slang-shaders/misc/shaders/scaling.slang

/*
    Scaling library v1.0 by fishku
    Copyright (C) 2023
    Public domain license (CC0)

    This file acts like a library and should be included in another shader to be
    used. For example usages, see the border/blur_fill shaders.

    It's recommended to use these functions in the vertex shader pass, and pass
    the data to the fragment pass.

    Features:
    - Cropping on each side
    - Centering of image after crop has been applied
    - Forcing of a certain aspect ratio
    - Forcing of either vert. or horiz. integer scaling, or both
    - Rotation support (0, 90, 180, 270 degrees).

    Refactored from the version that used to be in the blur_fill shader.

    Changelog:
    v1.0: Initial conversion from blur_fill release. Add rotation support.
*/

vec4 get_rotated_crop(vec4 crop, uint rotation) {
    switch (rotation) {
        case 0:
        default:
            return crop;
        case 1:
            return crop.yzwx;
        case 2:
            return crop.zwxy;
        case 3:
            return crop.wxyz;
    }
}

vec2 get_rotated_size(vec2 x, uint rotation) {
    switch (rotation) {
        case 0:
        case 2:
        default:
            return x;
        case 1:
        case 3:
            return x.yx;
    }
}

// Get 2 corners of input in texel space, spanning the input image.
// corners.x and .y define the top-left corner, corners.z and .w define the
// bottom-right corner.
vec4 get_input_corners(vec2 input_size, vec4 crop, uint rotation) {
    crop = get_rotated_crop(crop, rotation);
    return vec4(crop.y, crop.x, input_size.x - crop.w, input_size.y - crop.z);
}

// Get adjusted center in input pixel coordinate system.
vec2 get_input_center(vec2 input_size, vec4 crop, uint rotation,
                      float center_after_cropping) {
    crop = get_rotated_crop(crop, rotation);
    return center_after_cropping > 0.5
               ? 0.5 * vec2(crop.y + input_size.x - crop.w,
                            crop.x + input_size.y - crop.z)
               : vec2(0.49999) * input_size;
}

// Scaling from unit output to pixel input space.
vec2 get_scale_o2i(vec2 input_size, vec2 output_size, vec4 crop, uint rotation,
                   float center_after_cropping, float force_aspect_ratio,
                   vec2 aspect, vec2 force_integer_scaling,
                   bool output_size_is_final_viewport_size) {
    crop = get_rotated_crop(crop, rotation);
    if (output_size_is_final_viewport_size) {
        output_size = get_rotated_size(output_size, rotation);
    }
    aspect = get_rotated_size(aspect, rotation);
    // Aspect ratio before cropping.
    // lambda_1 * input_pixels.x, lambda_2 * input_pixels.y,
    // possibly corrected for forced aspect ratio
    aspect = (force_aspect_ratio < 0.5
                  ? output_size * input_size.yx
                  : (aspect.x < 0.5 || aspect.y < 0.5
                         ? vec2(1.0)
                         : vec2(aspect.x, aspect.y) * input_size.yx));
    // Pixels in input coord. space, after cropping.
    input_size = input_size -
                 (center_after_cropping > 0.5
                      ? vec2(crop.y + crop.w, crop.x + crop.z)
                      : 2.0 * vec2(min(crop.y, crop.w), min(crop.x, crop.z)));

    force_integer_scaling = get_rotated_size(force_integer_scaling, rotation);
    float scale_x, scale_y;
    if (output_size.x / (input_size.x * aspect.x) <
        output_size.y / (input_size.y * aspect.y)) {
        // Scale will be limited by width. Calc x scale, then derive y scale
        // using aspect ratio.
        scale_x = output_size.x / input_size.x;
        if (force_integer_scaling.x > 0.5 && scale_x > 1.0) {
            scale_x = floor(scale_x);
        }
        scale_y = scale_x * aspect.y / aspect.x;
        if (force_integer_scaling.y > 0.5 && scale_y > 1.0) {
            scale_y = floor(scale_y);
        }
    } else {
        // Scale will be limited by height.
        scale_y = output_size.y / input_size.y;
        if (force_integer_scaling.y > 0.5 && scale_y > 1.0) {
            scale_y = floor(scale_y);
        }
        scale_x = scale_y * aspect.x / aspect.y;
        if (force_integer_scaling.x > 0.5 && scale_x > 1.0) {
            scale_x = floor(scale_x);
        }
    }
    return output_size.xy / vec2(scale_x, scale_y);
}

// From unit output to pixel input space.
// coord_in_input_space = o2i(coord_in_output_space)
// This is used to sample from the input texture in the output pass.
// Version where scale is passed in.
vec2 o2i(vec2 x, vec2 input_size, vec4 crop, uint rotation,
         float center_after_cropping, vec2 scale_o2i) {
    return (x - 0.49999) * scale_o2i +
           get_input_center(input_size, crop, rotation, center_after_cropping);
}

// Version that computes scale.
vec2 o2i(vec2 x, vec2 input_size, vec2 output_size, vec4 crop, uint rotation,
         float center_after_cropping, float force_aspect_ratio, vec2 aspect,
         vec2 force_integer_scaling, bool output_size_is_final_viewport_size) {
    return o2i(x, input_size, crop, rotation, center_after_cropping,
               get_scale_o2i(input_size, output_size, crop, rotation,
                             center_after_cropping, force_aspect_ratio, aspect,
                             force_integer_scaling,
                             output_size_is_final_viewport_size));
}

// From pixel input to unit output space.
// Version where scale is passed in.
vec2 i2o(vec2 x, vec2 input_size, vec4 crop, uint rotation,
         float center_after_cropping, vec2 scale_o2i) {
    return (x - get_input_center(input_size, crop, rotation,
                                 center_after_cropping)) /
               scale_o2i +
           0.49999;
}

// Version that computes scale.
vec2 i2o(vec2 x, vec2 input_size, vec2 output_size, vec4 crop, uint rotation,
         float center_after_cropping, float force_aspect_ratio, vec2 aspect,
         vec2 force_integer_scaling, bool output_size_is_final_viewport_size) {
    return (x - get_input_center(input_size, crop, rotation,
                                 center_after_cropping)) /
               get_scale_o2i(input_size, output_size, crop, rotation,
                             center_after_cropping, force_aspect_ratio, aspect,
                             force_integer_scaling,
                             output_size_is_final_viewport_size) +
           0.49999;
}