学步园

In general, uncompressed image data is either 24bpp (RGB) or 32bpp (RGBA) flatrate. PVRTC is 4bpp (or 2bpp) flatrate so there is a compression of 6 or 8 (12 or 16) times compared to this.

A requirement for graphics hardware to use textures natively is that the format of the texture must be random accessible for the hardware. PVRTC is this kind of format, PNG is not and this is why PNG can achieve
greater compression ratios. PVRTC is a runtime, deployment format; PNG is a storage format.

PVRTC compression is carried out on 4x4 blocks of pixels at a time and at a flat bit rate so it is easy to calculate where in memory to retrieve the data required to derive a particular texel's value from and there is only one access to memory required. There
is dedicated circuitry in the graphics core which will decode this 4x4 block and give the texel value to your shader/texture combiner etc.

PNG compression does not work at a flat bitrate and is more complicated to retrieve specific values from; memory needs to be accessed from multiple locations in order to retrieve a single colour value and far more memory and processing would be required every
single time a texture read occurs. So it's not suitable for use as a native texture format and this is why your textures must be decompressed before the graphics hardware will use them. This increases bandwidth use when compared to PVRTC, which requires no
decompression for use.

So for offline storage (the size of your application on disk), PNG is smaller than PVRTC which is smaller than completely uncompressed. For runtime memory footprint and performance, PVRTC is smaller and faster than PNG which, because it must be decompressed,
is just as large and slow as uncompressed textures. You might gain some advantage with PNG at initialisation for disk access, but then you'd lose time for decompression.

If you want to reduce the storage footprint of PVRTC you could try zip-style compression on the texture files and expand these when you load from disk.

PVRTC (PowerVR Texture Compression) is a texture compression format. On devices using PowerVR e.g. most higher end mobile phones including the iPhone and other ARM-based gadgets like the iPod it is very fast to
draw since drawing it is hardware accelerated. It also uses much less memory since images are represented in their compressed form and decoded each draw, whereas a PNG needs to be decompressed before being drawn.

PNG is lossless compression.

PVRTC is lossy compression meaning it approximates the image. It has a completely different design criteria.

PVRTC will 'compress' (by approximating) any type of artwork, giving a fixed bits per texel, including photographic images.

PNG does not approximate the image, so if the image contains little redundancy it will hardly compress at all. On the other hand, a uniform image e.g. an illustration
will compress best with PNG.

Its apples and oranges.

dramatically reduce your memory consumption.

If you images are, say, 64x64, then you can place 256 of them on a 1024x1024 texture in a 16x16 arrangement.

With a little effort, images do not need to be all the same size, just so long as you keep track in the code of the rectangle in the texture that each image is at.

This is how iPhone game developers do it.

I agree with Will. There is no point in the question. I read the question 3 times, but I still don't know what Sankar want to know. It's just a complain, no question.

The only thing I can advice, don't use PVRTC if you mind to use it.It offers performance gain and saves VRAM, but it won't help you in this case. Because what you
want is just reducing game volume, not a consideration about trade-off between performance and quality.