Texture fetching

Textures give access to specialized hardware on GPUs that is usually used in graphics processing. In particular, textures use a different way of accessing their underlying device memory. Memory accesses to textures are routed through a special read-only texture cache, that is optimized for logical spatial locality, e.g. locality in 2D grids. This can also benefit certain algorithms used in GPGPU computing, when the access pattern is the same as used when accessing normal textures.

Additionally, textures can be indexed using floating-point values. This is used in graphics applications to interpolate between neighboring values of a texture. Depending on the interpolation mode the index can be in the range of 0 to size - 1 or 0 to 1. Textures also have a way of handling out-of-bounds accesses.

Depending on the value of the index, texture filtering or texture addressing is performed.

Here is the example texture used in this document for demonstration purposes. It is 2x2 texels and indexed in the [0 to 1] range.

Texture used as example

In HIP textures objects are of type hipTextureObject_t and created using hipCreateTextureObject().

For a full list of available texture functions see the HIP texture API reference.

A code example for how to use textures can be found in the ROCm texture management example

Texture filtering

Texture filtering handles the usage of fractional indices. When the index is a fraction, the queried value lies between two or more texels (texture elements), depending on the dimensionality of the texture. The filtering method defines how to interpolate between these values.

The filter modes are specified in hipTextureFilterMode.

The various texture filtering methods are discussed in the following sections.

Nearest point filtering

This filter mode corresponds to hipFilterModePoint.

In this method, the modulo of index is calculated as:

tex(x) = T[floor(x)]

This is also applicable for 2D and 3D variants.

This doesn’t interpolate between neighboring values, which results in a pixelated look.

The following image shows a texture stretched to a 4x4 pixel quad but still indexed in the [0 to 1] range. The in-between values are the same as the values of the nearest texel.

Texture upscaled with nearest point filtering

Linear filtering

This filter mode corresponds to hipFilterModeLinear.

The linear filtering method does a linear interpolation between values. Linear interpolation is used to create a linear transition between two values. The formula used is (1-t)P1 + tP2 where P1 and P2 are the values and t is within the [0 to 1] range.

In the case of linear texture filtering the following formulas are used:

• For one dimensional textures: tex(x) = (1-α)T[i] + αT[i+1]

• For two dimensional textures: tex(x,y) = (1-α)(1-β)T[i,j] + α(1-β)T[i+1,j] + (1-α)βT[i,j+1] + αβT[i+1,j+1]

• For three dimensional textures: tex(x,y,z) = (1-α)(1-β)(1-γ)T[i,j,k] + α(1-β)(1-γ)T[i+1,j,k] + (1-α)β(1-γ)T[i,j+1,k] + αβ(1-γ)T[i+1,j+1,k] + (1-α)(1-β)γT[i,j,k+1] + α(1-β)γT[i+1,j,k+1] + (1-α)βγT[i,j+1,k+1] + αβγT[i+1,j+1,k+1]

Where x, y, and, z are the floating-point indices. i, j, and, k are the integer indices and, α, β, and, γ values represent how far along the sampled point is on the three axes. These values are calculated by these formulas: i = floor(x'), α = frac(x'), x' = x - 0.5, j = floor(y'), β = frac(y'), y' = y - 0.5, k = floor(z'), γ = frac(z') and z' = z - 0.5

The following image shows a texture stretched out to a 4x4 pixel quad, but still indexed in the [0 to 1] range. The in-between values are interpolated between the neighboring texels.

Texture upscaled with linear filtering

Texture addressing

The texture addressing modes are specified in hipTextureAddressMode.

The texture addressing mode handles out-of-bounds accesses to the texture. This can be used in graphics applications to e.g. repeat a texture on a surface multiple times in various ways or create visible signs of out-of-bounds indexing.

The following sections describe the various texture addressing methods.

Address mode border

This addressing mode is set using hipAddressModeBorder.

This addressing mode returns a border value when indexing out of bounds. The border value must be set before texture fetching.

The following image shows the texture on a 4x4 pixel quad, indexed in the [0 to 3] range. The out-of-bounds values are the border color, which is yellow.

Texture with yellow border color.

The purple lines are not part of the texture. They only denote the edge, where the addressing begins.

Address mode clamp

This addressing mode is set using hipAddressModeClamp.

This mode clamps the index between [0 to size-1]. Due to this, when indexing out-of-bounds, the values on the edge of the texture repeat. The clamp mode is the default addressing mode.

The following image shows the texture on a 4x4 pixel quad, indexed in the [0 to 3] range. The out-of-bounds values are repeating the values at the edge of the texture.

Texture with clamp addressing

The purple lines are not part of the texture. They only denote the edge, where the addressing begins.

Address mode wrap

This addressing mode is set using hipAddressModeWrap.

Wrap mode addressing is only available for normalized texture coordinates. In this addressing mode, the fractional part of the index is used:

tex(frac(x))

This creates a repeating image effect.

The following image shows the texture on a 4x4 pixel quad, indexed in the [0 to 3] range. The out-of-bounds values are repeating the original texture.

Texture with wrap addressing.

The purple lines are not part of the texture. They only denote the edge, where the addressing begins.

Address mode mirror

This addressing mode is set using hipAddressModeMirror.

Similar to the wrap mode the mirror mode is only available for normalized texture coordinates and also creates a repeating image, but mirroring the neighboring instances.

The formula is the following:

tex(frac(x)), if floor(x) is even,

tex(1 - frac(x)), if floor(x) is odd.

The following image shows the texture on a 4x4 pixel quad, indexed in The [0 to 3] range. The out-of-bounds values are repeating the original texture, but mirrored.

Texture with mirror addressing

The purple lines are not part of the texture. They only denote the edge, where the addressing begins.