Image - Basics

Imaging drawing with AGG is a bit complicated. It works quite similar to how SVG draws image. Instead of drawing the source image at the target image buffer location with specified scaling as in most 2D graphics APIs, AGG does the reverse lookup of source image pixels needed to draw the target image pixel.

The following types are needed.

typedef agg::span_interpolator_linear<agg::trans_affine> InterpolatorType;
typedef agg::image_accessor_clip<PixelFormat> ImageAccessorType;
typedef agg::span_image_filter_rgba_nn<ImageAccessorType, InterpolatorType> SpanGeneratorType;
typedef agg::span_allocator<agg::rgba8> SpanAllocatorType;

Then we need to compute the reverse lookup matrix. In the following code, matrix would be the normal parameters needed to draw images using convention 2D graphics API. invertMatrix would be the reverse lookup matrix.

agg::trans_affine matrix;
matrix.scale(xScale, yScale);
matrix.translate(x, y);

// from the matrix, compute the invert matrix.  It will
// be the lookup of src pixels from the dst location.
agg::trans_affine invertMatrix (matrix);
invertMatrix.invert();

We also need a path to specify the target region to do the reverse lookup.

double rect[] =
{
    (double)x, (double)y,
    (double)x + width, (double)y,
    (double)x + width, (double)y + height,
    (double)x, (double)y + height,
    (double)x, (double)y
};

// The target region to draw the image.
SimplePath path(rect, sizeof(rect) / sizeof(double));

Finally, hook everything up and draw the image.

agg::rasterizer_scanline_aa<> ras;
agg::scanline_p8 scanline;

InterpolatorType interpolator (invertMatrix);
ImageAccessorType ia (src.ren(), transparentWhiteColor);
SpanGeneratorType sg (ia, interpolator);
SpanAllocatorType sa;

ras.reset();
ras.add_path(path);
agg::render_scanlines_aa(ras, scanline, dst, sa, sg);

Example

tutorial_image_1.cpp first generates the following image icon.

Then it draws this image with various scaling, translation etc.

An equivalent SVG is