Contact
-
Jan-Harald Fredriksen janharaldfredriksen-arm
Description
The existing specification requires that low dynamic range (LDR) ASTC textures are decompressed to FP16 values per component. In many cases, decompressing LDR textures to a lower precision intermediate result gives acceptable image quality. Source material for LDR textures is typically authored as 8-bit UNORM values, so decoding to FP16 values adds little value. On the other hand, reducing precision of the decoded result reduces the size of the decompressed data, potentially improving texture cache performance and saving power.
The goal of this extension is to enable this efficiency gain on existing ASTC texture data. This is achieved by giving the application the ability to select the intermediate decoding precision.
Three decoding options are provided:
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Decode to
VK_FORMAT_R16G16B16A16_SFLOATprecision: This is the default, and matches the required behavior in the core API. -
Decode to
VK_FORMAT_R8G8B8A8_UNORMprecision: This is provided as an option in LDR mode. -
Decode to
VK_FORMAT_E5B9G9R9_UFLOAT_PACK32precision: This is provided as an option in both LDR and HDR mode. In this mode, negative values cannot be represented and are clamped to zero. The alpha component is ignored, and the results are as if alpha was 1.0. This decode mode is optional and support can be queried via the physical device properties.
New Enum Constants
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VK_EXT_ASTC_DECODE_MODE_EXTENSION_NAME -
VK_EXT_ASTC_DECODE_MODE_SPEC_VERSION -
Extending VkStructureType:
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VK_STRUCTURE_TYPE_IMAGE_VIEW_ASTC_DECODE_MODE_EXT -
VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_ASTC_DECODE_FEATURES_EXT
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Issues
1) Are implementations allowed to decode at a higher precision than what is requested?
RESOLUTION: No. If we allow this, then this extension could be exposed on all implementations that support ASTC. But developers would have no way of knowing what precision was actually used, and thus whether the image quality is sufficient at reduced precision.
2) Should the decode mode be image view state and/or sampler state?
RESOLUTION: Image view state only. Some implementations treat the different decode modes as different texture formats.
Example
Create an image view that decodes to VK_FORMAT_R8G8B8A8_UNORM
precision:
VkImageViewASTCDecodeModeEXT decodeMode =
{
.sType = VK_STRUCTURE_TYPE_IMAGE_VIEW_ASTC_DECODE_MODE_EXT,
.pNext = NULL,
.decodeMode = VK_FORMAT_R8G8B8A8_UNORM
};
VkImageViewCreateInfo createInfo =
{
.sType = VK_STRUCTURE_TYPE_IMAGE_VIEW_CREATE_INFO,
.pNext = &decodeMode,
// flags, image, viewType set to application-desired values
.format = VK_FORMAT_ASTC_8x8_UNORM_BLOCK,
// components, subresourceRange set to application-desired values
};
VkImageView imageView;
VkResult result = vkCreateImageView(
device,
&createInfo,
NULL,
&imageView);Document Notes
For more information, see the Vulkan Specification
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