded to the GPU in its entirety.

The PVR format is the same format that the PVRTexTool from the Imagination Technologies PowerVR SDK produces, and requires parsing to obtain the actual texture data. See the PVRTextureLoader sample for an example of working with texture data in this format.

• Height and Width must be a power of 2.

• Must be square (height==width).

• Source images must be in a format that Image IO accepts. This includes PNG, JPG, TIFF and others.

Listing 2  Encoding images into the PVRTC compression format.

Encode Image.png into PVRTC using linear weights and 4 bpp, and saving as ImageL4.pvrtc
user$ texturetool -e PVRTC --channel-weighting-linear --bits-per-pixel-4 -o ImageL4.pvrtc Image.png

Encode Image.png into PVRTC using perceptual weights and 4 bpp, and saving as ImageP4.pvrtc
user$ texturetool -e PVRTC --channel-weighting-perceptual --bits-per-pixel-4 -o ImageP4.pvrtc Image.png

Encode Image.png into PVRTC using linear weights and 2 bpp, and saving as ImageL2.pvrtc
user$ texturetool -e PVRTC --channel-weighting-linear --bits-per-pixel-2 -o ImageL2.pvrtc Image.png

Encode Image.png into PVRTC using perceptual weights and 2 bpp, and saving as ImageP2.pvrtc
user$ texturetool -e PVRTC --channel-weighting-perceptual --bits-per-pixel-2 -o ImageP2.pvrtc Image.png

Listing 3  Encoding images into the PVRTC compression format while creating a preview.

Encode Image.png into PVRTC using linear weights and 4 bpp, and saving the output as ImageL4.pvrtc and a PNG preview as ImageL4.png
user$ texturetool -e PVRTC --channel-weighting-linear --bits-per-pixel-4 -o ImageL4.pvrtc -p ImageL4.png Image.png

Encode Image.png into PVRTC using perceptual weights and 4 bpp, and saving the output as ImageP4.pvrtc and a PNG preview as ImageP4.png
user$ texturetool -e PVRTC --channel-weighting-perceptual --bits-per-pixel-4 -o ImageP4.pvrtc -p ImageP4.png Image.png

Encode Image.png into PVRTC using linear weights and 2 bpp, and saving the output as ImageL2.pvrtc and a PNG preview as ImageL2.png
user$ texturetool -e PVRTC --channel-weighting-linear --bits-per-pixel-2 -o ImageL2.pvrtc -p ImageL2.png Image.png

Encode Image.png into PVRTC using perceptual weights and 2 bpp, and saving the output as ImageP2.pvrtc and a PNG preview as ImageP2.png
user$ texturetool -e PVRTC --channel-weighting-perceptual --bits-per-pixel-2 -o ImageP2.pvrtc -p ImageP2.png Image.png

The sample images produced by these settings are shown below.

Listing 4 demonstrates how you can upload Raw PVRTC data to the GPU via the texImage2DPVRTC sample function. Typically on iPhone OS you would use NSBundle and NSData methods to load data from a file. For a complete example demonstrating how to upload texture data from a PVR formatted data file, see the PVRTextureLoader sample.

Listing 4  Example of uploading PVRTC data to the graphics chip.

void texImage2DPVRTC(GLint level, GLsizei bpp, GLboolean hasAlpha, GLsizei width, GLsizei height, void *pvrtcData)
{
    GLenum format;
    GLsizei size = width * height * bpp / 8;
    if(hasAlpha) {
        format = (bpp == 4) ? GL_COMPRESSED_RGBA_PVRTC_4BPPV1_IMG : GL_COMPRESSED_RGBA_PVRTC_2BPPV1_IMG;
    } else {
        format = (bpp == 4) ? GL_COMPRESSED_RGB_PVRTC_4BPPV1_IMG : GL_COMPRESSED_RGB_PVRTC_2BPPV1_IMG;
    }
    if(size < 32) {
        size = 32;
    }
    glCompressedTexImage2D(GL_TEXTURE_2D, level, format, width, height, 0, size, data);
}

For sample code see the PVRTextureLoader sample.

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