First import.

Makefile

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+imcat: imcat.c
+	cc -Wall -g -o imcat imcat.c -lm
+
+run: imcat
+	./imcat ~/Desktop/*.png
+
+clean:
+	rm -f ./imcat
+

imcat.c

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+#include <stdio.h>
+#include <stdlib.h>
+#include <assert.h>
+#include <math.h>
+
+#define STB_IMAGE_IMPLEMENTATION
+#include "stb_image.h"
+
+static int termw=0, termh=0;
+
+#define RESETALL  "\x1b[0m"
+
+
+static void print_image( int w, int h, unsigned char* data )
+{
+	const int linesz = 16384;
+	char line[ linesz ];
+	unsigned char* reader = data;
+
+	for ( int y=0; y<h; ++y )
+	{
+		line[0] = 0;
+		for ( int x=0; x<w; ++x )
+		{
+			strncat( line, "\x1b[48;2;", sizeof(line) - strlen(line) - 1 );
+			char tripl[80];
+			unsigned char r = *reader++;
+			unsigned char g = *reader++;
+			unsigned char b = *reader++;
+			unsigned char a = *reader++;
+			(void) a;
+			snprintf( tripl, sizeof(tripl), "%d;%d;%dm ", r,g,b );
+			strncat( line, tripl, sizeof(line) - strlen(line) - 1 );
+		}
+		strncat( line, RESETALL, sizeof(line) - strlen(line) - 1 );
+		puts( line );
+	}
+	assert( reader = data + w * h * 4 );
+}
+
+
+static int process_image( const char* nm )
+{
+	int imw=0,imh=0,n=0;
+	unsigned char *data = stbi_load( nm, &imw, &imh, &n, 4 );
+	if ( !data )
+		return -1;
+	//fprintf( stderr, "%s has dimension %dx%d w %d components.\n", nm, imw, imh, n );
+
+	float aspectratio = imw / (float) imh;
+	float pixels_per_char = imw / (float)termw;
+	if ( pixels_per_char < 1 ) pixels_per_char = 1;
+	int kernelsize = (int) floorf( pixels_per_char );
+	if ( (kernelsize&1) == 0 ) kernelsize--;
+	if ( !kernelsize ) kernelsize=1;
+	const int kernelradius = (kernelsize-1)/2;
+
+	const int outw = imw < termw ? imw : termw;
+	const int outh = (int) roundf( outw / aspectratio );
+
+	//fprintf( stderr, "pixels per char: %f, kernelsize: %d, out: %dx%d\n", pixels_per_char, kernelsize, outw, outh );
+
+	unsigned char out[ outh ][ outw ][ 4 ];
+	for ( int y=0; y<outh; ++y )
+		for ( int x=0; x<outw; ++x )
+		{
+			const int cx = (int) roundf( pixels_per_char * x );
+			const int cy = (int) roundf( pixels_per_char * y );
+			int acc[4] = {0,0,0,0};
+			int numsamples=0;
+			int sy = cy-kernelradius;
+			sy = sy < 0 ? 0 : sy;
+			int ey = cy+kernelradius;
+			ey = ey >= imh ? imh-1 : ey;
+			int sx = cx-kernelradius;
+			sx = sx < 0 ? 0 : sx;
+			int ex = cx+kernelradius;
+			ex = ex >= imw ? imw-1 : ex;
+			for ( int yy = sy; yy <= ey; ++yy )
+				for ( int xx = sx; xx <= ex; ++xx )
+				{
+					unsigned char* reader = data + ( yy * imw * 4 ) + xx * 4;
+					acc[ 0 ] += reader[0];
+					acc[ 1 ] += reader[1];
+					acc[ 2 ] += reader[2];
+					acc[ 3 ] += reader[3];
+					numsamples++;
+				}
+			out[ y ][ x ][ 0 ] = acc[ 0 ] / numsamples;
+			out[ y ][ x ][ 1 ] = acc[ 1 ] / numsamples;
+			out[ y ][ x ][ 2 ] = acc[ 2 ] / numsamples;
+			out[ y ][ x ][ 3 ] = acc[ 3 ] / numsamples;
+		}
+
+	stbi_image_free( data );
+	data = 0;
+
+	print_image( outw, outh, (unsigned char*) out );
+	return 0;
+}
+
+
+int main( int argc, char* argv[] )
+{
+	if ( argc == 1 || !strcmp( argv[1], "--help" ) )
+	{
+		fprintf( stderr, "Usage: %s image [image2 .. imageN]\n", argv[0] );
+		exit( 0 );
+	}
+
+	// Step 1: figure out the width and height of terminal.
+	FILE* f = popen( "stty size", "r" );
+	if ( !f )
+	{
+		fprintf( stderr, "%s: Failed to determine terminal size using stty.\n", argv[0] );
+		exit( 1 );
+	}
+	const int num = fscanf( f, "%d %d", &termh, &termw );
+	assert( num == 2 );
+	pclose( f );
+
+	fprintf( stderr, "Your terminal is size %dx%d\n", termw, termh );
+
+	for ( int i=1; i<argc; ++i )
+	{
+		const char* nm = argv[ i ];
+		int rv = process_image( nm );
+		if ( rv < 0 )
+			fprintf( stderr, "Could not load image %s\n", nm );
+	}
+
+	return 0;
+}
+