Basic raster drawing primitives and utilities

This module provides fundamental drawing operations including antialiased lines, color conversion, geometric calculations, and pixel blending.

Pixel Coordinate System Conventions

The module uses a 1-based coordinate system consistent with Fortran arrays: - Pixel (1,1) is at the top-left corner - x-coordinates increase rightward, y-coordinates increase downward - All coordinates are internally converted using nint() for consistency

Antialiasing Approach

All drawing functions use distance-based antialiasing: - Calculate exact distance from pixel center to geometric shape - Alpha value derived from distance: alpha = 1.0 - max(0, distance - radius/half_width) - Sub-pixel accuracy maintained through real-valued intermediate calculations - Final pixel blending uses consistent coordinate rounding (nint)

Line-Marker Coordinate Alignment

Critical design principle: Lines and markers must align precisely at data points. Both line drawing (draw_line_distance_aa) and marker positioning use identical coordinate rounding via nint() in blend_pixel to prevent visual misalignment. This addresses the centering issue where markers appeared offset from line endpoints.

Color Representation

• Input colors: real values in range [0.0, 1.0]
• Storage: signed bytes representing unsigned values [0, 255]
• Conversion: color_to_byte handles proper range mapping and byte encoding
• Alpha blending: standard over-operation with clamped alpha values

Author: fortplot contributors

Functions

public function color_to_byte(color_val) result(byte_val)

Convert floating-point color value [0,1] to byte [0,255]

Arguments

Type	Intent	Optional	Attributes		Name
real(kind=wp),	intent(in)			::	color_val

Return Value integer(kind=1)

public function distance_point_to_line_segment(px, py, x1, y1, x2, y2) result(distance)

Calculate minimum distance from point to line segment

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Arguments

Type	Intent	Optional	Attributes		Name
real(kind=wp),	intent(in)			::	px
real(kind=wp),	intent(in)			::	py
real(kind=wp),	intent(in)			::	x1
real(kind=wp),	intent(in)			::	y1
real(kind=wp),	intent(in)			::	x2
real(kind=wp),	intent(in)			::	y2

Return Value real(kind=wp)

public function ipart(x) result(i)

Integer part of floating-point number

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Arguments

Type	Intent	Optional	Attributes		Name
real(kind=wp),	intent(in)			::	x

Return Value integer

public function fpart(x) result(f)

Fractional part of floating-point number

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Arguments

Type	Intent	Optional	Attributes		Name
real(kind=wp),	intent(in)			::	x

Return Value real(kind=wp)

public function rfpart(x) result(rf)

Reverse fractional part (1 - fractional part)

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Arguments

Type	Intent	Optional	Attributes		Name
real(kind=wp),	intent(in)			::	x

Return Value real(kind=wp)

Subroutines

public subroutine blend_pixel(image_data, img_w, img_h, x, y, alpha, new_r, new_g, new_b)

Alpha blend a pixel with existing pixel data

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Arguments

Type	Intent	Optional	Attributes		Name
integer(kind=1),	intent(inout)			::	image_data(:)
integer,	intent(in)			::	img_w
integer,	intent(in)			::	img_h
real(kind=wp),	intent(in)			::	x
real(kind=wp),	intent(in)			::	y
real(kind=wp),	intent(in)			::	alpha
real(kind=wp),	intent(in)			::	new_r
real(kind=wp),	intent(in)			::	new_g
real(kind=wp),	intent(in)			::	new_b

public subroutine draw_line_distance_aa(image_data, img_w, img_h, x0, y0, x1, y1, r, g, b, width)

Draw antialiased line using distance-based approach

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Arguments

Type	Intent	Optional	Attributes		Name
integer(kind=1),	intent(inout)			::	image_data(:)
integer,	intent(in)			::	img_w
integer,	intent(in)			::	img_h
real(kind=wp),	intent(in)			::	x0
real(kind=wp),	intent(in)			::	y0
real(kind=wp),	intent(in)			::	x1
real(kind=wp),	intent(in)			::	y1
real(kind=wp),	intent(in)			::	r
real(kind=wp),	intent(in)			::	g
real(kind=wp),	intent(in)			::	b
real(kind=wp),	intent(in)			::	width

public subroutine draw_filled_quad_raster(image_data, img_w, img_h, x_quad, y_quad, r, g, b)

Draw filled quadrilateral using scanline algorithm

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Arguments

Type	Intent	Optional	Attributes		Name
integer(kind=1),	intent(inout)			::	image_data(:)
integer,	intent(in)			::	img_w
integer,	intent(in)			::	img_h
real(kind=wp),	intent(in)			::	x_quad(4)
real(kind=wp),	intent(in)			::	y_quad(4)
real(kind=wp),	intent(in)			::	r
real(kind=wp),	intent(in)			::	g
real(kind=wp),	intent(in)			::	b