% SINERAMP - Generates sine on a ramp colour map test image % % The test image consists of a sine wave superimposed on a ramp function The % amplitude of the sine wave is modulated from its full value at the top of the % image to 0 at the bottom. % % The image is useful for evaluating the effectiveness of different colour maps. % Ideally the sine wave pattern should be equally discernible over the full % range of the colour map. In addition, across the bottom of the image, one % should not see any identifiable features as the underlying signal is a smooth % ramp. In practice many colour maps have uneven perceptual contrast over their % range and often include 'flat spots' of no perceptual contrast that can hide % significant features. % % Usage: im = sineramp(sze, amp, wavelen, p) % im = sineramp; % % Arguments: sze - [rows cols] specifying size of test image. If a % single value is supplied the image is square. % Defaults to [256 512]; Note the number of columns is % nominal and will be ajusted so that there are an % integer number of sine wave cycles across the image. % amp - Amplitude of sine wave. Defaults to 12.5 % wavelen - Wavelength of sine wave in pixels. Defaults to 8. % p - Power to which the linear attenuation of amplitude, % from top to bottom, is raised. For no attenuation use % p = 0. For linear attenuation use a value of 1. For % contrast sensitivity experiments use larger values of % p. The default value is 2. % % The ramp function that the sine wave is superimposed on is adjusted slightly % for each row so that each row of the image spans the full data range of 0 to % 255. Thus using a large sine wave amplitude will result in the ramp at the % top of the test image being reduced relative to the slope of the ramp at the % bottom of the image. However, the adjustment ensures that, at the lower edge % of the image, the full colour map is displayed. % % To start with try % >> im = sineramp; % % This is equivalent to % >> im = sineramp([256 512], 12.5, 8, 2); % % View it under 'gray' then try the 'jet', 'hsv', 'hot' etc colour maps. The % results may cause you some concern! % % If you are wishing to evaluate a cyclic colour map, say hsv, it is suggested % that you use the test image generated CIRCLESINERAMP. However you can use % this function to perform a basic evaluation of a cyclic colour map by % displaying two copies of the SINERAMP test image concatenated side-by-side. % % >> show([sineramp sineramp]), colormap(map_to_be_tested) % % However, note that despite there being an integer number of sine wave cycles % across the image and that each row has been adjusted to span the full data % range there will be a slight cyclic discontinuity at the top of the image, % though this is progressively removed as you move down the test image. % % See source code comments for more details on the default wavelength and % amplitude. % % See also: CIRCLESINERAMP, CHIRPLIN, CHIRPEXP, EQUALISECOLOURMAP, CMAP % The Default Wavelength: % The default wavelength is 8 pixels. On a computer monitor with a nominal % pixel pitch of 0.25mm this corresponds to a wavelength of 2mm. With a monitor % viewing distance of 600mm this corresponds to 0.19 degrees of viewing angle or % approximately 5.2 cycles per degree. This falls within the range of spatial % frequencies (3-7 cycles per degree ) at which most people have maximal % contrast sensitivity to a sine wave grating (this varies with mean luminance). % A wavelength of 8 pixels is also sufficient to provide a reasonable discrete % representation of a sine wave. The aim is to present a stimulus that is well % matched to the performance of the human visual system so that what we are % primarily evaluating is the colour map's perceptual contrast and not the % visual performance of the viewer. % % The Default Amplitude: % This is set at 12.5 so that from peak to trough we have a local feature of % magnitude 25. This is approximately 10% of the 256 levels in a standard % colour map. It is not uncommon for colour maps to have perceptual flat spots % that can hide features of this magnitude. % Copyright (c) 2013-2014 Peter Kovesi % Centre for Exploration Targeting % The University of Western Australia % peter.kovesi at uwa edu au % % Permission is hereby granted, free of charge, to any person obtaining a copy % of this software and associated documentation files (the "Software"), to deal % in the Software without restriction, subject to the following conditions: % % The above copyright notice and this permission notice shall be included in % all copies or substantial portions of the Software. % % The Software is provided "as is", without warranty of any kind. % July 2013 Original version. % March 2014 Adjustments to make it better for evaluating cyclic colour maps. % June 2014 Default wavelength changed from 10 to 8. function im = sineramp2(sze, amp, wavelen, p) if ~exist('sze','var'), sze = [256 512]; end if ~exist('amp','var'), amp = 12.5; end if ~exist('wavelen','var'), wavelen = 8; end if ~exist('p','var'), p = 2; end if length(sze) == 1 rows = sze; cols = sze; elseif length(sze) == 2 rows = sze(1); cols = sze(2); else error('size must be a 1 or 2 element vector'); end % Adjust width of image so that we have an integer number of cycles of % the sinewave. This is helps should one be using the test image to % evaluate a cyclic colour map. However you will still see a slight % cyclic discontinuity at the top of the image, though this will % disappear at the bottom of the test image cycles = round(cols/wavelen); cols = cycles*wavelen; % Sine wave x = 0:cols-1; fx = amp*sin( 1/wavelen * 2*pi*x); % Vertical modulating function A = ([(rows-1):-1:0]/(rows-1)).^p; im = A'*fx; % Add ramp ramp = meshgrid(0:(cols-1), 1:rows)/(cols-1); im = im + ramp*(255 - 2*amp); % Now normalise each row so that it spans the full data range from 0 to 255. % This ensures that, at the lower edge of the image, the full colour map is % displayed. It also helps with the evaluation of cyclic colour maps though % a small cyclic discontinuity will remain at the top of the test image. for r = 1:rows im(r,:) = normalise(im(r,:)); end im = im * 255;