1// Copyright 2018-2024 The Khronos Group Inc. 2// 3// SPDX-License-Identifier: CC-BY-4.0 4 5include::{generated}/meta/{refprefix}VK_EXT_astc_decode_mode.adoc[] 6 7=== Other Extension Metadata 8 9*Last Modified Date*:: 10 2018-08-07 11*Contributors*:: 12 - Jan-Harald Fredriksen, Arm 13 14=== Description 15 16The existing specification requires that low dynamic range (LDR) ASTC 17textures are decompressed to FP16 values per component. 18In many cases, decompressing LDR textures to a lower precision intermediate 19result gives acceptable image quality. 20Source material for LDR textures is typically authored as 8-bit UNORM 21values, so decoding to FP16 values adds little value. 22On the other hand, reducing precision of the decoded result reduces the size 23of the decompressed data, potentially improving texture cache performance 24and saving power. 25 26The goal of this extension is to enable this efficiency gain on existing 27ASTC texture data. 28This is achieved by giving the application the ability to select the 29intermediate decoding precision. 30 31Three decoding options are provided: 32 33 * Decode to ename:VK_FORMAT_R16G16B16A16_SFLOAT precision: This is the 34 default, and matches the required behavior in the core API. 35 * Decode to ename:VK_FORMAT_R8G8B8A8_UNORM precision: This is provided as 36 an option in LDR mode. 37 * Decode to ename:VK_FORMAT_E5B9G9R9_UFLOAT_PACK32 precision: This is 38 provided as an option in both LDR and HDR mode. 39 In this mode, negative values cannot be represented and are clamped to 40 zero. 41 The alpha component is ignored, and the results are as if alpha was 1.0. 42 This decode mode is optional and support can be queried via the physical 43 device properties. 44 45include::{generated}/interfaces/VK_EXT_astc_decode_mode.adoc[] 46 47=== Issues 48 491) Are implementations allowed to decode at a higher precision than what is 50requested? 51 52 RESOLUTION: No. 53 If we allow this, then this extension could be exposed on all 54 implementations that support ASTC. 55 But developers would have no way of knowing what precision was actually 56 used, and thus whether the image quality is sufficient at reduced 57 precision. 58 592) Should the decode mode be image view state and/or sampler state? 60 61 RESOLUTION: Image view state only. 62 Some implementations treat the different decode modes as different 63 texture formats. 64 65=== Example 66 67Create an image view that decodes to ename:VK_FORMAT_R8G8B8A8_UNORM 68precision: 69 70[source,c++] 71---- 72 VkImageViewASTCDecodeModeEXT decodeMode = 73 { 74 .sType = VK_STRUCTURE_TYPE_IMAGE_VIEW_ASTC_DECODE_MODE_EXT, 75 .pNext = NULL, 76 .decodeMode = VK_FORMAT_R8G8B8A8_UNORM 77 }; 78 79 VkImageViewCreateInfo createInfo = 80 { 81 .sType = VK_STRUCTURE_TYPE_IMAGE_VIEW_CREATE_INFO, 82 .pNext = &decodeMode, 83 // flags, image, viewType set to application-desired values 84 .format = VK_FORMAT_ASTC_8x8_UNORM_BLOCK, 85 // components, subresourceRange set to application-desired values 86 }; 87 88 VkImageView imageView; 89 VkResult result = vkCreateImageView( 90 device, 91 &createInfo, 92 NULL, 93 &imageView); 94---- 95 96=== Version History 97 98 * Revision 1, 2018-08-07 (Jan-Harald Fredriksen) 99 ** Initial revision 100 101