Proposed: Use godot rgbaf textures to store integers

Metadata

• Status: proposed
• Deciders: V-Sekai,
• Tags: V-Sekai,

Context and Problem Statement

Use godot rgbaf textures to store integer.

Describe the proposed option and how it helps to overcome the problem or limitation

Godot Shader to extract the raw 16-bit integer in a FORMAT_RGBAH texture by Lyuma.

Describe how your proposal will work, with code, pseudo-code, mock-ups, or diagrams

shader_type spatial;
render_mode unshaded;
uniform usampler2D data_tex;

/*
Use with the following GDScript code:
    var data_img = Image.new()
    var data := PoolByteArray()
    var indexin = 0x8400
    data.append(indexin&0xff)
    data.append((indexin>>8)&0xff)
    data_img.create_from_data(1, 1, false, Image.FORMAT_RH, data)
    var data_tex = ImageTexture.new()
    data_tex.create_from_image(data_img, 0)
    self.get_surface_material(0).set_shader_param("data_tex", data_tex)
*/

// Extract the raw 16-bit integer assigned in a GL_RGBA16F or similar texture format
// (in Godot, Image.FORMAT_RGBAH or Image.FORMAT_RH)

// vectorized ?:
// In GLSL, you are supposed to do this using a mix(uvec4, uvec4, bvec4)
// However, in Godot, they forgot this overload existed, so we reimplement it.

uvec4 godot_hack_mix(uvec4 a, uvec4 b, bvec4 c) {
    return uvec4(c.x ? b.x : a.x, c.y ? b.y : a.y, c.z ? b.z : a.z, c.w ? b.w : a.w);
}

void fragment() {
    // Fetch half precision texture using usampler2D.
    // For some reason, it first converts to 32-bit float (but losslessly????)
    // and so we have to convert the 32-bit float back to 16-bits.
    uvec4 data = texelFetch(data_tex, ivec2(0,0), 0);

    // Calculate bit shifted exponent
    uvec4 exps = godot_hack_mix(
        // Adjust floating point from 8-bit in float32 (127 bias) to 5-bit in half (15 bias)
        uvec4((uvec4((ivec4((data & uvec4(0x7F800000)) >> uvec4(23)) - ivec4(127)) + ivec4(15)) & uvec4(0x1F)) << uvec4(10)),
        // Inf and NaN will remain Inf and Nan (maximum half exponent = 31)
        uvec4(ivec4(31 << 10)),
        // Check for maximum float32 exponent = 255
        equal((data) & uvec4(0x7F800000), uvec4(0x7F800000)));

    // calculate final data.
    data = godot_hack_mix(
        // mantissa | exponent | signbit
        ((data & uvec4(0x007FFFFF)) >> uvec4(13)) | exps | (((data >> uvec4(31)) & uvec4(1)) << uvec4(15)),
        // Denormalized case: exponent is all over the place, so instead we add the smallest non-denormalized half, then extract the mantessa bits.
        // Finally, or the sign bit (and exponent = 0)
        (((floatBitsToUint(uintBitsToFloat(data & uvec4(0x7FFFFFFF)) + 1.0/16384.0) & uvec4(0x007FFFFF)) >> uvec4(13))) | (((data >> uvec4(31)) & uvec4(1)) << uvec4(15)),
        // Denormalized case can be detected with half exponent <= -15.
        lessThanEqual((data) & uvec4(0x7F800000), uvec4(0x38000000)));

    // Parse out individual bits if needed
    ivec2 datar = ivec2((uvec2(data.r) >> uvec2(0,8)) & uvec2(0xFF));
    ivec2 datag = ivec2((uvec2(data.g) >> uvec2(0,8)) & uvec2(0xFF));
    ivec2 datab = ivec2((uvec2(data.b) >> uvec2(0,8)) & uvec2(0xFF));
    ivec2 dataa = ivec2((uvec2(data.a) >> uvec2(0,8)) & uvec2(0xFF));

    // Example code: check for precise 16-bit value that matches GDScript.
    ALBEDO = vec3(0,0,0);
    if (data.r == uint(0xFFFF)) {
        ALBEDO = vec3(1,1,1);
    }
}

Positive Consequences

We are able to use V-Sekai easier.

Negative Consequences

We cannot get other features in.

Option graveyard

Use 8 bit colors.

If this enhancement will not be used often, can it be worked around with a few lines of script?

This is a few lines of script.

Is there a reason why this should be core and done by us?

Wanted to record this.

References

• V-Sekai